Steric Factors in Amide-Directed Metalations of N,N-Dialkyl-6-methoxynaphthalene-2-carboxamides: Synthesis of a Sterically Perturbed Acylnaphthol

The powerful ortho-metalating directive effect of the N,N- dialkylcarboxamide group can be used for the preparation of C-1 alkyl-substituted 2,6-acylnaphthols. The alternative reaction pathways of C-1 metalation vs. acylation (carbonyl addition) in the reaction of alkyllithium reagents with N,N-dialkyl-6-methoxy-naphthalenecarboxamides depends upon the cumulative steric nature of the N-alkyl and lithium alkyl groups: The N,N-dimethylamide 7 undergoes carbonyl addition with n-BuLi and t-BuLi; the N,N-diethylamide 8, carbonyl addition with n-BuLi, but metalation with t-BuLi; and the N,N-diisopropylamide 9 only C-1 metalation with n-BuLi. Subsequent reaction of the N,N-diethyl-1-ethyl-6-rnethoxynaphthalene-2-carboxamide (13) with n-BuLi gives the desired 2,6-acylnaphthyl methyl ether, whereas the corresponding ethylated diisopropylamide 14 undergoes additional metalation on the ethyl group. These 1-ethyl-2-carboxamidonaphthalenes are very sterically crowded and show evidence in the proton NMR of hindered rotation about both the amide bond and the ethyl group. The UV and fluorescence spectra of the acylnaphthols 17 and 18 show the consequences of this steric crowding through reduced conjugation between the acyl group and the naphthalene group. The more hindered 1-ethylated acylnaphthol 18 shows lower molar absorptivity, and it fluoresces only in basic solution.