Abstract
The detailed studies on the stereochemistry of the Diels-Alder adducts of 2, 5-dialkyl-cyclohex-2-en-1-one with butadiene in the presence of Lewis acid were made. The experimental results indicated that addition of butadiene to the dienophile took place stereoselectively opposite to the 5-alkyl side. This fact is in conflicting with the former results [see ref.4)] that addition of butadiene in the Diels-Alder reaction of carvone with butadiene occurred predominantly from the same side as that of the 5-isopropenyl group. The reported reaction, therefore, was re-investigated. It was concluded from the results of experiments that addition of butadiene took place predominantly opposite to the 5-isopropenyl side in a similar manner as our deduction. Consequently, the reported stereostructures [see ref.4)] should be revised in such a way that the structure (3b) for the major product is replaced by the formula (3a) and the structure (3a) for the minor product by the formula (3b). The reaction condition was also examined and the yield of the major product (3a) was much improved in the presence of 0.11 eq AlCl3 to the dienophile at room temperature under vigorous stirring.
