Stereochemistry of protonation at C(1) of nitronate adducts from 1,6-conjugate addition of Grignard reagents to 2-methoxy-1-nitronaphthalene

Abstract

C(1) protonation of 4-alkyl-2-methoxy-1,4-dihydronaphthalene-1-nitronate anions (7) gives a mixture of trans-(9) and cis-4-alkyl-2-methoxy-1-nitro-1,4-dihydronaphthalenes (8) with the latter isomer in excess. Isomerization experiments indicate that the cis compound is also the more stable isomer. Formation of the more stable isomer under kinetically controlled conditions confirms that the stereochemistry of the reaction is governed by the steric hindrance offered by the axial alkyl group to proton attack in the axial direction in a transition state whose geometry reflects a boat conformation for (7).