HETEROCYCLES

Paper | Special issue | Vol. 84, No. 1, 2012, pp. 473-492
Received, 1st April, 2011, Accepted, 20th May, 2011, Published online, 23rd May, 2011.
DOI: 10.3987/COM-11-S(P)8

■ Model Studies Towards the Total Synthesis of the Stemona Alkaloid 1-Hydroxyprotostemonine: Synthesis of ent-1-Hydroxystemoamide

Nalivela Kumara Swamy and Stephen G. Pyne*

School of Chemistry, Faculty of Science, University of Wollongong, Wollongong NSW 2522 , Australia

Abstract

As part of a model study towards the total synthesis of Stemona alkaloid 1-hydroxyprotostemonine 1, we have achieved the synthesis an A-B-C ring precursor, ent-1-hydroxystemoamide. Key steps involve an ene-yne RCM reaction and a diastereoselective dihydroxylation-lactonization reaction.

The Stemona family of alkaloids includes more than 139 different natural products,1-3 which have been structurally classified by Pilli into eight different groups.1,3 The pyrrolo[1,2-a]azepine nucleus is common to six of these, while a pyrido[1,2-a]azepine ring system is found in the more recently discovered Stemocurtisine group of Stemona alkaloids.1-6 Greger, on the other hand, has classified the Stemona alkaloids into three groups on the basis of biosynthetic considerations.2 In 2010 the first two Stemona alkaloids with a pyrido[1,2-a]azonine skeleton were reported.7 The Stemona alkaloid oxyprotostemonine 14,5 was isolated in 2004 and more recently we reported the isolation of the structurally related alkaloid, 1-hydroxyprotostemonine 28 which can be considered as a dihydro-derivative of 1 or a hydroxylated-derivative of protostemonine (Figure 1).
In this paper we report on a study into the synthesis of alkaloids 1 and 2 based on the retrosynthetic analysis shown in Scheme 1. This analysis suggested that the tricyclic compound 3 would be a suitable

advanced intermediate to achieve the synthesis of the target molecules. This compound is the 1R-hydroxy analogue of stemoamide for which several successful syntheses have been reported for this natural product.9 A formal synthesis of its enantiomer10 and syntheses of the racemate have also been reported.11 Our retrosynthetic analysis suggested that D-malic acid would be a suitable starting material. A ring-closing metathesis (RCM) reaction of the ene-yne 5 was expected to give the pyrrolo[1,2-a]azepine 5 based on Mori’s earlier synthesis of stemoamide via de-benzyloxy 5.9g,h However since L-malic acid is significantly less expensive than its D-form we performed our model studies on the enantiomeric series to that shown in Scheme 1(a). Here we report on the synthesis of the tricyclic compound 7 (Scheme 1(b)), the enantiomer of 1R-hydroxylstemoamide 3, from L-malic acid.
Our initial studies to prepare a 4,5-cis-hydroxyl-alkynyl pyrrolidinone like 5 (in the enantiomeric series) involved a study of the synthesis of the 4,5-cis-hydroxyl-ethynylpyrrolidinone 11 (Scheme 2). A mixture of L-malic acid and 4-pentenylamine12 in xylene were heated at 140 oC for 5 h to give the desired N-4-pentenylsuccinimide 9 in 62% yield. Regioselective reduction of 9 with NaBH4 at -40 oC13 gave the hemi-aminal 10 as the 4,5-cis-stereoisomer as evident from the relatively large geminal coupling constant J4,5 of 6.5 Hz.13 Treatment of the diol 10 with trimethylsilylethynyltrifluoroborate (3.0 equiv)/BF3●OEt2 (4.0 equiv) gave the desired cis-alkyne 11. Unfortunately the best yield we could obtain was 25% when the reaction was performed at 0 oC – rt with MeNO2 as the solvent. When we used CH2Cl2 as the solvent no reaction was observed. The 4,5-cis stereochemistry was evident from the magnitude of J4,5 which was 6.5 Hz.
An alternative route was then sort. The known N-PMB succinimide 12, prepared in two steps from L-malic acid,14 was treated with CAN in acetonitrile/water (3:1) to provide the N-deprotected succinimide 13 in 82% yield (Scheme 3) which was further transformed to the N-4-pentenylsuccinimide 8 (84% yield)

by treatment with 4-penten-1-ol under Mitsunobu’s reaction conditions15 (Scheme 3). Treatment of 8 with lithium 2-trimethylsilylacetylide gave a diastereomeric mixture (dr = 67:33) of carbinol adducts 14 which upon reduction with NaBH3CN (5.0 eq) in AcOH afforded a separable mixture of the cis and trans products 16 (52% yield) and 17 (26% yield), respectively (Scheme 3). The major isomer 16 showed J4,5 = 6.0 Hz, indicative of 4,5-cis stereochemistry13 while the minor trans isomer had J4,5 = 0.0 Hz. Under similar reaction conditions the reaction of 8 and lithium 3-tert-butyldimethylsilyloxypropynilide followed by treatment of 15 (dr = 70:30) with NaBH3CN (5.0 eq) in AcOH afforded a 3:1 inseparable mixture of the cis and trans TBS deprotected products 19 and 20, respectively, in 58% yield. The stereochemistry of the major isomer was confirmed from the magnitude of its coupling constant between H4 and H5 in its 1H NMR spectrum. The major isomer 10 showed J4,5 = 6.5 Hz, indicative of 4,5-cis stereochemistry.13 However obtaining the coupling constant J4,5 for the minor isomer proved difficult because of overlapping signals.

Compound 16 underwent a smooth ene-yne RCM reaction with Grubbs’ II catalyst to give the desired pyrrolo[1,2-a]azepine 20 in 81% yield (Scheme 4). We had planned to convert 20 to the diol 21 which upon oxidation was expected to produce the desired lactone C ring. However, attempts to convert 20 to the diol 21 by chemoselective hydroboration reactions using BH3•DMS were not successful due to competing reduction of the lactam carbonyl. Somfi reported similar difficulties during his studies on the synthesis of stemoamide.9b
In contrast to ene-yne 16, the mixture of ene-ynes 18/19 was unreactive to either Grubbs’ I or II catalyst (Scheme 5). This mixture was then oxidized with Jones reagent16 and the crude acid was not purified but

was esterified17 using DCC (1.0 eq), DMAP (0.1 eq) and EtOH (1.3 eq) in CH2Cl2. This gave a mixture (3:1) of separable cis and trans products from which the major cis isomer 22 (J4,5 = 6.5 Hz) could be isolated in 47% overall yield. When 22 was treated with Grubbs’ I catalyst in CH2Cl2 at rt for 5 h then the desired pyrrolo[1,2-a]azepine 23 was isolated in 82% yield (Scheme 5). Compound 23 was converted to the tricyclic compound 25 using the procedures employed by Mori in the synthesis of stemoamide.9g,h Reduction of the terminal alkene of 22 went as expected and gave 24 was an inseparable 4:1 mixture of diastereomers. The ester group was then hydrolysed to the corresponding acid which was treated with CuBr2 on alumina at 65 oC according to Mori’s bromo-lactonization procedure.9g,h The resulting mixture was then treated with Et3N to effect complete elimination of HBr from the bromo-lactone intermediate. Unfortunately, this resulted in a 1:1 diastereomeric mixture of the tricyclic molecule 25 and 26 which could be separated by column chromatography in low overall yields (Scheme 5). In the de-benzyloxy series of Mori this reaction was highly diastereoselective in favour of the desired stereoisomer.9g,h Clearly the extra O-benzyl group in our substrate 24 is responsible for the poor diasteoselectivity of this reaction sequence. Evidence for the configuration of 25 was obtained from NOESY NMR experiments which showed a significant correlation between H3a and H10a. In compound 26 however, no NOESY correlation between H3a and H10a was observed. Correlations were also observed between H10 and H10a in both compounds (Figure 2).

