1,3-oxazin-2-ones vs tetrahydrofurans by iodocyclisation of 2-alkoxycarbonylamino-3-alken-1-ols

Iodocyclisation of primary homoallylic alcohols 2a-d, containing either a 2-t-butoxy- or a benzyloxy-carbonylamino group, was studied, in order to establish the nucleophilic group involved in cyclofunctionalisation. In fact, the N-t-Boc derivative 2a gave the oxazinone 3, exclusively, whereas starting from the N-Cbz derivative 2b a diastereomeric mixture of substituted tetrahydrofurans 4 and 5 resulted in ratios depending upon the reaction conditions. These results were rationalised by means of computational methods. On the contrary, migration of both the t-butyl or benzyl group to the hydroxy group was observed when both 2c and 2d underwent cyclisation to give the corresponding 5-alkoxymethyl oxazolidin-2-ones 7a,b in low yield, but with high regio- and stereocontrol. (C) 2000 Elsevier Science Ltd. All rights reserved.