Due to the importance of the la&one framework in numerous natural productst, much effort has been devoted to their access.* Among the plethora of syntheses already described. the conversion of alkenoates or unsaturated amides has indeed been achieved but requires in general a two step procedure. For example, iodolactonizatiorts of unsaturated alkanoates or amides leads first to iodocompounds and needs a further reductive step with s&mrane hydridea't Ctherwise, selenolactonization presents the same inconveniance and generates toxic selenium derivatives. We anticipated that the cleavage of g,y-unsaturated esters into the corresponding acids followed by a cationic lactoniration pmcedut6 could represent another efticient strategy.
In coMection with our previous work concerning the photodeconjugation of a,f3unsaturated esters6J 1, we have investigated the possibility of transforming g,y-unsaturated carboxylic derivatives 2 into butanohdes or spirofactones 3 whose the synthesis is presently of interest* (Scheme 1).
2 scheme 1
The a,g-unsaturated esters 1. readily prepared by a Wittig-Homer reaction9 have been submitted to irradiation at CPC in methylene chloride and in the pmsence of one equivalent of diethylamine. Diethylamine which is easily removable under vacuum constitutes a convenient base to promote the keto-enohc rearrangement of the photodienolic intermediate.tc Under these conditions, the g,y-unsatma ted compounds 2 have been isolated in high yields (table 1).