Osmium catalysed syn-dihydroxylation of 24 resulted in formation of the lactone 27 (dr 5:1) in a highly diastereoselective process since dehydration of 27 under acid conditions gave the unsaturated lactone 28 as a single diastereomer in 70% yield (Scheme 6). No other diastereomer of 27 could be detected by NMR analysis of the crude reaction mixture or could be isolated. The conversion of 27 to 28 also involved cleavage of the O-benzyl protecting group. Finally, reduction of 28 with NaBH4/NiCl29g,h gave in 78% yield an inseparable mixture (dr = 87:13) of 7 and its diastereomer 29 (Scheme 6). The stereochemical assignment of 7 was based on the NOESY correlations observed between H-10 and H-10a, H-10a and H-10b and H-10a and the C-1 Me group and the lack of correlations between H-3a and H-10b (Scheme 6 (inset)). Further supportive evidence for the stereochemical relationship between H-1 and H-10b in 7 was the magnitude of J1,10b (12.3 Hz) which was almost identical to that found in stemoamide (12.4 Hz).18 The stereochemical relationship between 26 and 28 came from the O-debenzylation reaction of 26 with BBr3 which gave, in 67% yield, a product identical to compound 28 that was prepared according to Scheme 6 (Scheme 7). The stereochemistry assigned to 29 is not certain because a diastereomerically pure sample could not be obtained. Its stereochemistry at C-10b is based on literature precedent for the reduction of related molecules.9g,h

In conclusion, as part of a model study towards the total synthesis of the Stemona alkaloid 1-hydroxyprotostemonine 1, we have achieved the synthesis an A-B-C ring precursor, ent-1-hydroxystemoamide 7 starting with L-malic acid. The bromolactonization method employed by Mori in the synthesis of stemoamide was not diasteoselective in the corresponding O-benzyl analogues prepared in this study. An alternative route was developed using a highly diastereoselective syn-dihydroxylation reaction to prepare the lactone C-ring of 7. In principle this method could be used to prepare compound 3 from D-malic acid which could then be converted to 1-hydroxyprotostemonine 1 by the addition of the two butyrolactone rings.19-21

EXPERIMENTAL
General methods
All reactions were performed in oven dried glassware under an atmosphere of nitrogen, unless otherwise stated. Anhydrous CH2Cl2 and MeOH were obtained from Sigma-Aldrich Chemical Co. Anhydrous THF was obtained by distillation from sodium wire/benzophenone. “Evaporation” refers to the removal of solvent under reduced pressure using a rotory evaporator and then the removal of the last traces of solvent under high vacuum. Commercial substances were used without further purification. Petrol refers to the hydrocarbon fraction of bp 45-55 oC. 1H and 13C NMR spectra were recorded on a Varian Inova NMR Spectrometer (1H NMR at 500 MHz and 13C NMR at 125 MHz) or Varian Unity-300 (1H NMR at 300 (1H NMR at 300 MHz, 13C NMR at 75 MHz) instruments. CDCl3 (internal reference at 7.26 for 1H NMR and δ 77.00 for 13C NMR) was used as the NMR unless otherwise stated. The following abbreviations were used; s = singlet, d = doublet, t = triplet, m = multiplet, dd = doublet of doublet, br = broad. NMR assignments were based on COSY, HSQC, HMBC, NOESY and DEPT experiments. TLC analyses were performed using aluminium backed Merk silica gel TLC plates. Flash column chromatography was performed using Merk silica gel (40 – 63 μm) packed by the slurry method. Melting points were obtained using a Gallenkamp MF-370 capillary tube melting point apparatus and are uncorrected. Optical rotations were measured using a 1 cm cell, in a Jasco DIP-370 digital polarimeter. Specific rotations were calculated by using the average value of 10 optical rotation measurements. Low-resolution mass spectra were obtained on a Shimadzu GC mass spectrometer (EI) or Waters LCZ single quadropole (ESI). High-resolution mass spectra (exact masses) were obtained on a VG Autospec mass spectrometer (EI) or Water QTOF (ESI). HRMS were obtained in lieu of elemental analysis and 1H and 13C NMR spectroscopy were used as the criteria for purity. Infrared spectra were obtained as neat samples on a Smart Omni-Sampler Avator ESP Nicolet-Brand.

(S)-3-Hydroxy-1-(pent-4-enyl)pyrrolidine-2,5-dione (9)
To a solution of L-malic acid (0.10 g, 0.74 mmol) in xylene (50 mL) in a round bottom flask equipped with a Dean-Stark trap and a condenser was added pent-4-en-1-amine12 (0.095 g, 1.12 mmol) at rt. The resulting suspension was heated at 140 oC for 5 h, and then xylene was removed in vacuo. The crude product was purified by column chromatography (2:1, EtOAc/petrol) to give compound 9 (0.085 g, 62%) as a pale yellow oil. Rf = 0.52 (2:1, EtOAc/petrol). [α]27D -65.4 (c 3.62, CHCl3). IR (neat, νmax/cm-1): 3449, 2939, 1694, 1404, 1182, 909. 1H NMR (500 MHz, CDCl3): δ 5.82 – 5.70 (1H, m, H4`), 5.04 (1H, dd, J = 1.0, 17.0 Hz, H5`), 5.01 (1H, d, J = 10.0 Hz, H5`), 4.65 (1H, dd, J = 5.0, 8.5 Hz, H3), 4.24 (1H, bs, OH), 3.52 (2H, t, J = 7.5 Hz, H1`), 3.06 (1H, dd, J = 8.5, 18.0 Hz, H4), 2.67 (1H, dd, J = 5.0, 18.0 Hz, H4), 2.08 – 2.04 (2H, m, H3`), 1.71 – 1.65 (2H, m, H2`). 13C NMR (125 MHz, CDCl3): δ 178.7 (C5), 174.3 (C2), 136.9 (C4`), 115.4 (C5`), 66.7 (C3), 38.4 (C1`), 37.1 (C4), 30.8 (C3`), 26.5 (C2`). ESIMS m/z 184 [(M+H)+ 100%]. HRESIMS calcd. for C9H14NO3, (M+H)+ 184.0937, found: 184.0933.

(4S,5S)-4,5-Dihydroxy-1-(pent-4-enyl)pyrrolidin-2-one (10)
The cyclic imide 9 (0.085 g, 0.46 mmol) was dissolved in EtOH (20 mL), and the solution was cooled to –40 oC. NaBH4 (0.088 g, 2.36 mmol) was added portionwise and the resulting suspension was stirred at –40 oC for 30 min. Then the reaction mixture was quenched with saturated aqueous NaHCO3 solution (30 mL) and was extracted with EtOAc (3 x 70 mL). The combined organic extracts were dried (MgSO4) and concentrated in vacuo. The crude product was purified by column chromatography (0.5:10, MeOH/EtOAc) to give product 10 (0.068 g, 80%) as a colourless oil. Rf = 0.40 (0.5:10, MeOH/EtOAc). [α]23D+ 29.3 (c 0.12, CHCl3). IR (neat, νmax /cm-1): 3329, 2960, 1664, 1465, 1413, 1259, 1081, 1016, 799. 1H NMR (500 MHz, CDCl3): δ 5.83-5.76 (1H, m, H4), 5.09-4.97 (2H, m H5`), 4.35 (1H, dd, J = 6.5, 7.0 Hz, H4), 4.21 (1H, d, J = 7.0 Hz, H5), 3.50-3.42 (1H, m, H1`), 3.22-3.13 (1H, m, H1), 2.83 (1H, dd, J = 6.5, 17.5 Hz, H3), 2.09-2.04 (2H, m, H3), 1.73-1.60 (1H, m , H2`). 13C NMR (125 MHz, CDCl3): δ 173.7 (C2), 137.5 (C4`), 115.3 (C5`), 82.9 (C5), 71.8 (C4), 39.7 (C1`), 38.5 (C3), 31.0 (C3`), 26.7 (C2`). EIMS m/z 185 (M +, 70%). HREIMS calcd. for C9H15NO3 (M +) 185.1044, found: 185.1038.

