Stereocontrolled Synthesis of a C 1 –C 10 Building Block (“Southwestern Moiety”) for the Unnatural Enantiomers of the Polyene Polyol Antibiotics Filipin III and Pentamycin: A Sultone‐Forming Ring‐Closing Metathesis for Protection of Homoallylic Alcohols

In C 2 ‐symmetric diol 10 one OH group was substituted by crotonic acid and the other was incorporated in an ethenesulfonate. A twofold ring‐closing metathesis under forcing conditions provided unsaturated lactone/unsaturated sultone 24 . Conjugate addition of Li 2 (PhMe 2 Si) 2 Cu(CN) to both C=C bonds was followed by opening of the sultone moiety with Bu 4 N + F – . Resultant homoallylic alcohol 28 was carried on to acetonide‐containing lactone 31 . Its CpTiCl 2 enolate underwent a completely diastereoselective aldol addition to hexanal. Functional group interconversions rendered “southwestern” C 1 –C 10 building block 37 . The steps (1) ethenesulfonylation of a homoallylic alcohol, (2) sultone formation, (3) Li 2 (PhMe 2 Si) 2 Cu(CN) addition, and (4) alcohol release by fluoride‐induced fragmentation, from our main sequence represent a new protection/deprotection stragegy for a secondary homoallylic alcohol, as demonstrated with a model compound, too.