Stereoselective Synthesis of Steroids and Related Compounds, IV['l Addition of Cerium Reagents Derived from (Trimethylsily1)propargyl Bromide to Aldehydes

The addition of (trimethylsily1)propargylmagnesium bromide (9) to aldehydes afforded predominantly the allenylic alcohols rac-10. The regioselectivity of this addition changes dramatically when organocerium reagents were used for the addition. In this case nearly exclusive formation of the homopropargylic alcohols rac-8 was observed. Highest selectivity for this process was obtained when cerium(II1) isopropoxide (13) was used as a precursor for the formation of the organocerium reagent.