(4S,5S)-5-Ethynyl-4-hydroxy-1-(pent-4-enyl)pyrrolidin-2-one (11)
To a stirred solution of 10 (0.60 g, 3.00 mmol) and potassium trimethylsilylethynyltrifluoroborate (1.84 g, 9.00 mmol) in MeNO2 (1.3 mL) at 0 oC under a N2 atmosphere was added BF3·Et2O (1.55 mL, 12.0 mmol). The resulting mixture was stirred for 1 h before warming to rt and stirring for a further 12 h. The reaction mixture was then diluted with EtOAc (8 mL) and washed with saturated aqueous NaHCO3 (8 mL). The separated organic phase was dried (MgSO4) then concentrated under reduced pressure. The resulting residue was dissolved in THF (5 mL) then treated with LiOH (2 mL of a saturated aqueous solution) and the resulting mixture was stirred for 1 h before being diluted with EtOAc (8 mL). The separated organic layer was dried (MgSO4) and the solvent removed in vacuo. The crude residue was purified by column chromatography (2:1, EtOAc/petrol) to afford the title compound 11 (0.15 g, 25%) as a pale yellow liquid. Rf = 0.53 (2:1, EtOAc/petrol). [α]26D -12.6 (c 1.5, CHCl3). IR (neat, νmax/cm-1): 3287, 2930, 2361, 2332, 1676, 1425, 1261, 826. 1H NMR (500 MHz, CDCl3): δ 5.84 – 5.77 (1H, m, H4`), 5.05 (1H, d, J = 17.5 Hz, H5`), 4.98 (1H, d, J = 10.5 Hz, H5`), 4.48 (1H, dd, J = 2.0, 5.5 Hz, H5), 4.44 (1H, apparent q, J = 5.5 Hz, H4), 3.67 – 3.61 (1H, m, H1`), 3.19 – 3.13 (1H, m, H1`), 2.68 – 2.61 (2H, m, H3, H7`), 2.06 (2H, apparent q, J = 7.0 Hz, H3`), 1.74 – 1.61 (2H, m, H2`). 13C NMR (125 MHz, CDCl3): δ 172.1 (C2), 137.4 (C4`), 115.1 (C5`), 77.7, 76.4 (C≡CH), 65.7 (C4), 56.1 (C5), 40.7 (C1`), 39.1 (C3), 30.9 (C3`), 26.3 (C2`). ESIMS m/z 194 [(M+H)+ 100%]. HRESIMS calcd. for C11H16NO2, (M+H)+ 194.1122, found: 194.1127.

(S)-3-(Benzyloxy)pyrrolidine-2,5-dione (13)
To a solution of 1214 (6.20 g, 19.0 mmol) in MeCN/H2O (3:1, 400 mL) at rt was added CAN (41.81 g, 76.00 mmol). The reaction mixture was stirred at rt for 1 h. The mixture was diluted with water and extracted with EtOAc (3 x 50 mL). The organic extracts were washed with saturated aqueous NaHCO3 solution (30 mL), dried (MgSO4) and the solvent was concentrated in vacuo. The crude product was purified by column chromatography (1:1, EtOAc/petrol ) to afford compound 13 (3.24 g, 82%) as a colourless oil. Rf = 0.45 (2:1, EtOAc/petrol). [α]25D -98.5 (c 5.75, CHCl3). IR (neat, νmax/cm-1): 3232, 3084, 2360, 1790, 1712, 1329, 1191, 1113, 741, 698. 1H NMR (500 MHz, CDCl3): δ 9.17 (1H, bs, NH), 7.36 – 7.30 (5H, m, ArH), 4.93 (1H, d, J = 11.5 Hz, H1`), 4.74 (1H, d, J = 11.5 Hz, H1`), 4.37 (1H, dd, J = 4.0, 8.0 Hz, H3), 2.94 (1H, dd, J = 8.0, 18.0 Hz, H4), 2.69 (1H, dd, J = 4.0, 18.0 Hz, H4). 13C NMR (125 MHz, CDCl3): δ 176.7 (C5), 174.8 (C2), 136.5 (ArC), 128.5 (ArCH), 128.2 (ArCH), 128.18 (ArCH), 72.9 (C1`), 72.8 (C3), 37.2 (C4). ESIMS m/z 206 [(M+H)+ 100%]. HRESIMS calcd. for C11H12NO3, (M+H)+ 206.0779, found: 206.0777.

(S)-3-(Benzyloxy)-1-(pent-4-enyl)pyrrolidine-2,5-dione (8)
To a solution of 13 (0.10 g, 0.48 mmol) in THF (5 mL) were added 4-penten-1-ol (0.06 mL, 0.58 mmol), PPh3 (0.153 g, 0.58 mmol), and DIAD (0.12 mL, 0.58 mmol) at 0 oC under a N2 atmosphere. The mixture was stirred for 5 min and then warmed to rt and stirring was continued for 4 h. The solvent was removed in vacuo and the crude product was purified by column chromatography (1:9, EtOAc/petrol) to afford compound 8 (0.112 g, 84%) as a colourless oil. Rf = 0.67 (2:8, EtOAc/petrol). [α]25D-61 (c 4.26, CHCl3). IR (neat, νmax/cm-1): 2936, 2360, 1704, 1403, 1269, 1113, 741, 698. 1H NMR (500 MHz, CDCl3): δ 7.41 – 7.31 (5H, m, ArH), 5.82 – 5.74 (1H, m, H4`), 5.04 (1H, d, J = 17.5 Hz, H5`), 4.98 (1H, d, J = 11.5 Hz, H5`), 4.98 (1H, d, J = 11.5 Hz, H6`), 4.78 (1H, d, J = 11.5 Hz, H6`), 4.34 (1H, dd, J = 4.0, 8.0 Hz, H3), 3.51 (2H, t, J = 8.5 Hz, H1`), 2.92 (1H, dd, J = 8.0, 18.0 Hz, H4), 2.63 (1H, dd, J = 4.0, 18.0 Hz, H4), 2.05 (2H, apparent q, J = 7.0 Hz, H3`), 1.71 – 1.65 (2H, m, H2`). 13C NMR (125 MHz, CDCl3): δ 175.8 (C5), 174.1 (C2), 137.0 (ArC), 136.6 (C4`), 128.5 (ArCH), 128.2 (ArCH), 128.18 (ArCH), 128.1 (ArCH), 115.3 (C5`), 72.9 (C6`), 72.0 (C3), 38.2 (C1`), 36.1 (C4), 30.8 (C3`), 26.5 (C2`). ESIMS m/z 274 [(M+H)+ 100%]. HRESIMS calcd. for C16H20NO3, (M+H)+ 274.1403, found: 274.1394.

(4S)-4-(Benzyloxy)-5-hydroxy-1-(pent-4-enyl)-5-((trimethylsilyl)ethynyl)pyrrolidin-2-one (14)
To a solution of trimethylsilylacetylene (3.58 g, 36.63 mmol) in dry THF (20 mL) at – 78 oC, was added dropwise n-BuLi (2.5 M in hexanes, 11.7 mL, 29.0 mmol). After stirring for 15 min a solution of 8 (4.00 g, 14.0 mmol) in dry THF (5 mL) was added dropwise over 15 min at – 78 oC. Then stirring was continued for 1 h at – 78 oC under a N2 atmosphere. Saturated aqueous NH4Cl solution (15 mL) was added at – 78 oC and then the mixture was extracted with EtOAc (3 x 40 mL). The combined organic extracts were washed with brine, dried (MgSO4) and concentrated in vacuo. The residue was purified by chromatography (2.5:1 EtOAc/petrol)) to afford 14 as a 67:33 mixture of two diastereomeric products (3.80 g, 70%) as a pale yellow gum. Rf = 0.52 (2.5:1, EtOAc/petrol). IR (neat, νmax/cm-1): 3278, 2944, 2359, 2344, 1698, 1404, 1327, 1251, 845, 738, 698. 1H NMR (500 MHz, CDCl3): δ major isomer 7.37 – 7.26 (5H, m, ArH), 5.85 – 5.77 (1H, m, H4`), 5.04 (1H, dd, J = 1.0, 17.0 Hz, H5`), 4.96 (1H, d, J = 10.5 Hz, H5`), 4.90 (1H, d, J = 11.5, H6`), 4.66 (1H, d, J = 11.5, H6`), 4.04 – 4.00 (2H, m, H4, OH), 3.46 – 3.32 (2H, m, H1`), 2.74 (1H, dd, J = 6.5, 17.0 Hz, H3), 2.41 (1H, dd, J = 6.5, 17.0 Hz, H3), 2.10 (2H, apparent q, J = 7.0 Hz, H3`), 1.83 – 1.77 (2H, m, H2`), 0.18 (9H, s, (CH3)3Si); minor isomer 7.39 – 7.31 (5H, m, ArH), 5.85 – 5.77 (1H, m, H4`), 5.03 (1H, dd, J = 1.0, 17.0 Hz, H5`), 4.95 (1H, d, J = 10.5 Hz, H5`), 4.77 (1H, d, J = 11.5, H6`), 4.75 (1H, d, J = 11.5, H6`), 4.22 (1H, t, J = 5.5, 7.0 Hz, H4), 4.09 (1H, s, OH), 3.48 – 3.33 (2H, m, H-1`), 2.61 (1H, dd, J = 7.0, 17.0 Hz, H3), 2.44 (1H, dd, J = 5.5, 17.0 Hz, H3), 2.08 (2H, apparent q, J = 7.5 Hz, H3`), 1.82 – 1.75 (2H, m, H2`), 0.25 (9H, s, (CH3)3Si).
13C NMR (125 MHz, CDCl3): δ major isomer 172.6 (C2), 138.3 (ArC), 138.1 (C4`), 128.8 (ArCH), 128.3 (ArCH), 128.2 (ArCH), 128.0 (ArCH), 115.4 (C5`), 100.3 (C≡C), 94.9 (C5), 88.8 (C≡C), 81.2 (C4), 72.8 (C6`), 40.4 (C1`), 36.7(C3), 31.8 (C3`), 28.3 (C2`), -0.00 (CH3)3Si); minor isomer 170.8 (C2), 137.8 (ArC), 136.3 (C4`), 128.6 (ArCH), 128.4 (ArCH), 128.0 (ArCH), 114.7 (C5`), 102.2 (C≡C), 91.6 (C5), 83.8 (C≡C), 78.1 (C4), 72.7 (C6`), 40.0 (C1`), 35.3 (C3), 31.2 (C3`), 27.7 (C2`), -0.45 (CH3)3Si). ESIMS m/z 372 [(M+H)+ 100%].
HRESIMS calcd. for C21H30NO3Si, (M+H)+ 372.1985, found: 372.1995.

