The long-known "rule of five" predicts 1,5 (crossed) cyclization for 1,5-hexadienes, and indeed there have been rather few exceptions to this behaviour reported over the past fifteen years. We have now studied the effect of structure on the regiochemistry of closure in three classes of carbonyl-substituted hexadienes --1,5-hexadien-3-ones, 1-acyl-I,5-hexadienes, and 2-acyl-1,5-hexadienes. We find that for the first two of these classes regiochemistry is a function of structure and that this can

more »

... be controlled to yield predictably regiospecific 1,5 or 1,6 closure or a combination of the two modes. The third class, 2-acyl-1,5-hexadienes, undergoes 1,5 closure regardless of these structural variations. Mechanistic implications and synthetic applications of these findings are discussed.