Synthesis and X-ray structure of functionalised proline mimics

Methyl (2 R )-(1-benzoyl-2-benzyl-1,2,3,6-tetrahydropyridin-2-yl)carboxylate 1 reacts with osmium tetroxide to give two isomeric syn diols 2a and 2b and also with Br 2 to give two isomeric anti dibromides 4a and 4b . The major dibromide 4a gave rise to the lactone 5 on standing, the crystal structure of which is reported.


Introduction
For some time, we [1][2][3][4] and others, [5][6][7] have had an interest in the design and preparation of analogues of proline of varying ring size and substitution.This interest stems from the fact that proline, and its hydroxylated derivatives, are known to induce secondary structure in peptides and proteins. 8This induced secondary structure plays an important role in structures such as collagen, [9][10][11] an insoluble fibrous protein found in bone, teeth, blood vessels, connective tissue, tendons, cartilage, and hide.In addition, many bioactive peptides and natural products contain one or more functionalised proline, or proline-like, residues that are critical to their biological activity. 12,13As a result, there has been much interest in the synthesis and functionalisation of proline analogues.However, of the many mimetics known few possess substitution to the αcarbon.With this in mind we prepared the ring-expanded and benzyl-substituted proline mimetic, methyl (2R)-(1-benzoyl-2-benzyl-1,2,3,6-tetrahydropyridin-2-yl)carboxylate (1), and also solved its X-ray structure. 1We now present results of studies on the functionalisation of its constituent olefin.

Results and Discussion
In the first instance, the peptidomimetic 1 was treated with potassium osmate and Nmethylmorpholine-N-oxide (NMO) to give a mixture of syn diols in a ratio of 3:1 by 1 H NMR (Scheme 1).The crude mixture was purified by column chromatography to give the major isomer in 55% yield and an enriched sample of the minor isomer (2:1, minor to major).The major isomer was assigned the absolute configuration 2a based on the known configuration of 1 and the fact that osmylation would be expected to predominate from the less hindered face of 1, i.e. the side opposite the pseudo axial benzyl group as revealed in the X-ray structure (see Fig 1).The assignment of absolute configuration to 2a, and hence 2b, was supported by 1 H NMR spectral data.In particular, the resonance at 3.96 ppm (H5) gave a positive nOe to the benzyl methylene group at 3.06 ppm.This is consistent with a syn relationship between these two groups as in 2a but not 2b.The assignment of the resonances at 4.20 and 3.96 to H4 and H5, respectively, was consistent with observed COSY cross peaks between (i) the resonance at 4.20 ppm and the C3 methylene at 2.04 and 2.27 ppm and (2) the resonance at 3.96 ppm and the C6 methylene at 2.37 and 3.51 ppm.A positive nOe was also observed between the H4 and one of the C3 methylene resonances.With this result in hand we next turned our attention to the bromination of 1 (Scheme 2).Treatment of 1 with a slight excess of bromine gave two anti dibromides in a ratio of 4:1 by 1 H NMR. Purification of this mixture by silica chromatography resulted in isolation of the major isomer in 67% yield.Bromination of 1 would be expected to initially give rise to the two isomeric bromonium ions 3a and 3b, and again addition anti to the pseudo axial C2 benzyl group would be expected to predominate to give 3a as the major isomer.The bromonium ion 3a would then give rise to 4a via anti bromide attack at the less hindered C5 position.A similar bromide attack on 3b would give rise to 4b.Thus the major dibromide isomer is assigned the absolute configuration 4a. 1 H NMR and nOe data is again consistent with this assignment: as for 2a the resonance for H4 (4.61 ppm) of 4a is downfield relative to that of H5 (4.03 ppm) and a diagnostic positive nOe was observed between H4 and the C3 methylene.Unlike 2a, an nOe was not observed between C2 benzyl methylene and H5 since these groups are syn in 2a but anti in 4a.The major isomer 4a was left to stand for 6 months at which time a single crystal was obtained and its structure solved by X-ray crystallography (Figure 2).This revealed, somewhat to our surprise, that lactonisation had occurred to give 5 with the absolute configuration being assigned based on the known absolute configuration of C2 in 1. Lactonisation of 4a to give 5 requires hydrolysis of the methyl ester followed by displacement of the C4 bromide, which would be expected to occur with inversion of configuration at that centre.However, this is not possible since the methoxycarbonyl group and the C4 bromide of 4a are syn.In addition, while attack of the axial 'carboxylate' of 4b at C4 is possible, it would give rise to a lactone with the incorrect configuration at C5. Therefore, lactonisation to give 5 must occur by reversion to the bromonium ions 3a and 3b, of which only 3b has suitable stereochemistry for cyclisation.This is supported by the observation that 5 and 3b have the same absolute configuration at the stereogenic centre bearing the bromine.
Lactone 5 crystallized in the chiral, orthorhombic space group P2 1 2 1 2 1 with four molecules in the unit cell (see Figure 2 for a perspective view and atom numbering).It is apparent that the piperidine ring atoms represented by N, C3-C7 adopt a distinct boat conformation, with a fivemembered lactone ring bridge existing between C5 and the carboxyl group extending from C3.A distorsion of normal tetrahedral geometry (bond angle is 109.5º) is observed about C3 due to the formation of this lactone.Bond angles around C3, internal to the ring system, for C2-C3-C4, C2-C3-N and C4-C3-N, are 100.Purification by radial chromatography (ethyl acetate/petroleum ether, 1:3) gave the major isomer 4a (15 mg, 67%).

Figure 1 .
Figure 1.Compound 1 and perspective view of its X-ray crystal structure.
Bromine was added dropwise to a solution of 1 (15 mg, 0.045 mmol) in CH 2 Cl 2 (5 mL) under nitrogen, until a permanent brown colour remained and the resulting mixture was stirred under nitrogen for 3 h.The solvent was removed under reduced pressure to give a brown residue containing a mixture of 4a and 4b (4:1 by 1 H NMR).

Inc Experimental Section General Procedures.
Proton NMR and nOe spectra were recorded on a Varian 500 MHz NMR spectrometer.Carbon NMR spectra were recorded on a Varian 300 MHz NMR spectrometer operating at 75 MHz.Chromatography was carried out on a Harrison and Harrison chromatotron using Merk type P.F.254 silica gel.The magnesium silicate was removed by filtration and the filtrate adjusted to pH 7 with 1N H 2 SO 4 .Acetone was removed under reduced pressure and the resulting aqueous phase acidified to pH 2 with 1M aqueous HCl.The aqueous phase was then saturated with NaCl and extracted with ethyl acetate (2 x 2 mL).The organic extracts were combined, dried (MgSO 4 ) and the solvent removed under reduced pressure to give a crude mixture of 2a and 2b (40 mg, 3:1 by 1 H NMR). The residue was purified by radial chromatography (ethyl acetate/dichloromethane 1:1) to give a fraction containing 27 mg (55%) of the major cis isomer 3a, and a second fraction containing 13 mg of a 3:1 mixture of 2a and 2b which could not be separated further.Selected data for 2b (from mixture): 1 H NMR (CDCl 3 ) δ 18, 2.58 (dd J=14.2 and 1.5Hz, 1H), 3.01 (d J=15.4Hz, 1H, CH a Ph), 3.75 (d J=15.4Hz, 1H), 3.86 (s, 3H, OMe), 3.97 (d J=14.2Hz, 1H, CH b Ph).