(4S,5S)-4-(Benzyloxy)-1-(pent-4-enyl)-5-((trimethylsilyl)ethynyl)pyrrolidin-2-one (16) and (4S,5R)-4-(Benzyloxy)-1-(pent-4-enyl)-5-((trimethylsilyl)ethynyl)pyrrolidin-2-one (17)
To a solution of 14 (6.00 g, 16.0 mmol) in AcOH (40 mL) was added NaCNBH3 (5.00 g, 80 mmol) portionwise and the mixture was stirred at rt for 18 h. The mixture was quenched with water (50 mL) and extracted with EtOAc (2 x 100 mL). The combined extracts were washed with saturated aqueous NaHCO3 solution, dried (MgSO4) and the solvent was removed in vacuo. The crude product, which was a 2:1 mixture of cis:trans diastereomers, was purified by column chromatography (2:1 EtOAc/petrol) to give the cis compound 16 (2.99 g, 52%) as pale yellow gum and the trans compound 17 (1.51 g, 26%) as a pale yellow gum.
(16): Rf = 0.52 (2:1, EtOAc/petrol). [α]25D -7.7 (c 0.96, CHCl3). IR (neat, νmax/cm-1): 2936, 2176, 2359, 2340, 1701, 1251, 1108, 843, 789, 697. 1H NMR (500 MHz, CDCl3): δ 7.39 – 7.27 (5H, m, ArH), 5.84 – 5.76 (1H, m, H4`), 5.04 (1H, d, J = 17.0 Hz, H5`), 4.97 (1H, d, J = 10.0 Hz, H5`), 4.73 (1H, d, J = 11.5, H6`), 4.57 (1H, d, J = 11.5, H6`), 4.51 (1H, d, J = 6.0, H5), 4.21 (1H, apparent q, J = 6.5, H4), 3.62 – 3.56 (1H, m, H1`), 3.23 – 3.17 (1H, m, H-1`), 2.63 – 2.54 (2H, m, H3), 2.07 (2H, apparent q, J = 7.0 Hz, H3`), 1.75 – 1.59 (2H, m, H2`), 0.18 (9H, s, (CH3)3Si). 13C NMR (125 MHz, CDCl3): δ 172.0 (C2), 137.5 (ArC), 137.4 (C4`), 128.4 (ArCH), 128.3 (ArCH), 127.6 (ArCH), 115.0 (C5`), 98.4 (C≡CSi), 93.6 (C≡CSi), 72.6 (C4), 71.6 (C6`), 54.8 (C5), 40.8 (C1`), 37.0 (C3), 31.0 (C3`), 28.3 (C2`), -0.29 (CH3)3Si). ESIMS m/z 356 [(M+H)+ 100%]. HRESIMS calcd. for C21H30NO2Si, (M+H)+ 356.2032, found: 356.2046.
(17): Rf = 0.54 (2:1, EtOAc/petrol). [α]25D +7.8 (c 4.05, CHCl3). 1H NMR (500 MHz, CDCl3): δ 7.39 – 7.26 (5H, m, ArH), 5.84 – 5.76 (1H, m, H4`), 5.04 (1H, d, J = 17.0 Hz, H5`), 4.97 (1H, d, J = 10.0 Hz, H5`), 4.63 (2H, s, H6`), 4.28 (1H, s, H5), 4.20 – 4.17 (1H, m, H4), 3.62 – 3.50 (1H, m, H1`), 3.17 – 3.11 (1H, m, H1`), 2.75 (1H, dd, J = 7.0, 17.0 Hz, H3), 2.48 (1H, dd, J = 3.0, 17.0 Hz, H3), 2.07 (2H, q, J = 4.0 Hz, H3`), 1.72 – 1.61 (2H, m, H2`), 0.17 (9H, s, H9`). 13C NMR (125 MHz, CDCl3): δ 172.2 (C2), 137.9 (ArC), 137.5 (C4`), 128.8 (ArCH), 128.3 (ArCH), 128.0 (ArCH), 115.4 (C5`), 101.2 (C≡C), 92.2 (C≡C), 78.0 (C4), 71.7 (C6`), 56.7 (C5), 40.8 (C1`), 37.6 (C3), 31.2 (C3`), 26.5 (C2`), 0.02 (C9`). ESIMS m/z 356 [(M+H)+ 100%].

