Réaction d'ylures stabilisés sur des aldéhydosucres: influence de divers facteurs dont la structure de l'aldéhydosucre sur le pourcentage d'isomères géométriques obtenu

Reactions of stabilized ylides with aldehydosugars: influence of some factors, particularly the structure of the aldehydosugar, on the ratio of the geometrical isomers formed Aldehydosugars bearing no cis ‐substituent on the C‐atom β to the carbonyl group ( 2, 3, 6, 7 ) or bearing a substituent without an electron lone‐pair ( 5 ) gave, upon treatment with acetylmethylidenetriphenylphosphorane, exclusively the E ‐isomer, this representing the classical behaviour of aldehydes. On the other hand, aldehydosugars having a lone‐pair bearing substituent ( 1, 4, 8, 9 ) gave a mixture of E ‐ and Z ‐isomers. In the case of most of the aldehydosugars of the latter type (giving some Z ‐isomer) a partial epimerization of the C‐atom α to the carbonyl group took place when the solvent of the Wittig reaction was HCONMe 2 . It is probable that the presence of an electron lone‐pair in the adequate position inhibits the reversion of the kinetic erythro ‐betaine, allowing the formation of the Z ‐isomer and, in particular solvent conditions, a cycloelimination leading to the epimeric aldehydosugar which ultimately reacts to give the epimeric E ‐enone.