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

✍ Scribed by Jean M. J. Tronchet; Bernard Gentile

• Publisher: John Wiley and Sons
• Year: 1979
• Tongue: German
• Weight: 497 KB
• Volume: 62
• Category: Article
• ISSN: 0018-019X
• DOI: 10.1002/hlca.19790620707

✦ Synopsis

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.