(4S)-4-(Benzyloxy)-5-(3-(tert-butyldimethylsilyloxy)prop-1-ynyl)-5-hydroxy-1-(pent-4-enyl)pyrrolidin- 2-one (15)
To a solution of tert-butyldimethyl(prop-2-ynyloxy)silane (1.86 g, 10.0 mmol) in dry THF (10 mL) at – 78 oC, was added dropwise n-BuLi in hexane (2.93 mL, 2.5 M, 7.32 mmol). After stirring for 15 min a solution of 8 (1.0 g, 3.66 mmol) in dry THF (5 mL) was added dropwise over 15 min at – 78 oC. Then stirring was continued for 1 h at – 78 oC under a N2 atmosphere. Saturated aqueous NH4Cl solution (15 mL) was added at – 78 oC and then the mixture was extracted with EtOAc (3 x 40 mL). The combined organic extracts were washed with brine, dried (MgSO4) and concentrated in vacuo. The crude product was purified by chromatography (1:3, EtOAc/petrol)) to afford 15 as a 70:30 mixture of two diastereomeric products (1.00 g, 68%) and as a pale yellow gum. Rf = 0.67 (1:3, EtOAc/petrol). IR (neat, νmax/cm-1): 3089, 2928, 1659, 1400, 1367, 1254, 1099, 834, 779, 738, 696. 1H NMR (500 MHz, CDCl3): δ major isomer 7.27 – 7.16 (5H, m, ArH), 5.74 – 5.63 (1H, m, H4`), 4.90 (1H, dd, J = 1.5, 17.0 Hz, H5`), 4.84 (1H, dd, J = 2.0, 10.0 Hz, H5`), 4.65 (1H, d, J = 12.0 Hz, H6`), 4.58 (1H, d, J = 12.0 Hz, H6`), 4.25 (2H, s, H9`), 4.09 (1H, dd, J = 5.5, 7.0 Hz, H4), 3.39 – 3.15 (2H, m, H1`), 2.51 (1H, dd, J = 7.0, 17.0 Hz, H3), 2.35 (1H, dd, J = 5.5, 17.0 Hz, H3), 1.98 -1.91 (2H, m, H3`), 1.69 – 1.63 (2H, m, H2`), 0.92 (9H, s, (C(CH3)3), 0.00 (6H, s, (CH3)2Si); minor isomer 7.26 – 7.16 (5H, m, ArH), 5.74 – 5.66 (1H, m, H4`), 4.93 (1H, dd, J = 1.5, 17.0 Hz, H5`), 4.86 (1H, dd, J = 2, 10.0 Hz, H5`), 4.77 (1H, d, J = 11.5 Hz, H6`), 4.55 (1H, d, J = 11.5 Hz, H6`), 4.27 (2H, s, H9`), 3.93 (1H, dd, J = 5.5 7.0 Hz, H4), 3.36 – 3.20 (2H, m, H1`), 2.63 (1H, dd, J = 7.0, 17.0 Hz, H3), 2.30 (1H, dd, J = 5.5, 17.0 Hz, H3), 2.00 – 1.96 (2H, m, H3`), 173 – 1.65 (2H, m, H2`), 0.79 (9H, s, (C(CH3)3), 0.00 (6H, s, (CH3)2Si).
13CNMR (125 MHz, CDCl3): δ major isomer 170.7 (C2), 137.8 (ArC), 136.3 (C4`), 128.5 (ArCH), 128.3 (ArCH), 128.2 (ArCH), 128.0 (ArCH), 127.9 (ArCH), 114.7 (C5`), 84.8, 84.1 (C≡C), 81.8 (C5), 78.0 (C4), 72.7 (C6`), 51.3 (C9`), 40.0 (C1`), 35.3 (C3), 31.1 (C3`), 27.8 (C2`), 25.6 (C(CH3)3), 18.1 (C(CH3)3), -5.2 (CH3)2Si); minor isomer 171.7 (C2), 137.8 (ArC), 137.5 (C4`), 128.3 (ArCH), 127.7 (ArCH), 127.6 (ArCH), 114.8 (C5`), 84.4, 84.0 (C≡C), 80.8 (C4), 79.7 (C5), 72.3 (C6`), 51.4 (C9`), 39.8 (C1`), 36.1 (C3), 31.2 (C3`), 27.8 (C2`), 25.6 (C(CH3)3), 18.1 (C(CH3)3), -5.3 (CH3)2Si). ESIMS m/z 444 [(M+H)+ 100%]. HRESIMS calcd. for C25H38NO4Si, (M+H)+ 444.2559, found: 444.2570.

(4S)-4-(Benzyloxy)-5-(3-hydroxyprop-1-ynyl)-1-(pent-4-enyl)pyrrolidin-2-one (18/19)
To a solution of 15 (1.0 g, 2.26 mmol) in HOAc (25 mL) was added NaCNBH3 (071 g, 11.0 mmol) portionwise and the mixture was stirred at rt for 18 h. The mixture was quenched with water (50 mL) and extracted with EtOAc (2 x 80 mL). The combined extracts were washed with saturated aqueous NaHCO3 solution, dried (MgSO4) and the solvent was removed in vacuo. The crude product was purified by column chromatography (1:1, EtOAc/petrol)) to afford inseparable mixture (cis:trans = 3:1) of two diastereomeric products 18/19 (0.41 g, 58%) and as a colourless gum. Rf = 0.58 (1:1, EtOAc/petrol). IR (neat, νmax/cm-1): 3375, 2935, 2360, 2337, 1674, 1454, 1409, 1073, 1027, 742, 697. 1H NMR (500 MHz, CDCl3): δ major cis diastereomer 7.38 – 7.26 (5H, m, ArH), 5.85 – 5.75 (1H, m, H4`), 5.04 (1H, d, J = 16.5 Hz, H5`), 4.98 (1H, d, J = 10.0 Hz, H5`), 4.70 (1H, d, J = 11.5 Hz, H6`), 4.60 (1H, d, J = 11.5 Hz, H6`), 4.54 (1H, d, J = 6.5 Hz, H5), 4.32 (2H, s, H9`), 4.24 (1H, apparent q, J = 7.0 Hz, H4), 3.70 – 3.61 (1H, m, H1`), 3.16 – 3.01 (1H, m, H1`), 2.60 (2H, d, J = 7.5 Hz, H3), 2.09 – 2.02 (2H, m, H3`), 1.73 – 1.59 (2H, m, H2`); minor trans diastereomer (in part) 7.38 – 7.26 (5H, m, ArH), 4.29 (2H, s, H9`), 3.70 – 3.61 (1H, m, H1`), 3.16 – 3.01 (1H, m, H1`), 2.77 (1H, d, J = 7.0, 17.0 Hz, H3), 2.50 (1H, d, J = 17.0 Hz, H3), 2.09 – 2.02 (2H, m, H3`), 1.73 – 1.59 (2H, m, H2`). 13C NMR (125 MHz, CDCl3): δ major cis diastereomer 171.7 (C2), 137.5 (ArC), 137.0 (C4`), 128.4 (ArCH), 128.1 (ArCH), 128.0 (ArCH), 127.9 (ArCH), 127.7 (ArCH), 115.1 (C5`), 86.5, 78.5 (C≡C), 72.5 (C4), 71.8 (C6`), 54.0 (C5), 50.8 (C9`), 40.7 (C1`), 36.8 (C3), 30.9 (C3`), 26.3 (C2`); minor trans diastereomer (in part) 171.7 (C2), 137.5 (ArC), 137.0 (C4`), 128.4 (ArCH), 128.1 (ArCH), 128.0 (ArCH), 127.9 (ArCH), 127.7 (ArCH), 71.4 (C6`), 55.8 (C5), 50.6 (C9`), 40.4 (C1`), 37.2 (C3), 30.7 (C3`), 26.1 (C2`). ESIMS m/z 314 [(M+H)+ 100%]. HRESIMS calcd. for C19H24NO3, (M+H)+ 314.1743, found: 314.1756.

(1S,9aS)-1-(Benzyloxy)-9-vinyl-5,6,7,9a-tetrahydro-1H-pyrrolo[1,2-a]azepin-3(2H)-one (20)
A solution of 16 (2.00 g, 5.63 mmol) and Grubbs’ 2nd generation catalyst (0.034 g, 0.56 mmol) in CH2Cl2 (400 mL) was heated at reflux for 5 h under a N2 atmosphere. The solution was then concentrated and the residue was purified by column chromatography (1:1, EtOAc/petrol) to give a title compound 20 (1.29 g, 81%) as a pale yellow oil. Rf = 0.54 (1:1, EtOAc/petrol). [α]25D -42.4 (c 1.13, CHCl3). IR (neat, νmax/cm-1): 2931, 1674, 1453, 1404, 1267, 1069, 735, 697. 1H NMR (500 MHz, CDCl3): δ 7.31 – 7.21 (5H, m, ArH), 6.38 (1H, dd, J = 11.0, 18.0 Hz, H1`), 6.09 (1H, t, J = 7.0 Hz, H8), 5.08 (1H, d, J = 18.0 Hz, H2`), 5.01 (1H, d, J = 11.0 Hz, H2`), 4.65 (1H, d, J = 4.0 Hz, H9a), 4.48 (2H, s, H3`), 4.23 (1H, apparent t, J = 4.5 Hz, H1), 4.07 (1H, dd, J = 8.5, 14.0 Hz, H5), 2.92 – 2.85 (1H, m, H5), 2.79 – 2.71 (1H, m, H7), 2.65 – 2.56 (2H, m, H2), 2.11 – 2.04 (1H, m, H6), 2.02 – 1.96 (1H, m, H7), 1.67 – 1.60 (1H, m, H6). 13C NMR (125 MHz, CDCl3): δ 172.5 (C3), 139.0 (C1`), 138.0 (C9), 134.8 (C8), 134.2 (ArC), 128.2 (ArCH), 127.4 (ArCH), 127.2 (ArCH), 110.7 (C2`), 74.1 (C1), 71.5 (C3`), 66.0 (C9a), 38.6 (C2), 38.1 (C5), 23.9 (C6), 22.1 (C7). ESIMS m/z 284 [(M+H)+ 100%]. HRESIMS calcd. for C18H22NO2, (M+H)+ 284.1626, found: 284.1651.

