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

Abstract

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 product 6. The mechanism of the process presumably involved neighbouring group participation of the dioxolane acetal function. 1H NMR and molecular mechanics 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 process.