D-Glucose based synthesis of proline–serine C–C linked central and right hand core of a kaitocephalin-a glutamate receptor antagonist

Abstract

The synthesis of the proline–serine core of kaitocephalin 1 was accomplished starting from D-glucose wherein, C2 of D-glucose provided the carboxylic acid functionality of serine; while the amino- and β-hydroxy groups of the serine fragment were amenable from the C3 and C4 hydroxy groups of sugar. The key intermediate to construct substituted proline with the appropriate quaternary carbon framework of the target molecule was constructed using the Jocic–Reeve and Corey–Link reaction sequence with the desired stereochemistry from the C5-centre of D-glucose.

---

Introduction

Kaitocephalin 1 was isolated from the fungus Eupenicillium shearii in 1997 by Shin-ya et al.¹ It exhibits strong antagonist activity against AMPA (2-amino-3-(3-hydroxy-5-methyl-isoxazole-4-ylpropionate)), KA (kainic acid) and NMDA (N-methyl-D-aspartate) glutamate receptors, which play a key role in many physiological processes such as neural plasticity, memory and learning,² thus rendering potential chemotherapeutic applications in epilepsy, stroke, and Alzheimer's and Parkinson's diseases.³ The molecular architecture of 1 is truncated in to three amino acid segments, wherein proline is the central part to which N-acyl alanine (left hand segment) and serine (right hand fragment) are connected to the α′- and α-position of proline, respectively, through C–C bonds (Scheme 1). The scarce availability of 1 in nature, its importance in neurobiological studies and intriguing structural features has attracted ample attention for its synthesis. To date, seven groups have independently reported the total synthesis of 1 or its analogues/fragments.⁴ The challenging aspect in the synthesis of 1 is the construction of the central trisubstituted proline core, which bears the quaternary stereogenic center at C4 connected to the hydroxymethyl carbon of serine with the requisite stereochemistry. In this direction, the commonly used approach for the introduction of the central and right hand segment of 1 is the alkylation of substituted proline/pyroglutamate with Garner's aldehyde or its equivalent to obtain a C4 quaternary stereocenter with a C3-hydroxy group (Scheme 1). In the other three asymmetric methods, a central proline ring with requisite functionalities is constructed using (a) palladium catalyzed cyclization of oxiranylacrylate, (b) stereoselective desymmetrization of protected serinol and (c) rhodium catalyzed C–H amination protocols. As a part of our continuing efforts in the synthesis of natural products and their analogues using carbohydrate chemistry,⁵ herein we report a hitherto unknown chiral pool approach for the synthesis of a trisubstituted proline segment (central) connected through C–C bonds to the right hand serine fragment of kaitocephalin 1, starting from D-glucose.

Scheme 1 Retrosynthesis of 1.

Results and discussion

Recently, we reported the synthesis of α-geminal dihydroxymethyl substituted piperidine and pyrrolidine iminosugars from the key intermediate I (Scheme 1), which was obtained from D-glucose by utilizing the Jocic–Reeve and Corey–Link reaction sequence.⁶ Examination of intermediate I revealed the presence of C–C bonded proline and serine core (IV) of 1 with requisite absolute configuration at the C-5′quaternary center (Scheme 1).

Thus, the proline core of 1 could be obtained from C-5′ azido aldehyde I by Wittig olefination to obtain II, followed by the lactamization and oxidation of primary alcohol to obtain the trisubstituted proline core (III). While the rest of the D-glucose pendant could be judiciously manipulated to the serine fragment, (a) the carboxylic group of serine (C1 of 1) could be derived by the excision of the reducing end (using oxidative cleavage of hemiacetal) followed by oxidation and (b) the amino functionality of serine could be achieved by the S_N2 displacement of C3′-OTs by the azido functionality of D-glucohexofuranose. The C4′-hydroxy of glucose is suitably placed with the required absolute configuration of the target molecule, which justifies the selection of D-glucohexofuranose as the starting material.

As shown in Scheme 2, the synthesis commenced with the dichloro D-glucohexofuranose 2, which is obtained from D-glucose in 38% overall yield.⁷ The conversion of 2 to C5′ azido aldehyde 3 using NaN₃ in the presence of 1,3-dimethyl-3,4,5,6-tetrahydro-2-pyrimidinone (DMPU) and 15-crown-5 ether is known to give a low yield.⁷ To improve the yield, we modified the reaction conditions. Thus, compound 2 on treatment with NaN₃ in DMF using 1.0 equiv. of DBU afforded C5′-azidoaldehyde 3 in 93% yield.

Scheme 2 Synthesis of 11.