Ethyl 3-((4S,5S)-4-(benzyloxy)-2-oxo-1-(pent-4-enyl)pyrrolidin-5-yl)propiolate (22)
To a solution of alcohols 18/19 (50 mg, 0.16 mmol) in acetone (6 mL), was added Jones reagent16 at 0 oC dropwise until disappearance of starting material by TLC analysis. The reaction mixture was diluted with water (10 mL), extracted with EtOAc (3 x 30 mL) and the combined organic extracts washed with brine solution, dried (MgSO4) and concentrated in vacuo. The crude product (48 mg, 92%) was dissolved in CH2Cl2 (10 mL) and treated with EtOH (0.011 mL, 0.19 mmol), DCC (30 mg, 0.14 mmol) and DMAP (1.8 mg, 0.014 mmol) at 0 oC under a N2 atmosphere. The reaction mixture was warmed to rt and stirring was continued for 6 h. The mixture was diluted with water (5 mL) and extracted with CH2Cl2 (3 x 20 mL). The combined organic extracts were washed with brine and dried (MgSO4) and concentrated in vacuo. The crude compound was purified by column chromatography (2 : 1, EtOAc/petrol), to give the cis diastereomer 22 (26 mg, 47%) as pale yellow gum and the trans diastereomer (8.5 mg, 15%) as a pale yellow gum.
(22): Rf = 0.57 (2:1, EtOAc/petrol). [α]27D +10 (c 1.4, CHCl3); IR (neat, νmax/cm-1): 2930, 2359, 2334, 2236, 1705, 1408, 1250, 1073, 749, 698. 1H NMR (500 MHz, CDCl3): δ 7.40 – 7.30 (5H, m, ArH), 5.83 -5.75 (1H, m, H4`), 5.05 (1H, dd, J = 1.5, 17.0 Hz, H5`), 4.99 (1H, d, J = 10.5 Hz, H5`), 4.70 (1H, d, J = 11.5 Hz, H6`), 4.60 (1H, d, J = 11.5 Hz, H6`), 4.59 (1H, d, J = 6.5 Hz, H5), 4.30 – 4.23 (3H, m, H4 and H10`), 3.67 – 3.61 (1H, m, H1`), 3.18 – 3.13 (1H, m, H1`), 2.62 (2H, d, J = 7.5 Hz, H3), 2.11 – 2.01 (2H, m, H3`), 1.74 – 1.57 (2H, m, H2`), 1.32 (3H, t, J = 7.0 Hz, H11`). 13C NMR (125 MHz, CDCl3): δ 171.2 (C2), 152.8 (C9`), 137.3 (ArC), 136.8 (C4`), 128.5 (ArCH), 128.1 (ArCH), 127.9 (ArCH), 115.4 (C5`), 80.4, 79.3 (C≡C), 72.5 (C4), 72.0 (C6`), 62.2 (C10`), 53.9 (C5), 40.9 (C1`), 36.7 (C3), 30.9 (C3`), 26.3 (C2`), 13.9 (C11`). ESIMS m/z 356 [(M+H)+ 100%]. HRESIMS calcd. for C21H26NO4, (M+H)+ 356.1862, found: 356.1864.

Ethyl 2-((1S,9aS)-1-(benzyloxy)-3-oxo-2,3,5,6,7,9a-1H-pyrrolo[1,2-a]azepin-9-yl)acrylate (23)
A solution of 22 (0.15 g, 0.42 mmol) and Grubbs’ 1st generation catalyst (0.034 g, 0.042 mmol) in CH2Cl2 (30 mL) was stirred at rt for 5 h under a N2 atmosphere. The solution was quenched by opening to the air and was then concentrated in vacuo. The residue was purified by column chromatography on silica gel (2:1, EtOAc/petrol) to give a title compound 23 (0.122 g, 82%) as a light brown gum. Rf = 0.52 (2:1, EtOAc/petrol). [α]22D -11.8 (c 0.76, CHCl3). IR (neat, νmax/cm-1): 2936, 2359, 2337, 1671, 1453, 1270, 1069, 910, 731, 699. 1H NMR (500 MHz, CDCl3): δ 7.33 – 7.24 (5H, m, ArH), 6.07 (1H, s, H1`), 6.00 (1H, t, J = 6.0 Hz, H8), 5.61 (1H, s, H1`), 4.57 (1H, d, J = 6.0 Hz, H9a), 4.55 (1H, d, J = 12.0, H6`), 4.41 (1H, d, J = 12.0 Hz, H6`), 4.26 – 4.17 (2H, m, H4`), 4.09 – 4.04 (2H, m, H1, H5), 3.16 – 3.09 (1H, m, H5), 2.75 – 2.67 (1H, m, H7), 2.60 (1H, d, J = 17.0 Hz, H2), 2.52 (1H, dd, J = 5.0, 17.0 Hz, H2), 2.09 -1.38 (3H, m, H7, H6), 1.31 (3H, t, J = 7.0 Hz, H5`). 13C NMR (125 MHz, CDCl3): δ 172.3 (C3), 166.6 (C3`), 141.8 (C2`), 137.9 (ArC), 134.1 (C8), 133.2 (C9), 128.3 (ArCH), 127.5 (ArCH), 127.1 (ArCH), 126.6 (C1`), 74.5 (C1), 71.1 (C6`), 67.5 (C9a), 61.1 (C4`), 38.2 (C5), 37.8 (C2), 24.0 (C6), 22.2 (C7), 14.1 (C5`). ESIMS m/z 356 [(M+H)+ 100%]. HRESIMS calcd. for C21H26NO4, (M+H)+ 356.1860, found: 356.1862.

Ethyl 2-((1S)-1-(benzyloxy)-3-oxo-2,3,5,6,7,9a-hexahydro-1H-pyrrolo[1,2-a]azepin-9-yl)propanoate (24)
To a solution of 23 (0.12 g, 0.33 mmol) in MeOH (10 mL) was added NaBH4 (0.10 g, 2.70 mmol) at 0 oC, and the solution was stirred at the same temperature for 3 h. After completion of the reaction, saturated aqueous NH4Cl solution was added, and the aqueous layer was extracted with EtOAc (3 x 30 mL). The organic layer was dried over MgSO4 and concentrated. The residue was purified by column chromatography (2:1, EtOAc/petrol) to give 24 (0.09 g, 75%) as a 4:1 inseparable mixture of diastereomeric products and as a colourless oil. Rf = 0.50 (2:1, EtOAc/petrol). IR (neat, νmax/cm-1): 2937, 2360, 2342, 1722, 1688, 1454, 1181, 1095, 1071, 741, 697. 1H NMR (500 MHz, CDCl3): δ major diastereomer 7.32 – 7.23 (5H, m, ArH), 5.95 (1H, t, J = 8.5 Hz, H8), 4.28 (1H, d, J = 11.5 Hz, H6`), 4.45 (1H, d, J = 11.5 Hz, H6`), 4.28 (1H, d, J = 4.5 Hz, H9a), 4.16 (1H, t, J = 4.5 Hz, H1), 4.10 (2H, q, J = 7.0 Hz, H4`), 4.05 – 3.97 (1H, m, H5), 3.15 (1H, q, J = 7.5 Hz, H2`), 2.97 – 2.90 (1H, m, H5), 2.71 – 2.65 (1H, m, H7), 2.64 (1H, d, J = 17.0 Hz, H2), 2.55 (1H, dd, J = 5.0, 17.0 Hz, H2), 2.04 – 1.59 (3H, m, H7, 6), 1.26 (3H, d, J = 7.0 Hz, H1`), 1.20 (3H, t, J = 7.0 Hz, H5`); minor diastereomer (in part) 7.32 – 7.23 (5H, m, ArH), 5.93 (1H, t, J = 8.5 Hz, H8), 4.54 (1H, d, J = 12.0 Hz, H6`), 4.38 (1H, d, J = 12.0 Hz, H6`), 4.35 (1H, d, J = 5.5 Hz, H9a), 4.19 (1H, t, J = 5.0 Hz, H1), 4.13 (2H, q, J = 6.5 Hz, H4`), 4.05 – 4.02 (1H, m, H5), 2.98 – 2.88 (2H, m, H5, 2`), 2.71 – 2.54 (3H, m, H2, H7), 2.06 – 1.58 (3H, m, H6, H7), 1.32 (3H, d, J = 7.5 Hz, H1`), 1.21 (3H, t, J = 7.5 Hz, H5`). 13C NMR (125 MHz, CDCl3): δ major diastereomer 174.2 (C3), 172.3 (C3`), 137.9 (ArC), 134.0 (C9), 130.4 (C8), 128.2 (ArCH), 127.5 (ArCH), 127.4 (ArCH), 127.1 (ArCH), 127.0 (ArCH), 75.6 (C1), 70.7 (C-Bn), 68.0 (C9a), 60.8 (C4`), 46.3 (C2`), 37.9 (C5), 37.7 (C2), 23.6 (C6), 21.5 (C7), 16.6 (C1`), 14.1 (C5`). ESIMS m/z 358 [(M+H)+ 100%]. HRESIMS calcd. for C21H28NO4, (M+H)+ 358.2009, found: 358.2018.

