| Popis: |
Reaction of a series of 21 glycosides unprotected at O-2 and featuring various configurations with DAST (diethylaminosulfur trifluoride) was monitored by 19F NMR spectroscopy. By means of the diacritical set of data (shift values and coupling constants) thus obtained for each product, identification of the operative mechanisms was possible. By correlation of these findings with stereochemical details from the structure of the educts, new parameters governing the choice of the reaction paths could be deduced. This evaluation led to the result that ring contraction after attack at C-2 of the ring oxygen and entry of the fluoride at C-1 is strongly favoured over all other possibilities. Exceptions are all derivatives of the manno series as well as all members of the trans-decalin type structure as present after 4,6-O-benzylidene acetal formation with educts having their ring substituents at C-4 and C-5 in diequatorial (trans) orientation. From these, α-D-mannopyranosides generally and of the trans-decalin types those with an additional 1,2-trans-configuration are prone to 1,2-aglycon migration and, again, entry of the fluoride at C-1. Additional pathways like alkoxy group migration, substitution under retention of configuration, or orthoester formation, are possible by participation of a suitably located neighbouring group at C-3 inasmuch as an alkoxy group interferes from an antiperiplanar orientation to the leaving group at C-2 and an acyloxy functionality attacks in a diequatorial relationship to the latter. The generally intended nucleophilic substitution by fluoride under inversion of configuration is of minor importance. |
