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 benzyloxycarbonylamino 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.