(3aR,10S,10aS)-10-(Benzyloxy)-1-methyl-3a,4,5,6,10,10a-hexahydro-2H-furo[3,2-c]pyrrolo[1,2-a]-azepine-2,8(9H)-dione (25) and (3aS,10S,10aS)-10-(Benzyloxy)-1-methyl-3a,4,5,6,10,10a-hexahydro-2H-furo[3,2-c]pyrrolo[1,2-a]azepine-2,8(9H)-dione (26)
A solution of 24 (0.20 g, 0.56 mmol) in MeOH (8 mL) and aqueous 1N NaOH solution (5.92 mL) was stirred at 0 oC for 8 h. The solution was acidified with 10% HCl and the aqueous layer was extracted with EtOAc (3 x 30 mL). The combined organic extracts were washed with brine, dried (MgSO4), and concentrated in vacuo. The residue was dissolved in CHCl3 (50 mL), and CuBr2 on alumina9g,h (3.0 g) was added. The mixture was heated at 65 oC for 60 h. After filtration, the solid was washed with MeOH, and the filterate was concentrated. Water was added to the residue, and the aqueous layer was extracted with EtOAc (3 x 40 mL). The combined organic extracts were washed with brine solution, dried (MgSO4) and concentrated. The residue was dissolved in EtOAc (15 mL), and Et3N (0.155 mL, 1.10 mmol) was added and the solution was stirred at rt for 16 h. The reaction solution was then washed with 10% HCl, brine and saturated NaHCO3 solution. The organic layer was dried (MgSO4) and concentrated. The residue was purified by column chromatography (2:1, EtOAc/petrol) to give 25 (0.0247 g, 13.5%) as a colourless gum and 26 (0.0247 g, 13.5%) as a colorless gum.
(25): Rf = 0.53 (2:1, EtOAc/petrol). [α]26D +27.4 (c 2.81, CHCl3). 1H NMR (500 MHz, CDCl3): δ 7.33 – 7.16 (5H, m, ArH), 5.00 (1H, d, J = 11.5 Hz, H3a), 4.71 (1H, d, J = 5.0 Hz, H10a), 4.62 (1H, d, J = 11.5 Hz, H2`), 4.57 – 4.54 (1H, m, H10), 4.37 (1H, d, J = 11.5 Hz, H2`), 4.20 – 4.16 (1H, m, H6), 2.90 – 2.83 (1H, m, H6), 2.78 (1H, dd, J = 3.0, 17.0 Hz, H9), 2.63 (1H, dd, J = 6.0, 17.0 Hz, H9), 2.22 – 2.16 (1H, m, H5), 1.96 – 1.84 (2H, m, H4, H5), 1.83 (3H, s, H1`, Me), 1.79 – 1.68 (1H, m, H4). 13C NMR (125 MHz, CDCl3): 173.6 (C2), 171.6 (C8), 156.5 (1a), 136.6 (ArC), 128.5 (ArCH), 128.2 (ArCH), 127.4 (ArCH), 124.2 (C1), 80.6 (C3a), 73.0 (C10), 71.3 (C2`), 61.5 (C10a), 40.0 (C6), 36.3 (C9), 26.1 (C5), 22.0 (C4), 10.4 (C1` Me). ESIMS m/z 328 [(M+H)+ 100%].
(26): Rf = 0.54 (2:1, EtOAc/petrol). [α]26D +18.0 (c 1.6, CHCl3). IR (neat, νmax/cm-1): 2927, 2360, 2337, 1749, 1689, 1454, 1255, 1108, 737, 698. 1H NMR (500 MHz, CDCl3): δ 7.33 – 7.11 (5H, m, ArH), 5.01 (1H, d, J = 11.5 Hz, H-3a), 4.80 (1H, d, J = 5.0 Hz, H10a), 4.56 (1H, d, J = 12.0 Hz, H2`), 4.32 (1H, d, J = 14.0 Hz, H6), 4.23 – 4.14 (2H, m, H10, H2`), 2.63 (1H, d, J = 17.0 Hz, H9), 2.57 (1H, dd, J = 5.0, 17.0 Hz, H9), 2.51 – 2.42 (2H, m, H4, H6), 1.90 – 1.87 (1H, m, H5), 1.67 – 1.57 (4H, m, H5, H1`). 13C NMR (125 MHz, CDCl3): δ 173.1 (C2), 172.2 (C8), 160.4 (1a), 136.3 (ArC), 128.6 (ArCH), 128.3 (ArCH), 127.9 (ArCH), 124.8 (C1), 83.8 (C3a), 72.1 (C10), 71.1 (C2`), 62.0 (C10a), 42.7 (C6), 37.2 (C9), 34.7 (C4), 25.6 (C5), 8.6 (C1` Me). ESIMS m/z 328 [(M+H)+ 100%]. HRESIMS calcd. for C21H26NO4, (M+H)+ 328.1534, found: 328.1549.

(3aS,10S,10aR,10bR)-10-(Benzyloxy)-10b-hydroxy-1-methylhexahydro-1H-furo[3,2-c]pyrrolo[1,2-a]azepine-2,8(9H, 10bH)-dione (27)
To a solution of 24 (0.20 g, 0.56 mmol) in acetone/water (9:1) was added K2OsO4.2H2O (12 mg, 0.03 mmol) and NMO (98.0 mg, 0.84 mmol) at rt and reaction mixture warmed to 60 oC and stirred for 24 h. The solution was cooled to rt and the reaction mixture was quenched with saturated aqueous potassium bisulfite solution (15 mL), After stirring for 10 min, water (30 mL) was added and the mixture was extracted with EtOAc (3x50 mL). The organic extracts were combined, dried (MgSO4) and the solvent concentrated in vacuo. The crude product was purified by flash column chromatography (0.5:10, MeOH/EtOAc) to give the title compound 27 as a 5:1 mixture of inseperable diastereomers (0.109 g, 56%) and as a colourless liquid.
Rf = 0.45 (0.5:10, MeOH/EtOAc). [α]27D +98.1 (c 1.1, CHCl3). IR (neat, νmax/cm-1): 3327, 2935, 2960, 2337, 1770, 1670, 1454, 1330, 1197, 1012, 975, 739, 697. 1H NMR (500 MHz, CDCl3): δ major isomer 7.38 – 7.20 (5H, m, ArH), 4.79 (1H, dd, J = 6.0, 10.0 Hz, H3a), 4.60 (1H, d, J =11.5 Hz, H2`), 4.28 (1H, d, J = 11.5 Hz, H2`), 4.20 -4.14 (1H, m, H6), 4.08 (1H, apparent t, J = 4.0 Hz, H10), 3.75 (1H, d, J = 3.5 Hz, H10a), 3.19 (1H, q, J = 7.5 Hz, H1), 3.05 (1H, t, J = 13.0 Hz, H6), 2.67 (1H, d, J = 17.0 Hz, H9), 2.47 (1H, dd, J = 4.0, 17.0 Hz, H9), 2.04 – 1.98 (2H, m, H4), 186 – 1.83 (1H, m, H5), 1.49 – 1.40 (1H, m, H5), 1.09 (3H, d, J = 7.5 Hz, H1` Me); minor isomer (in part) 7.37 – 7.20 (5H, m, ArH), 5.02 (1H, dd, J = 4.0, 12.0 Hz, H3a), 4.51 (1H, d, J = 12.0 Hz, H2`), 4.38-4.36 (1H, m, H10), 4.27 (1H, d, J = 12.0 Hz, H2`), 4.21-4.18 (1H, m, H6), 4.07 (1H, d, J = 4.0 Hz, H10a), 3.29 (1H, t, J = 13.0 Hz, H6), 2.93 (1H, q, J = 7.0 Hz, H1), 2.70 (1H, d, J = 17.0 Hz, H9), 2.45 (1H, dd, J = 5.0, 17.0 Hz, H9), 2.07 – 1.80 (2H, m, H4), 1.48 (3H, d, J = 8.0 Hz, H1` Me), 1.40 – 1.16 (2H, m, H5). 13C NMR (125 MHz, CDCl3): δ 177.3 (C2), 172.1 (C8), 135.8 (ArC), 128.8 (ArCH), 128.5 (ArCH), 128.2 (ArCH), 83.9 (C3a), 80.2 (C10b), 72.9 (C10a), 70.4 (C2` Bn), 66.1 (C10), 42.6 (C1), 40.2 (C6), 36.8 (C9), 26.2 (C4), 25.7 (C5), 7.7 (C1`); minor isomer (in part) 178.7 (C2), 172.1 (C8), 136.4 (ArC), 128.6 (ArCH), 128.0 (ArCH), 127.3 (ArCH), 84.2 (C3a), 80.7 (C10b), 74.9 (C10), 69.6 (C2` Bn), 67.6 (C10a), 52.0 (C1), 40.1 (C6), 36.5 (C9), 26.7 (C4), 26.2 (C5), 12.8 (C1`). ESIMS m/z 346 [(M+H)+ 100%]. HRESIMS calcd. for C19H23NO5, (M+H)+ 346.1645, found: 346.1654.

