Chain elongation by a seemingly stereospecific cyanohydrin synthesis: the preparation and configurational assignment of 3,7-anhydro-d-threo-l-talo- and -l-galacto-octose

2,6-Anhydro-d-glycero-l-manno-heptose (1) is converted by the cyanohydrin reaction into crystalline d-threo-l-talo-octononitrile (3), which shows mutarotation in water. The equilibrium mixture, as measured by ¹³C-n.m.r. spectroscopy, contains about equal amounts of 3 and its epimer, d-threo-l-galacto-octononitrile. On evaporation of the aqueous mixture, pure, crystalline 3 is again obtained. Labelling experiments in ³H₂O proved that epimerization proceeds through reversible deprotonation. Stabilization of 3 in the solid state is explained by intramolecular hydrogen-bonding. In pyridine, rapid isomerization of 3 occurs. When acetylation of 3 is conducted in this solvent, the yield of 2,4,5,6,8-penta-O-acetyl-3,7-anhydro-d-threo-l-talo-octono-nitrile (4) depends strongly on the conditions of acetylation. Acetylation after equilibration produces an equimolar mixture of 4 and its isomer 2,4,5,6,8-penta-O-acetyl-3,7-anhydro-d-threo-l-galacto-octononitrile. Structural assignment for both was achieved by 360-Mhz, ¹H- and ¹³C-n.m.r. spectroscopy. Reduction of 4 in pyridine-acetic acid-water in the presence of N,N-diphenylethylenediamine yields a 1:2.36 mixture of 2,4,5,6,8-penta-O-acetyl-3,7-anhydro-d-threo-l-talo-octose N,N-diphenylimidazolidine (6) and 2,4,5,6,8-penta-O-acetyl-3,7-anhydro-d-threo-l-galacto-octose N,N-diphenylimidazolidine (8). Compounds 6 and 8 could be separated and obtained as crystalline solids, and their structure proved by ¹H- and ¹³C-n.m.r. spectroscopy. Hydrolysis of 6 and 8 gave 2,4,5,6,8-penta-O-acetyl-3,7-anhydro-d-threo-l-galacto-octose and -d-threo-l-talo-octose.