INTRAMOLECULAR DIELS-ALDER REACTION OF CYCLENIC TRIENES - STEREOSELECTIVITY AND NMR STRUCTURE DETERMINATION

A series of trienes possessing an internally cyclenic dienophilic grou p undergo thermal intramolecular Diels-Alder (IMDA) reaction with high selectivity for the cis-fused products. A concentrated solution of Li ClO4 in diethyl ether catalyzes the IMDA reaction of cyclenic nitrotri enes, giving rise to the trans-fused compounds. The stereochemical out come of these various processes are rationalized in terms of a minimiz ation of the steric interactions between the ring and the chain on the one hand and the endo-stabilization from the nitro group on the other . The structures of the cycloadducts have been carefully determined by NMR H-1 and C-13 spectroscopy: dipolar interactions, detected via nuc lear Overhauser effects, and criteria based on scalar coupling and mor eover on chemical shifts have been employed. Conformational preference s were observed.