In the next step, the Wittig olefination of 3 with ethyl 2-(triphenylphosphoranylidene)acetate afforded α, β-unsaturated ester 4 in good yield and exclusively as the E-isomer. Hydrogenation of 4 with 10% Pd–C in methanol under an H₂ balloon atmosphere followed by the addition of K₂CO₃ (0.3 equiv.) in the same pot (after the complete disappearance of 4 on the TLC plate) and stirring for one hour at room temperature afforded lactam 5 in overall 80% yield in a four step one pot reaction.⁸ Subsequently, compound 5 was treated with NaH and p-toluenesulfonylchloride in dry THF at 0–30 °C for 2 h, which afforded C3′-O-tosylate 6 in 90% yield.⁹ In the next step, the C6′-silyl deprotection with anhydrous TBAF in THF gave primary alcohol 7, which on oxidation with Dess–Martin periodinane, followed by Pinnick oxidation (NaClO₂, NaH₂PO₄, 30% H₂O₂) and esterification (KHCO₃, MeI, DMF) gave methyl ester 8 in good yield. Fortunately, compound 8 was isolated as an off white solid and X-ray crystallography data (Fig. 1) indicated the absolute configuration at the quaternary carbon as 5′R. Lactam 8 was then protected as its N-benzyl derivative 9 by reacting with NaH and benzyl bromide in THF at 0–30 °C.¹⁰ Finally, the hydrolysis of the 1,2-O-isopropylidene group in 9 using TFA : H₂O (3 : 1) gave hemiacetal that was immediately subjected to oxidative cleavage using NaIO₄ followed by Pinnick oxidation and esterification (KHCO₃, MeI, DMF) to furnishe O-formate 10.¹¹ The S_N2 displacement of O-tosylate in 10 using NaN₃ in DMF at 50 °C afforded N-benzyl protected proline and serine segment 11 of kaitocephalin 1. The construction of the left hand alanine fragment on the N-debenzylated proline–serine segment 11, which leads to the formation of kaitocephalin 1, is known.^4b

Fig. 1 ORTEP diagram of compound 8.

Conclusions

In conclusion, we developed a new chiral pool strategy for the synthesis of the C–C linked proline–serine fragment of kaitocephalin from D-glucose. Our methodology is totally different from that of known methods and it addresses the synthesis of the challenging central tri-substituted proline core, which has a quaternary stereogenic center, using the Jocic–Reeve and Corey–Link approach with 5-ketoglucohexose followed by building the proline ring with requisite functionalities. Easy access for the manipulation of the functional groups in D-glucohexofuranose and simple chemical transformations with complete diastereoselectivity make this strategy useful for the total synthesis of kaitocephalin or its analogues and work in this direction is in progress.

Experimental

General experimental methods

Melting points were recorded using a Thomas Hoover melting point apparatus and are uncorrected. IR spectra were obtained using FTIR with samples as a thin film or KBr pellets and are expressed in cm⁻¹. ¹H (300, 500 MHz) and ¹³C (125 MHz) NMR spectra were obtained using CDCl₃/D₂O as the solvent. Chemical shifts are reported in δ units (ppm) with reference to TMS as an internal standard and J values are given in Hz. High resolution mass spectra (HRMS) were obtained in positive ion electrospray (ESI) mode using a TOF (time of flight) analyzer. Optical rotations were measured on a JASCO P-1020 digital polarimeter with sodium light (589.3 nm) at 25–30 °C. Thin layer chromatography was performed on pre-coated plates. Column chromatography was carried out with silica gel (100–200 mesh). Reactions were carried out in oven-dried glassware under dry N₂. Methanol and THF were purified and dried before use. Distilled n-hexane and ethyl acetate were used for column chromatography. After quenching of the reaction with water, the work-up involved the washing of combined organic layers with water and brine, drying over anhydrous sodium sulphate and evaporation of the solvent at reduced pressure.

1,2-O-Isopropylidene-3-O-benzyl-5-deoxy-5-azido-5-C(S)-(tert-butyldimethylsilyloxy)methyl-α-D-gluco-hexodialdo-furanose 3. To a stirred solution of 2 (2 g, 3.94 mmol) in dry DMF (15 mL) under an N₂ blanket, NaN₃ (1.28 g, 19.70 mmol) was added followed by the addition of DBU (0.58 mL, 3.9 mmol). The resulting pale brown solution was then heated at 90 °C until the starting material was consumed (2 h, monitored by TLC). The reaction mixture was then poured in water and extracted with ethyl acetate (3 × 20 mL) and the organic layer washed with water and brine and then dried over anhydrous sodium sulphate. Evaporation of the solvent and purification using column chromatography (n-hexane/ethyl acetate = 9.5/0.5) gave 3 (1.75 g, 93.5%) as a viscous oil: R_f 0.55 (n-hexane/ethyl acetate = 9/1); [α]_D^28.5 = −24 (c 1.03, CHCl₃); IR (neat) 1732 cm⁻¹, 2123 cm⁻¹; ¹H NMR (300 MHz, CDCl₃) δ 9.59 (s, 1H, CHO), 7.20–7.42 (m, 5H, Ar–H), 5.98 (d, J = 3.6 Hz, 1H, H1), 4.60–4.7 (m, 2H, H4, CH₂a–Ph), 4.47 (d, J = 3.6 Hz, 1H, H2), 4.45 (d, J = 11.5 Hz, 1H, CH₂b–Ph), 4.00–4.10 (m, 2H, H3, CH₂aOTBS), 3.88 (d, J = 10.5 Hz, 1H, CH₂bOTBS), 1.46 (s, 3H, CH₃), 1.33 (s, 3H, CH₃), 0.84 (s, 9H, 3CH₃), 0.03 (s, 3H, CH₃), −0.01 (s, 3H, CH₃); ¹³C NMR (75 MHz, CDCl₃) δ 196.8, 136.2, 128.6, 128.3, 128.2, 112.4, 105.0, 82.0, 81.9, 80.9, 72.2, 71.5, 64.0, 26.9, 26.5, 25.6 (strong, 3C), 25.6, 18.0, −5.8; HRMS (ESI-TOF) m/z: [M + Na]⁺ calcd for C₂₃H₃₅N₃NaO₆Si 500.2187; found 500.2187.

