Synthesis of D3-Dihydrothiapyran-1,1-dioxides and Their Ramberg-Bäcklund Conversion to Cyclopentadienes

Treatment of Z-sulfone with n-BuLi gives complete conversion to the cyclized anion 3. The anion 2 is protonated to 4. Reaction of 3 with hexachloroethane leads to a .mixture of 4, -5 and 6 .Ramberg-BBcklund rearrangement of 2 with one equivalent of n-BuLi affords 6 and 1.The same reaction with 6 -gives 8.During a synthetic study directed to conjugated isoprenoid polyenesl, we have come across the facile base-promoted ringclosure of sulfone L2 to the substituted A3dihydrothiapyran-1,l-dioxides 4, 2 and 6 (Scheme 11.The cyclization appears to be general for sulfones carrying a hydrogen at the a-position and a Z-butadienyl 2 moiety at the &position .It allows the simultaneous introduction of substituents at the positrons 3 and 6 of the thiapyran ring, while position 2 may be either left unsubstituted or be substituted by chlor~ne.The Ramberg-Bzcklund desulfurization of the chlorosulfones 2 and 6 -leads to the cyclopentadienes ?and 8 (Schemes 2 and 3 1 , respectively.The monochlorocyclopentadienyl derivative L may be considered as a cyclopentenone precursor.The overall reaction constitutes a cyclopentadiene synthesis startmg from an acyclic sulfone.