Stereoselective syntheses of conjugated hexatrienes - Application to the synthesis of galbanolenes by a modified Ramberg-Backlund reaction

(E, E ) -, (E, Z) - and (Z, Z) - diallylic sulfones (6) undergo stereospeci fic Ramberg Backlund reaction to give (E, E, E) -, (E, E, Z) - and (Z, E, Z ) - 1,3, 5 - hexatrienes (7), respectively, upon treatment with CBr2F2 in t he presence of alumina - supported KOH. The stereoselectivity of the reacti on depends on the solvent and temperature. Good (E) - selectivity ( > 95%) can usually be obtained by conducting the reaction at 0 degrees C in CH2Cl2 but in some circumstances better (E) - stereoselectivity can be achieved w hen the reaction is performed at - 78 degrees C in V(t - BuOH) : V(CBr2F2) = 1:1 mixture. The stereoselective syntheses of galbanolenes (7m)and (7n) u sing this newly developed method are exemplified.