4-disilylation and the reductive duplication accompanied by O-silylation, can chiefly occur by action of the Me3SiClMgHMPT system on various α-ethylenic carbonylated compounds. When the sequence of such derivatives is conjugated with another group , the 1,4-disilylation is the main reaction and affords upon hydrolysis the corresponding derivative β-silylated with respect to the carbonylgroup (new illustrative