(3aS,10S)-10-Hydroxy-1-methyl-3a,4,5,6,10,10a-hexahydro-2H-furo[3,2-c]pyrrolo[1,2-a]azepine-2,8(9H)-dione (28)
Method 1: To a solution of 27 (0.08 g, 0.23 mmol) in toluene (8 mL) in a flask equipped with a Dean-Stark apparatus and a condenser, was added PTSA (66 .0 mg, 0.34 mmol) at rt. The resulting reaction mixture was heated at reflux temperature and stirred for 18 h. The reaction mixture was then cooled to rt and diluted with water (4 mL) and extracted with EtOAc (3x30 mL). The combined organic extracts were washed with saturated aqueous NaHCO3 solution and brine, dried (MgSO4) and the solvent concentrated in vacuo. The crude product was purified by column chromatography (0.5:10, MeOH/EtOAc) to give compound 28 (38.0 mg, 70%) as an off white solid. Mp = 210 – 212 oC. Rf = 0.46 (0.5:10, MeOH/EtOAc). [α]23D +20.5 (c 0.34, CHCl3). IR (neat, νmax/cm-1): 3401, 2932, 2359, 2343, 1742, 1697, 1429, 1058, 1016.
1H NMR (300 MHz, CDCl3): δ 5.07 – 5.02 (1H, m, H3a), 4.78 (1H, d, J = 4.2 Hz, H10a), 4.61 (1H, apparent q, J = 5.1 Hz, H10), 4.33 – 4.27 (1H, m, H6), 3.91 (1H, d, J = 4.2 Hz, OH), 2.74 (1H, dd, J = 5.7, 17.1 Hz, H9), 2.54 – 2.41 (3H, m, H4, H6, H9), 1.91 (3H, s, H1`, Me), 1.90 – 1.87 (1H, m, H5), 1.70 – 1.61 (1H, m, H5), 1.32 – 1.18 (1H, m, H4). 13C NMR (75 MHz, CDCl3): δ 174.1 (C2), 172.9 (C8), 161.2 (C1a), 125.1 (C1), 84.5 (C3a), 66.4 (C10), 63.5 (C10a), 42.9 (C6), 40.2 (C9), 34.6 (C4), 25.5 (C5), 8.9 (C1` Me). ESIMS m/z 238 [(M+H)+ 100%]. HRESIMS calcd. for C12H16NO4, (M+H)+ 238.1041, found: 238.1059.

Method 2: To a solution of 26 (0.015 g, 0.045 mmol) in dichloromethane (1.0 mL) at 0 oC was added borontribromide (0.101 g, 0.412 mmol). After 30 min the mixture was diluted with CH2Cl2 (10 mL) and the solution was washed with a saturated solution of NaHCO3 (2 x 10 mL) and then brine. The solution was then dried (MgSO4) and the solvent concentrated in vacuo. The crude product was purified by column chromatography (0.5:10, MeOH/EtOAc) to give compound 28 (6.6 mg, 68%) as an off white solid. The 1H NMR spectrum of this material was identical to that prepared via method 1.

(1R,3aS,10S,10aS,10bS)-10-Hydroxy-1-methylhexahydro-1H-furo[3,2-c]pyrrolo[1,2-a]azepine-2,8(9H,10bH)-dione (7) and (1S,3aS,10S,10aS,10bS)-10-Hydroxy-1-methylhexahydro-1H-furo[3,2-c]pyrrolo[1,2-a]azepine-2,8(9H,10bH)-dione (29)
To a solution of 28 (23.0 mg, 0.097 mmol) in MeOH (2.5 mL) was added NiCl2.6H2O (8.77 mg, 0.034 mmol) and NaBH4 (25.0 mg, 0.67 mmol) at -30 oC, and the mixture was stirred at same temperature for 2 h. The resulting mixture was then diluted with CH2Cl2 and the solution was washed with brine and saturated aqueous NaHCO3 solution, and dried (MgSO4) and concentrated in vacuo. The residue was purified by small column chromatography (0.6:10, MeOH/EtOAc) to give a 87:13 mixture of inseparable diastereomeric products 7 and 29 (18.0 mg, 78%) as a colourless solid. Mp = 150 – 152 oC. Rf = 0.43 (0.6:10, MeOH/EtOAc). [α]24D +7.8 (c 0.38, CHCl3). IR (neat, νmax/cm-1): 3402, 2961, 2360, 2340, 1759, 1670, 1439, 1167, 1009. 1H NMR (500 MHz, CDCl3): δ major diastereomer 4.92 – 4.86 (1H, m, H3a), 4.52 (1H, m, H10), 4.20 (1H, d, J = 14.5 Hz, H6), 3.85 – 3.82 (2H, m, H10a, OH), 3.30 (1H, dq, J = 12.3, 7.2 Hz, H1), 2.68 – 2.58 (2H, m, H6, H9), 2.39 (1H, d, J = 17.0 Hz, H9), 2.36 – 2.30 (2H, m, H1a, H4), 1.90 – 1.83 (1H, m, H5), 1.57 – 1.37 (2H, m, H5, H9), 1.32 (3H, d, J = 7.2 Hz, Me (H1`)); minor diastereomer (in part) 4.80 – 4.76 (1H, m, H3a), 4.29 (1H, apparent t, J = 6.5 Hz, H10), 4.10 (1H, d, J = 14.5 Hz, H6), 3.75 – 3.68 (1H, m, H10a), 3.23 – 3.18 (1H, m, H1), 2.84 (1H, dd, J = 7.5, 18.0 Hz, H9), 1.28 (3H, d, J = 7.0 Hz, Me (H1`)). 13C NMR (125 MHz, CDCl3): δ major diastereomer 178.7 (C2), 173.1 (C8), 80.3 (C3a), 67.2 (C10), 59.8 (C10a), 51.9 (C1a), 42.1 (C9), 40.5 (C6), 36.9 (C1), 35.0 (C4), 25.7 (C5), 14.6 (C1`); minor diastereomer (in part) 79.2 (C3a), 71.6 (C10), 62.1 (C10a), 43.7 (C9), 43.5 (C6), 37.4 (C1), 24.6 (C5), 12.6 (C1`) ESIMS m/z 240 [(M+H)+ 100%]. HRESIMS calcd. for C12H18NO4, (M+H)+ 240.1148, found: 240.1169.

ACKNOWLEDGEMENTS
We thank the Australian Research Council and the University of Wollongong for financial support.

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