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

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Abstract
Reactions of stabilized ylides with aldehydosugars: influence of some factors, particularly the structure of the aldehydosugar, on the ratio of the geometrical isomers formedAldehydosugars 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 HCONMe2. 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.