Ethyl-(1,2-O-isopropylidene-3-O-benzyl-5-deoxy-5-azido-5-C(S)-(tert-butyldimethylsilyloxy) methyl-6,7-dideoxy-6(E)-ene-α-D-gluco-octofurano)-uronate 4. Compound 3 (1.5 g, 3.14 mmol) was dissolved in DCM (20 mL), to which ethyl 2-(triphenylphosphoranylidene)acetate (1.36 g, 3.92 mmol) was added and the reaction mixture was refluxed for 5 h. After the reaction was completed, the solvent was evaporated and purification by column chromatography gave (n-hexane/ethyl acetate = 9/1) 4 (1.53 g, 89.5%) as a viscous oil: R_f 0.4 (n-hexane/ethyl acetate = 9/1); [α]_D^28.0 = 1 (c 0.5, CHCl₃); IR (neat) 1658 cm⁻¹, 1718 cm⁻¹, 2113 cm⁻¹; ¹H NMR (300 MHz, CDCl₃) δ 7.33–7.21 (m, 5H, Ar), 7.01 (d, J = 15.8 Hz, 1H, H8), 6.08 (d, J = 15.8 Hz, 1H, H7), 5.92 (d, J = 3.7 Hz, 1H, H1), 4.64 (d, J = 11.7 Hz, 1H, CHa–Ph), 4.56 (d, J = 3.7 Hz, 1H, H2), 4.45 (d, J = 3.1 Hz, 1H, H3), 4.41 (d, J = 11.7 Hz, 1H, CHb–Ph), 4.15 (q, J = 6.25 Hz 2H, O–), 3.99 (d, J = 3.1 Hz, 1H, H4), 3.71 (ABq, J = 10.1 Hz, 2H, H5), 1.43 (s, 3H), 1.27 (s, 3H), 1.22 (t, J = 7.1 Hz, 1H), 0.82 (s, 9H), 0.00 (s, 3H), −0.04 (s, 3H). ¹³C NMR (125 MHz, CDCl₃) δ 166.00 (C=O), 143.84 (Ar), 136.81 (Ar), 128.55 (Ar), 128.06 (C7), 127.91 (Ar), 123.36 (C8), 111.97 (C13), 104.68 (C1), 82.26 (C2), 81.66 (C3), 80.35 (C4), 71.96 (–Ph), 66.45 (C6), 60.48 (O–CH3), 26.83 (C5), 26.40 (C11), 25.69 (tBu), 18.06 (CH3), 14.22 (CH3), −5.63 (Si–), −5.72 (Si–C̲). HRMS (ESI-TOF) m/z: [M + Na]⁺ calcd for C₂₇H₄₁N₃NaO₇Si 570.2606; found 570.2600.

