α-Alkylation of the Diels–Alder cycloadducts of the thioaldehyde, ethyl thioxoacetate

Abstract

The cycloadduct 1a of anthracene and the labile thioaldehyde, ethyl thioxoacetate 2a, has been converted into the α-lithio derivative 1b with lithium diisopropylamide (LDA). Subsequent treatment with, separately, methyl iodide, ethyl iodide, allyl bromide and benzyl bromide gave the corresponding 12-‘alkyl’ derivatives 1c–f. Each derivative, when heated in toluene at 111 °C, dissociates to liberate anthracene and a thioketone 2, which can be trapped in situ with 2,3-dimethylbuta-1,3-diene 3 to afford the corresponding 2-substituted dihydrothiine 4 in good yield. The same set of products 4 was obtained directly by α-alkylation of the parent dihydrothiine 4a. Similarly, the 12-methyl anthracene adduct 1c gives, with cyclopentadiene and cyclohexadiene, the stereoisomeric cycloadducts 5 and 6, and 7 and 8 of the cyclic dienes and ethyl 2-thioxopropionate 2c. When treated with LDA and methyl iodide, the thioaldehyde adducts 9a and 13a of cyclopentadiene and cyclohexadiene undergo rearrangement and S-methylation, rather than C-methylation, and afford the cyclopropanecarboxylates 10a and 14a. The methyl and ethyl sulphonium salts 17a–b of the parent dihydrothiine 4a rearrange similarly to give the cyclopropanecarboxylates 19a–b. Methylation of the dilithio derivatives of the cycloadducts of thioxoacetic acid has also been investigated. The cyclopropanecarboxylic acid 14b was found to rearrange slowly in the crystalline state to give the epimeric γ-lactones 15 and 16.