Stereoselective synthesis of unhindered olefins by 2-fold extrusion reactions
摘要:
Two-fold extrusion reactions of 1,3,4-thiadiazolines 1 provide a convenient stereoselective route to unhindered Z-olefins. An improved preparation of the thiadiazolines involves reaction of an aldehyde with H2S-hydrazine, followed by in situ oxidation of the intermediate predominantly trans thiadiazolidine 2. Stereospecific extrusion of nitrogen yields the cis-thiirane which upon treatment with triphenylphosphine affords the corresponding Z-alkene in good yields.
The insertion of lithiated epoxides into boronic esters followed by thermal syn-elimination provides a stereospecific entry to alkenes. This process avoids transition metals and is amenable to iteration to provide higher substitution patterns.