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-tritnethylsilyloxyfuran (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.