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-substituted 2-acylcyclohexane-1,3-diones proceeds diastereoselectively yielding a 9R,16R:9S,16S pair of enantiomers rather than a possible mixture of four 8-aza-D-homogona-12,17a-dione stereoisomers; this stereoselectivity results from the impossibility of a threo-attack on the prochiral β,β’-triketone by azomethyne owing to the spatial structure of 2-acylcyclohexane-1,3-dione, on the one hand, and by the steric effect of the C(1) methyl group of 3,4-dihydroisoquinoline, on the other.