Abstract
We investigated π-facial selectivity in the Diels-Alder reaction of cross-conjugated ketones bearing a spiro-ring (spiro-lactone 1, spiro-ether 2, spiro-ketone 3) with dienes. Treatment of spiro lactone 1 with 2,3-dimethyl-1,3-butadiene and 1,3-cyclohexadiene afforded only single adducts of which was endo for the latter. The products resulted from attack of the dienes to the face bearing the oxygen atom of the oxa-spiro-ring. Treatment of dienones 1-3 with cyclopentadiene afforded a mixture of two endo-adducts. The Diels-Alder reaction of dienones 1,2 with cyclopentadiene in CH2Cl2, CH3CN, CF3CH2OH afforded preferentially the adducts which resulted from reaction of the face of the dienophile bearing the spiro ring oxygen atom(6.5:1-29:1). The Diels-Alder reaction of dienones 1,2 with cyclopentadiene was accelerated in CF3CH2OH. When CH3CN was used as a solvent, the facial selectivity in the Diels-Alder reaction of dienones 1,2 with cyclopentadiene slightly decreased. Both yield and π-facial selectivity in the Diels-Alder reaction of spiro-ketone 3 was very low relative to that of the reactions of dienones 1,2. Theoretical calculations imply that the dipole moment of the transition states and products, and orbital perturbations explained in terms of the Cieplak model may all contribute to the observed selectivities.
