OXYANION ORIENTATION IN ANIONIC OXY-COPE REARRANGEMENTS

Efficiency of chirality transfer in anionic oxy-Cope rearrangement depends solely on the orientational preference of the oxyanionic bond in the substrates with a single carbinol carbon chiral center. In chairlike transition-state conformations for the rearrangement of simplest substrates like anions generated from (E)-1-phenyl-1,5-hexadien-3-ol and (E)-1,5-heptadien-4-ol, the oxyanionic bond is more prone to adopt the pseudoaxial orientation. On the other hand, anions generated from (E)-2-methyl-1,5-heptadien-4-ol, (E)-5-tert-butyl-1-phenyl-1,5-hexadien-3-ol, and 4-(2'-methyl-1'-cyclohexenyl)-1-buten-3-ol undergo rearrangement via chairlike transition states in which the pseudoequatorial oxyanionic bond is favored. It can thus be surmized that there is a slight stereoelectronic preference for the pseudoaxial oxyanionic bond in the chairlike transition states for the rearrangement of substrates without steric constraints. Substitution at C5 of the basic 1,5-hexadien-3-ol framework of substrates, however, leads to 1,3-diaxial steric interaction in the chairlike transition states with pseudoaxial oxyanionic bond, and pseudoequatorial disposition of oxyanionic bond becomes more favorable.