1,2-O-Isopropylidene-3-hydroxy-5-deoxy-5-C(S)-(3-pyrolidinone)-6-(tertbutyldimethylsilyloxy)methyl-α-D-gluco-hexofuranose 5. To a stirred solution of 4 (1.5 g, 2.741 mmol) in methanol, 10% Pd/C was added and the reaction mixture was stirred under H₂ (balloon pressure) at rt. After 12 h, K₂CO₃ (0.15 g) was added to the reaction mixture and the reaction was stirred for an additional 1 h. The reaction mixture was then filtered over celite, and the filtrate was evaporated to give the crude product, which was purified using column chromatography (n-hexane/ethyl acetate = 1/1 then 1/9) to give lactone 5 (0.85 g, 80.18%) as a white solid: mp 203–205 °C, R_f 0.15 (n-hexane/ethyl acetate = 1/1) [α]_D^28.7 = −20.17 (c 0.23, CHCl₃); IR (neat) 1680.77 cm⁻¹, 2930.04 cm⁻¹, 3317.80 cm⁻¹ (broad). ¹H NMR (500 MHz, CDCl₃) δ 6.40 (brs, 1H, NH), 5.96 (d, J = 3.7 Hz, 1H, H1), 4.52 (d, J = 3.7 Hz, 1H, H2), 4.52–4.40 (bs, 1H, OH, exchanges with D₂O). 4.26–4.24 (m, 1H, H3, after D₂O exchange appeared as d with J = 2.70 Hz), 4.14 (d, J = 2.7 Hz, 1H, H4), 3.70 (d, J = 9.9 Hz, 1H, H6a), 3.63 (d, J = 9.9 Hz, 1H, H6b), 2.52–2.34 (m, 2H, H8), 2.29–2.16 (m, 1H, H7a), 2.12–2.02 (m, 1H, H7b), 1.49 (s, 3H, CH₃), 1.32 (s, 3H, CH₃), 0.91 (s, 9H, tBu), 0.11 (s, 6H, 2 × SiCH₃). ¹³C NMR (125 MHz, CDCl₃) δ 178.81 (CONH), 111.54 (C10), 104.59 (C1), 85.44 (C4), 81.97 (C2), 75.31 (C3), 67.32 (C6), 63.28 (C5), 30.04 (C8), 26.80 (CH₃), 26.17 (C7), 25.79 (tBu), 18.19 (CH₃), −5.53(Si–), −5.64 (Si–C̲). HRMS (ESI-TOF) m/z: [M + Na]⁺ calcd for C₁₈H₃₃NNaO₆Si 410.1969; found 410.1958.

1,2-O-Isopropylidene-3-O-tosyl-5-deoxy-5-C(S)-(2-(5-oxo-pyrolidine))-6-(tertbutyldimethylsilyloxy)methyl-α-D-gluco-hexofuranose 6. A solution of 5 (0.8 g, 2.06 mmol) in dry THF (10 mL) was added dropwise to a pre-cooled solution of NaH (1.08 g, 4.53 mmol) in dry THF (5 mL) under an N₂ blanket and stirring was continued for 20 min after the addition was complete. Then, p-tosyl chloride (0.430 g, 2.26 mmol) was added to the reaction mixture and the reaction mixture was stirred for an additional 1.5 h. After complete consumption of the starting material (monitored by TLC), a saturated solution of NH₄Cl was added carefully and extracted with ethyl acetate (3 × 10 mL). The organic phase was washed with brine, dried over anhydrous sodium sulphate, and evaporation of solvent and purification by column chromatography (n-hexane/ethyl acetate = 7/3) gave 6 (1 g, 90.1%) as a thick liquid: R_f 0.6 (n-hexane/ethyl acetate = 1/1); [α]_D^28.7 = 10.42 (c 0.47, CHCl₃); IR (neat) 1697.70 cm⁻¹, 2927.54 cm⁻¹. ¹H NMR (500 MHz, CDCl₃) δ 7.83 (d, J = 8.2 Hz, 1H, Ar), 7.41 (d, J = 8.2 Hz, 1H, Ar), 5.91 (d, J = 3.8 Hz, 1H, H1), 5.77 (brs, 1H, NH), 4.99 (d, J = 2.7 Hz, 1H, H3), 4.59 (d, J = 3.8 Hz, 1H, H2), 4.39 (d, J = 2.7 Hz, 1H, H4), 3.51 (ABq, J = 10.1 Hz, 2H, H6), 2.49 (s, 3H, CH₃–Ar), 2.33–2.24 (m, 2H, H8), 2.21–2.14 (m, 1H, H7a), 1.80–1.66 (m, 1H, H7b), 1.46 (s, 3H, CH₃), 1.28 (s, 3H, CH₃), 0.89 (s, 9H, tBu), 0.06 (s, 6H, 2 × Si–CH₃). ¹³C NMR (125 MHz, CDCl₃) δ 176.8 (CONH), 145.8 (Ar), 132.9 (Ar), 130.2 (Ar), 127.8 (Ar), 112.2 (C10), 104.0 (C1), 83.0 (C2), 81.7 (C3), 79.3 (C4), 67.5 (C6), 62.9 (C5), 29.9 (C8), 26.4 (CH₃), 26.2 (CH₃), 25.7 (t-Bu), 24.4 (C7), 21.7 (Ar–), 17.9 (C̲–(CH₃)), −5.6 (Si–), −5.6 (Si–C̲). HRMS (ESI-TOF) m/z: [M + Na]⁺ calcd for C₂₅H₃₉NNaO₈SSi 564.2058; found 564.2057.

