The condensation of 9-anthraldehyde with various monosubstituted ethylenes as dienophiles in the Diels-Alder reaction gave only vicinally substituted adductsj2 while 9-~ yanoanthracene~ and the methyl acetal of 9-anthraldehyde4 gave in some cases both the vicinal and non-vicinal adducts. In only one case did the non-vicinal 9, 11-disubstituted-9, 1O-dihydro-9, lo-ethanoanthracene predominate over the vicinal 9, 12-disubstituted ...