Glycosylidene Carbenes. Part 22. α‐ D ‐Selectivity in the Glycosidation by Carbenes Derived from 2‐Acetamido‐hexoses

Glycosylidene carbenes derived from the GlcNAc and AllNAc diazirines 1 and 3 were generated by the thermolysis or photolysis of the diazirines. The reaction of 1 with i‐PrOH gave exclusively the isopropyl α‐D‐glycoside of 5 besides some dihydrooxazole 9 (Scheme 2). A similar reaction with (CF3)2CHOH yielded predominantly the α‐D‐anomer of 6, while glycosidation of 4‐nitrophenol (→7) proceeded with markedly lower diastereoselectivity. Similarly, the Allo‐diazirine 3 gave the corresponding glycosides 12–14, but with a lower preference for the α‐D‐anomers (Scheme 3). The reactions of the carbene derived from 1 with Ph3COH (→8) and diisopropylideneglucose 10 (→11) gave selectively the α‐D‐anomers (Scheme 2). The αD‐selectivity increases with increasing basicity (decreasing acidity) of the alcohols. It is rationalized by an intermolecular H‐bond between the acetamido group and the glycosyl acceptor. This H‐bond increases the probability for the formation of a 1,2‐cis‐glycosidic C–O bond. The gluco‐intermediates are more prone to forming a N–H…︁(H)OR bond than the allo‐isomers, since the acetamido group in the N‐acetylallosamine derivatives forms an intramolecular H‐bond to the cis‐oriented benzyloxy group at C(3), as evidenced by δ/T and δ/c experiments.