1,2-O-Isopropylidene-3-O-tosyl-5-deoxy-5-C(S)(2-(5-oxo-pyrolidine))-α-D-gluco-hexofuranose 7. To a stirred solution of compound 6 (0.9 g, 1.66 mmol) in THF (10 mL), a 1 M solution of TBAF in THF (2 mL, 1.9 mmol) was added at 0 °C. The reaction mixture was stirred for 1 h, after the reaction was completed, it was quenched with a saturated solution of NH₄Cl (5 mL) and extracted with ethylacetate (3 × 10 mL). Evaporation of the solvent and purification via column chromatography (n-hexane/ethyl acetate = 1/1, 1/9) gave 7 (0.65 g, 91.5%) as a colorless solid: mp 120–123 °C R_f 0.45 (ethyl acetate) [α]_D^27.8 = 4.68 (c 0.32, CHCl₃); IR (CHCl₃) 1697.70 cm⁻¹, 2927.54 cm⁻¹. IR (neat) 1664.51 cm⁻¹, 3241.73 cm⁻¹ (broad). ¹H NMR (500 MHz, CDCl₃) δ 7.83 (d, J = 8.0 Hz, 1H, Ar), 7.42 (d, J = 8.0 Hz, 1H, Ar), 6.62 (brs, 1H, exchanges with D₂O), 5.92 (d, J = 3.8 Hz, 1H, H1), 5.03 (d, J = 2.8 Hz, 1H, H3), 4.62 (d, J = 3.8 Hz, 1H, H2), 4.37 (d, J = 2.8 Hz, 1H, H4), 3.62 (t, J = 8.0 Hz, 1H, exchanges with D₂O), 3.45–3.40 (m, 2H, H6, after D₂O exchanges appear as ABq, J = 11.6 Hz, 2H, H6), 2.48 (s, 3H, CH₃), 2.35–2.22 (m, 2H, H8), 2.20–2.11 (m, 1H, H7), 1.78–1.68 (m, 1H, H7), 1.47 (s, 3H, CH₃), 1.27 (s, 3H). ¹³C NMR (125 MHz, CDCl₃) δ 178.01 (CONH), 145.93 (Ar), 132.7 (Ar), 130.27 (Ar), 127.94 (Ar), 112.50 (C10), 104.05 (C1), 83.11 (C2), 81.71 (C3), 79.71 (C4), 66.90 (C6), 63.81 (C5), 30.36 (C8), 26.48 (CH₃), 26.20 (CH₃), 24.65 (C7), 21.79 (Ar–). HRMS (ESI-TOF) m/z: [M + Na]⁺ calcd for C₁₉H₂₅NNaO₈S 450.1193; found 450.1191.

Methyl-(1,2-O-isopropylidene-3-O-tosyl-5-deoxy-5-C(S)-(2(5-oxo-pyrolidine))-α-D-gluco-hexofurano)-uronate 8. Compound 7 (0.6 g, 1.40 mmol) was dissolved in DCM and cooled to 0 °C, then to this cooled solution, DMP (0.893 g, 2.10 mmol) was added and reaction mixture was stirred for 12 h while allowing it to warm up to rt. After the reaction was completed, the reaction mixture was washed with a saturated solution of NaHCO₃. The organic phase was dried and evaporated to give 0.59 g of crude aldehyde as a thick liquid. This aldehyde (0.59 g) was dissolved in acetonitrile (10 mL), to which a solution of NaH₂PO₄ (0.043 g, 0.276 mmol) and 30% H₂O₂ (0.17 mL, 1.51 mmol) in water (5 mL) was added. The mixture was cooled to 0 °C, and NaClO₂ (0.187 g, 2.07 mmol) in water (5 mL) was added dropwise over a period of 15 min and stirred at rt until the reaction was complete (12 h). The reaction mixture was treated with sodium sulfite and extracted with ethyl acetate (3 × 10 mL). Evaporation of the solvent gave an acid (0.6 g) as a thick foam. This acid was dissolved in dry DMF (8 mL), to which KHCO₃ was added (0.27 g, 2.7 mmol) and cooled to 0 °C. MeI (0.1 mL, 1.68 mmol) was added dropwise to the mixture, which was stirred for 2 h while allowing it to warm up to rt. After the reaction was complete, water was added and extracted with ethyl acetate. Evaporation of the solvent and purification via column chromatography (n-hexane/ethyl acetate = 7/3) gave compound 8 (0.5 g, 78.12%) as an off white solid: mp = 170–173 °C; R_f 0.6 (n-hexane/ethyl acetate = 1/1) [α]_D^28.0 = 12.22 (c 0.7, CHCl₃); IR (neat) 1695.70 cm⁻¹, 1737.70 cm⁻¹. ¹H NMR (500 MHz, CDCl₃) δ 7.83 (d, J = 8.2 Hz, 2H, Ar), 7.42 (d, J = 8.2 Hz, 2H, Ar), 5.93 (d, J = 3.7 Hz, 1H, H1), 5.91 (s, 1H, NH) 5.09 (d, J = 3.1 Hz, 1H, H3), 4.63 (d, J = 3.7 Hz, 1H, H2), 4.58 (d, J = 3.1 Hz, 1H, H4), 3.74 (s, 3H, CO₂), 2.49 (s, 3H, CH₃–Ar), 2.35–2.25 (m, 4H, H5, H6), 1.50 (s, 3H), 1.30 (s, 3H). ¹³C NMR (125 MHz, CDCl₃) δ 176.51 (COOMe), 171.88 (CONH), 145.95 (Ar), 132.36 (Ar), 130.16 (Ar), 128.16 (Ar), 112.82 (C10), 104.42 (C1), 83.06 (C3), 80.88 (C2), 80.18 (C4), 64.82 (C5), 53.23 (COO), 29.18 (C8), 27.15 (C7), 26.71 (CH₃), 26.31 (CH₃), 21.79 (Ar–). HRMS (ESI-TOF) m/z: [M + Na]⁺ calcd for C₂₀H₂₅NNaO₉S 478.1142; found 478.1149.

