Reactions of 3-nitro- and 3-bromo-3-nitroacrylates with 1,3-cyclohexadiene

Abstract

Nitroacrylates and their derivatives are known to be active dienophiles in the Diels–Alder reaction with acyclic [1, 2] and cyclic {cyclopentadiene [3, 4], furan [5, 6], β-furylnitroethene [7], β-(2-nitrovinyl)indole [8]} 1,3-dienes. In this work we investigate the reaction of the nitroand gem-bromonitroacrylates I, II with 1,3-cyclohexadiene. The study of these reactions is of interest for the development of convenient methods of the synthesis of functionalized bicycleoctenes, which are important intermediates in the synthesis of fragments of the natural compounds (hormones, vitamin D3, etc.) [1, 9]. Cyclopentadiene is known to react with nitroacrylate I at 0°C to give a mixture of endoand exodiastereomers of the corresponding nitronorbornene in a quantitative yield [3, 4]. In contrast to cyclopentadiene the reaction of 1,3-cyclohexadiene occurs in more rigid conditions (boiling the reaction mixture in benzene for 36 h) and results in a mixture of endoand exo-isomers of ethyl 3-nitrobicyclo[2.2.2]-5-octen2-ylcarboxylate IIIa, IIIb in 87% yield. The endoisomer IIIa was isolated in pure form by the repeated chromatography of the bicyclooctene isomers mixture. Bromonitroacrylate II reacts with cyclohexadiene under similar conditions (benzene, 80°C, 40 h) to afford a mixture of the difficultly separable products, from which by the column chromatography the endoIVa and exo-IVb isomers of ethyl 3-bromo-3-nitrobicyclo[2.2.2]-5-octen-2-ylcarboxylate in the ratio of 6:1 with an overall yield of 45%, and the products of DOI: 10.1134/S1070363212120213