Stereoelectronic control of oxazolidine ring-opening:: structural and chemical evidences

Ring opening of oxazolidines derived from tris(hydroxymethyl)aminomethane, L-serine and L-threonine was investigated. It was shown that n(N)-->sigma*(C-O) electron delocalization (endo-anomeric effect) occurring in the five-membered ring plays a major role in the cleavage of the intracyclic C-O bond. The present work establishes that when the nitrogen lone pair is conjugated with a carbonyl group (n(N)-->pi(C=O) delocalization) as happens in N-acyloxazolidines, both hydrolysis and reductive ring-opening become much more difficult as a consequence of a concomitant decrease of oxygen basicity and of an increase of the intracyclic C-O bond strength. (C) 2002 Elsevier Science Ltd. All rights reserved.