Stereoselectivity of the annelation of 3,4-dihydroisoquinolines by 5-monosubstituted 2-acylcyclohexane-1,3-diones

The annelation of 1-methyl-3,4-dihydroisoquinoline by prochiral 5-substitut ed 2-acylcyclohexane-1,3-diones proceeds diastereoselectively yielding a 9R ,16R:9S,16S pair of enantiomers rather than a possible mixture of four 8-az a-D-homogona-12,17a-dione stereoisomers; this stereoselectivity results fro m the impossibility of a three-attack on the prochiral beta,beta'-triketone by azomethyne owing to the spatial structure of 2-acylcyclohexane-1,3-dion e, on the one hand, and by the steric effect of the C(1) methyl group of 3, 4-dihydroisoquinoline, on the other.