Dioxolane acetal ring expansion during a sugar triflate displacement. Synthesis and assignment of diastereoisomer configuration of novel 9-crown-3 ether derivatives

Treatment of 2,5:3,6-dianhydro-6-thio-4-O-trifluoromethanesulfonyl-L-talose ethylene acetal (5) with lithium benzoate in boiling DMF unexpectedly gave the 9-crown-3 ether derivatives 7 and 8 instead of the substitution produc t 6. The mechanism of the process presumably involved neighbouring group pa rticipation of the dioxolane acetal function. H-1 NMR and molecular mechani cs calculations (MM3) provided the assignment of stereoisomer configuration since the results of semi-empirical PM3 calculations on postulated oxonium -ion intermediates reasonably explained the high stereoselectivity of the p rocess. (C) 1999 Elsevier Science Ltd. All rights reserved.