The preparation of Tricyclo[4,2,1,0^2,5]nona-3,7-dienes by cycloadditions onto 3,4-disubstituted cyclobutenes. Specificities in the Diels-Alder reaction

Abstract

The reaction between cis-3,4-dichlorocyclobutene and 2,5-dimethyl-3,4- diphenylcyclopenta-2,4-dienone forms a mixture of exo,anti (28) and endo,anti adducts (27) with the former predominating. At higher temperatures the endo,anti adduct (27) undergoes smooth decarbonylation to form cis-7,8-dicldoro-2,5-dimethyl-3,4-diphenylbicyclo[4,2,0]octa- 2,4-diene. The stereochemistry of each adduct was determined by a combination of spectral (especially p.m.r.) techniques and chemical trans-formations. Intramolecular nucleophilic displacements and electrophilically induced cyclizations have been instrumental in determining stereochemical relationships. Similar cycloadditions involving 1,2,3,4-tetrachloro-5,5-dimethoxycyclopenta-1,3-diene and 1,4-dimethyl-2,3-diphenylcyclopenta-1,3-diene as the diene components and cis-3,4-dichlorocyclobutene or dimethyl tricyclo[4,2,2,0^2,5]deca- 3,7,9-triene-7,8-dicarboxylate (10) as dienophiles have been studied and the stereoselectivities determined. A combination of steric and secondary orbital effects have been proposed to account for the observed stereospecificities1. A PMO-based treatment is used to account for the perispecificity of (10) and related alicyclic polyenes.