Syntheses of 2- and 3-Isopropyl-1-methylnaphthalenes from Cyclic β- and γ-Keto Esters

Abstract

3, 4-Dihydro-3-methoxycarbonyl-1 (2H)-naphthalenone [1] and 3, 4-dihydro-2-ethoxycar-bony1-1(2H)-naphthalenone [9a] were allowed to react with methylmagnesium iodide, and from their products 1, 3- or 1, 2-dialkylnaphthalenes and three naphthothiophene derivatives were synthesized. The reaction of the cyclic γ-keto ester [1] with methylmagnesium iodide yielded a mixture of the expected diol [2], keto alcohol [3], and 1, 2, 3, 4-tetrahydro-1, 1-dimethylnaphthalene-3-carboxylic acid [4a]. By heating [2] with sulfur, 3-isopropyl-1-methylnaphthalene [7a] was obtained. From [6], the product of the Grignard reaction with ester of [4a] by dehydrating and successive heating with sulfur, [7a] and 4, 5-dihydro-3, 5, 5-trimethylnaphtho[1, 2-b] thiophene [8] were obtained. By dehydration of [3] with formic acid, 3-isopropyl-1-naphthol [7b] was obtained. On the other hand, the cyclic β-keto ester [9a] reacted with Grignard reagent to yield the desired keto alcohol [10]. When it was dehydrated, α, β-unsaturated ketone [11a] was obtained. 2-Isopropyl-1-methylnaphthalene [7], 2-t-butyl-1-methylnaphthalene [7e], and 3, 3-dirnethyl-1, 3-dihydronaphtho[1, 2-c]thiophene [14] were obtained from [11a]. The melting point of picrate of [7a] was 105.5-106.5°C, but that shown in the literature is 84°C. In order to identify the hydrocarbon [7a], it was also synthesized from 4-methyl derivative of [9a], and in this synthesis 3, 5-dimethylnaphtho[1, 2-b]thiophene [15] was obtained as a by-product.