Silicon tetrachloride in organic synthesis: new applications for the vinylogous aldol reaction

This paper describes a novel and efficient methodology for vinylogous aldol reactions based on SiCl₄ catalysis. According to the nucleophilicity Mayr's scale, vinylogous aldol reaction of Chan's diene proved to be effective by using catalytic amount of SiCl₄, without any other promoter. On the contrary, the SiCl₄/Lewis base system has been conveniently exploited for the efficient and selective vinylogous reaction of less nucleophilic Danishefsky's diene and 2-trimethylsilyloxyfuran (TMSOF). Indeed, a number of Lewis bases, such as sulfoxides, formamides and phosphoramides have been successfully used as SiCl₄ promoters. TMSOF and silyloxydienes, resulting from 2,2,6-trimethyl-1,3]-dioxin-4-one derivatives, required stoichiometric amount of SiCl₄, while vinylogous aldol reaction of Chan's and Danishefsky's dienes took satisfactorily place in the presence of catalytic or sub-stoichiometric amount of catalyst.