Methyl-(1,2-O-isopropylidene-3-O-tosyl-5-deoxy-5-C(S)-(2-(1-benzyl-5-oxo-pyrolidine))-α-D-gluco-hexofurano)-uronate 9. A solution of 8 (0.15 g, 0.32 mmol) in dry THF (3 mL) was added dropwise to a pre-cooled solution of NaH 60% oil suspension (0.016 g, 0.41 mmol) in dry THF (3 mL) under an N₂ blanket and stirring was continued for 20 min after complete addition. Then, benzyl bromide (0.041 mL, 0.35 mmol) was added to the reaction mixture and the reaction mixture was stirred for an additional 1.5 h. After complete consumption of the starting material (monitored by TLC), a saturated solution of NH₄Cl (5 mL) was added carefully and extracted with ethyl acetate (3 × 5 mL). The organic phase was washed with brine, and dried over anhydrous sodium sulphate, and evaporation of the solvent and purification by column chromatography (n-hexane/ethyl acetate = 8/2) gave 9 (0.165 g, 91.66%) as a colorless solid: mp 80–85 °C R_f 0.4 (n-hexane/ethyl acetate = 7/3) [α]_D^28.0 = 9.7 (c 0.34, CHCl₃); IR (neat) 1695.65 cm⁻¹, 1738.73 cm⁻¹. ¹H NMR (500 MHz, CDCl₃) δ 7.74 (d, J = 8.2 Hz, 2H, Ar), 7.35 (d, J = 8.0 Hz, 2H, Ar), 7.35–7.10 (m, 5H), 5.94 (d, J = 3.8 Hz, 1H, H1), 5.21 (d, J = 3.2 Hz, 1H, H3), 4.85 (d, J = 15.7 Hz, 1H, CHaPh), 4.71 (d, J = 3.3 Hz, 1H, H4), 4.69 (d, J = 3.8 Hz, 1H, H2), 4.06 (d, J = 15.7 Hz, 1H, CHbPh), 3.10 (s, 3H, CO₂CH₃), 2.62–2.51 (m, 1H, H7a), 2.50–2.38 (m, 5H, CH₃–Ar, H7b, H6a), 2.1–2.0 (m, 1H, H6b), 1.41 (s, 3H, CH₃), 1.30 (s, 3H, CH₃). ¹³C NMR (125 MHz, CDCl₃) δ 175.93 (COOMe), 170.61 (CONH), 145.53, 135.87, 132.72, 130.03, 128.31, 128.23, 127.98, 127.46, 112.42 (C10), 104.13 (C1), 83.50 (C3), 83.27 (C2), 76.00 (C4), 68.73 (C5), 52.07 (COO), 43.99 (–Ph), 29.67 (C8), 26.77 (CH₃), 26.22 (CH₃), 24.43 (C7), 21.71 (Ar–). HRMS (ESI-TOF) m/z: [M + Na]⁺ calcd for C₂₇H₃₁NNaO₉S 568.1612; found 568.1617.

