STEREOSELECTIVE SYNTHESIS OF STEROIDS AND RELATED-COMPOUNDS .4. ADDITION OF CERIUM REAGENTS DERIVED FROM (TRIMETHYLSILYL)PROPARGYL BROMIDE TO ALDEHYDES

The addition of (trimethylsilyl)propargylmagnesium bromide (9) to alde hydes afforded predominantly the allenylic alcohols rac-10. The regios electivity of this addition changes dramatically when organocerium rea gents were used for the addition. In this case nearly exclusive format ion of the homopropargylic alcohols rac-8 was observed. Highest select ivity for this process was obtained when cerium(III) isopropoxide (13) was used as a precursor for the formation of the organocerium reagent .