Asymmetric synthesis with .alpha.,.beta.-bis[(methoxymethyl)oxy] ketones. Enantioselective total synthesis of natural (+)-indolizidine 195B (bicyclic gephyrotoxin 195B) and (-)-pinidine and their enantiomers from a common chiral synthon

Asymmetric synthesis of 3,5-disubstituted indolizidines by intermolecular addition of an allylsilane on an N-acyliminium ion

“…These values compare favourably with those obtained previously for synthetic samples. 10,15,17 In the same manner, the hydrogenation of 11b provided a mixture of isomers 3 and 4 in a 7:3 ratio. These could be separated by flash column chromatography.…”

“…They were identified as (À)-indolizidine 195B and (+)-monomorine, respectively, by comparison of their 1 H and 13 C NMR spectra with the literature. [15][16][17] The specific rotations ½a 22 D were determined to be À90.2 (c 0.75, MeOH) for 1 and +17.6 (c 0.21, hexane) for 2. These values compare favourably with those obtained previously for synthetic samples.…”

Synthesis of 2,6‐Disubstituted Piperidine‐Bearing α‐Amino Acid Side‐Chains

“…This synthetic pathway, HWE condensation followed by cyclization and reductive amination, was described for generation of pipecolic acid derivatives. [12] Starting from enantiomerically pure compound 6, we expected to obtain a single diastereomer with a cis configuration on the piperidine ring as already described [13] for other piperidine derivatives. Examination of LC-MS of the obtained piperidine compounds revealed the presence of two diastereomers in a ratio of about 60:40 which could be separated after removal of the Boc protecting group by acidolysis (Table 1).…”

“…1 H, 13 C, and 31 P NMR spectra were recorded in [D 6 ]DMSO at 400, 100, and 100 MHz respectively at 300 K. H 3 PO 4 was used as external reference for 31 P NMR spectra. Chemical shifts were reported as δ values (ppm) indirectly referenced to the solvent signal.…”

“…these two protons are in a cis disposition towards the piperidine ring. As C 6Ј epimerization has never been observed during such a ring-closure procedure, [12,13,15,16] we assumed that configuration of the starting amino acid derivative 5 was conserved. This allowed us to ascertain the configuration of asymmetric carbons C 2Ј and C 6Ј .…”