(R)-Methyl-1-benzyl-2-((1R,2S)-1-(formyloxy)-3-methoxy-3-oxo-2-(tosyloxy)propyl)-5-oxopyrrolidine-2-carboxylate 10. A solution of 9 (0.15 g, 0.27 mmol) in TFA–H₂O (3.00 mL, 3 : 1) was stirred at 0 °C for 6 h. Azeotropic removal of TFA with toluene in vacuo afforded the intermediate hemiacetal (0.135 g, thick liquid), which was taken in MeOH/water (6 mL, 9 : 1), cooled to 0 °C and NaIO₄ (0.088 g, 0.41 mmol) was added. After stirring for 5 h, the reaction mixture was concentrated in vacuo, the residue extracted with CHCl₃ (3 × 10 mL), and the extract was concentrated in vacuo to obtain the crude aldehyde (0.1 g, thick liquid). This was dissolved in MeCN (5 mL), treated successively with NaH₂PO₄ (0.006 g, 0.039 mmol) in H₂O (1 mL) and 30% H₂O₂ (0.021 mL, 0.218 mmol), cooled to 0 °C, and NaClO₂ (0.026 g, 0.298 mmol) in H₂O (1.5 mL) was added into it in 10 min. After stirring at 20 °C, until the reaction (10 h) was complete, the reaction mixture was treated with sodium sulphite and extracted with EtOAc (3 × 5 mL). Concentration of the extract in vacuo gave the acid (0.089 g). The crude acid was dissolved in dry DMF (5 mL), and NaHCO₃ (0.027 g, 0.32 mmol) was added followed by MeI (0.012 mL, 0.20 mmol) at 0 °C. After the reaction was completed, the usual workup and purification of the residue by column chromatography (n-hexane/ethyl acetate = 1/1) gave 10 (0.085 g, 60.71%) as a thick liquid: R_f 0.5 (n-hexane/ethyl acetate = 1/1), [α]_D^26.8 = 52.38 (c 1.09 CHCl₃); IR (neat) 1693.85 cm⁻¹, 1729.15 cm⁻¹, 1744.23 cm⁻¹. ¹H NMR (500 MHz, CDCl₃) δ 7.86 (s, 1H, OCH̲O), 7.78 (d, J = 7.7 Hz, 2H, Ar), 7.35 (d, J = 7.7 Hz, 2H, Ar), 7.30–7.20 (m, 5H, Ar), 6.13 (s, 1H, H1), 5.61 (s, 1H, H2), 4.92 (d, J = 15.7 Hz, 1H, CHaPh), 3.98 (d, J = 15.7 Hz, 1H, CHbPh), 3.64 (s, 3H, CO₂CH₃), 3.15 (s, 3H, CO₂CH₃), 2.70–2.49 (m, 4H, H4, H5), 2.46 (s, 3H, CH₃Ar). ¹³C NMR (125 MHz, CDCl₃) δ 176.29 (CONH), 170.05 (COOMe), 166.16 (COOMe), 158.48 (OC̲OH), 145.77, 135.66, 132.43, 129.85, 128.50, 128.32, 128.29, 127.64, 75.24 (C1), 69.32 (C2), 68.40 (C3), 53.12 (COO), 52.56 (COO), 43.86 (–Ph), 29.42 (C5), 23.42 (C4), 21.74 (Ar–). HRMS (ESI-TOF) m/z: [M + Na]⁺ calcd for C₂₅H₂₇NNaO₁₀S 556.1246; found 556.1253.

(R)-Methyl-2-((1S,2R)-2-azido-1-hydroxy-3-methoxy-3-oxopropyl)-1-benzyl-5-oxopyrrolidine-2-carboxylate 11. To a stirred solution of tosylate 10 (0.05) in DMF, NaN₃ was added and the resulting turbid solution was stirred at 50 °C for 6 h. After the reaction was completed, the reaction mixture was poured in water and extracted with ethyl acetate. Drying, evaporation of the organic extract in vacuo and purification of the residue by column chromatography (n-hexane/ethyl acetate = 1/1) gave 11 (0.029 g, 76.25%) as a thick liquid: R_f 0.56 (n-hexane/ethyl acetate = 1/1); [α]_D^29.1 = 32.1 (c 0.39 CHCl₃); IR (neat) 1693.85 cm⁻¹, 1744.23 cm⁻¹, 2120.21 cm⁻¹, 3400 cm⁻¹, ¹H NMR (300 MHz, CDCl₃) δ 7.28–7.26 (m, 5H, Ar), 4.62 (d, J = 15.24 Hz, 1H, Bn), 4.35 (d, J = 15.24 Hz, 1H, Bn), 4.00 (d, J = 2.22 Hz, 1H, H3), 3.75 (d, J = 2.2 Hz, 1H, H2), 3.55 (s, 3H, COO), 3.48 (s, 3H, COO), 2.65–2.34 (m, 4H). ¹³C NMR (125 MHz, CDCl₃) δ 176.51 (CONH), 171.88 (COOMe), 166.16 (COOMe), 145.95 (Ar), 132.32 (Ar), 129.85 (Ar), 128.50 (Ar), 128.43 (Ar), 75.29 (C2), 68.24 (C3), 62.56 (C1), 53.42 (COO), 52.56 (COO), 43.12 (–Ph), 29.86 (C5), 23.42 (C4). HRMS (ESI-TOF) m/z: [M + Na]⁺ calcd for C₁₇H₂₀N₄NaO₆ 399.1281; found 399.1278.

Acknowledgements

We are thankful to the Department of Science and Technology, New Delhi (Project File No. EMR/2014/000873) for providing financial support. Central Instrumentation Facility (CIF) for providing analytical support and crystal data. PRM thankful to the CSIR, New Delhi for Senior research fellowship.

Notes and references

1. K. Shin-ya, J. S. Kim, K. Furihata, Y. Hayakawa and H. Seto, Tetrahedron Lett., 1997, 38, 7079.
2. (a) Z. A. Bortolotto, Z. I. Bashir, C. H. Davies and G. L. Collingridge, Nature, 1994, 386, 740; (b) M. B. Kennedy, Cell, 1989, 59, 777; (c) D. Muller, M. Joly and G. Lynch, Science, 1988, 242, 1694; (d) K. Shin-ya, Biosci., Biotechnol., Biochem., 2005, 69, 867.
3. (a) C. E. Bigge, Curr. Opin. Chem. Biol., 1999, 3, 441; (b) M. L. Mayer, Curr. Opin. Neurobiol., 2005, 15, 282; (c) R. Dingledine, K. Borges, D. Bowie and S. F. Traynelis, Pharmacol. Rev., 1999, 51, 7; (d) D. R. Madden, Nat. Rev. Neurosci., 2002, 3, 91.
4. (a) D. Ma and J. Yang, J. Am. Chem. Soc., 2001, 123, 9706; (b) M. Okue, H. Kobayashi, K. Shin-ya, K. Furihata, Y. Hayakawa, H. Seto, H. Watanabe and T. Kitahara, Tetrahedron Lett., 2002, 43, 857; (c) H. Watanabe, M. Okue, H. Kobayashi and T. Kitahara, Tetrahedron Lett., 2002, 43, 861; (d) M. Kawasaki, T. Shinada, M. Hamada and Y. Ohfune, Org. Lett., 2005, 7, 4165; (e) R. G. Vaswani and A. R. Chamberlin, J. Org. Chem., 2008, 73, 1661; (f) Y. Gosei and K. Kyokaishi, J. Synthetic Org. Chem., 2007, 65, 511; (g) K. Takahashi, D. Yamaguchi, J. Ishihara and S. Hatakeyama, Synlett, 2008, 5, 671; (h) M. Hamada, T. Shinda and Y. Ohfune, Org. Lett., 2009, 11, 4664; (i) S. Yu, S. Zhu, X. Pan, J. Yang and D. Ma, Tetrahedron, 2011, 67, 1673; (j) K. Takahashi, D. Yamaguchi, J. Ishihara and S. Hatakeyama, Org. Lett., 2012, 14, 1644; (k) W. Lee, J. H. Youn and S. H. Kang, Chem. Commun., 2013, 49, 5231; (l) N. Chandan and M. G. Moloney, Tetrahedron Lett., 2013, 54, 1987; (m) P. Garner, L. Wirasinghe, I. Houten and J. Hu, Chem. Commun., 2014, 50, 4908.
5. (a) N. P. Karche, S. M. Jachak and D. D. Dhavale, J. Org. Chem., 2003, 68, 4531; (b) V. D. Chaudhari, A. Kumar and D. D. Dhavale, Org. Lett., 2005, 7(26), 5805; (c) N. S. Karanjule, S. D. Markad and D. D. Dhavale, J. Org. Chem., 2006, 71, 6273; (d) V. D. Chaudhari, A. Kumar and D. D. Dhavale, Tetrahedron, 2006, 62, 4349; (e) N. S. Karanjule, S. D. Markad, V. S. Shinde and D. D. Dhavale, J. Org. Chem., 2006, 71, 4667; (f) N. B. Kalamkar, V. M. Kasture and D. D. Dhavale, J. Org. Chem., 2008, 73, 3619; (g) R. S. Mane, S. Ghosh, B. A. Chopade, O. Reiser and D. D. Dhavale, J. Org. Chem., 2011, 76, 2892.
6. N. J. Pawar, V. Parihar, S. T. Chavan, R. Joshi, P. V. Joshi, S. G. Sabharwal, V. G. Puranik and D. D. Dhavale, J. Org. Chem., 2012, 77, 7873.
7. S. Deloisy, T. Thang, A. Olesker and G. Lukacs, Tetrahedron Lett., 1994, 35, 4783.
8. In this reaction, we isolated the un-cyclized product by the reduction of the azide and olefin groups along with C3 O-debenzylation, as evident from ¹H NMR spectrum. Addition of 0.3 equiv. K₂CO₃ in the same pot provided the required lactam.
9. Tosylation of compound 5 using p-tolunesulfonylchloride in dichloromethane, pyridine/TEA with a catalytic amount of DMAP under a variety of reaction conditions was unsuccessful.
10. Hydrolysis of the 1,2-O-isopropylidine group in 9 using TFA: H₂O gave hemiacetal, which upon oxidative cleavage using NaIO₄ followed by Pinnick oxidation gave a complex mixture. Therefore, we protected lactam 9 as its benzyl derivative.
11. Formation of O-formate was confirmed by NMR analysis, which showed a singlet at δ 7.86 in the ¹H NMR spectrum and δ 158.5 in the ¹³C NMR spectrum, which correspond to the formate group.

---

Footnote

† Electronic supplementary information (ESI) available: Figures giving ¹ H and ¹³C NMR spectra for all compounds and a CIF file giving crystallographic data for compound 8. CCDC 1415037. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c5ra17442b

---