Reactions of cyclohexadienes. Part XII. Some dienamines and dimethyl acetylenedicarboxylate

Abstract

Reaction of some N-(cyclohexa-I ,3-dienyl) morpholine derivatives with dimethyl acetylenedicarboxylate gives rise initially to cyclobutene derivatives by reaction with the enamine double bond. Whether these adducts are stable, or convertible into the isomeric cyclo-octatrienes, depends on the presence and nature of a substituent a t the 4-position of the cyclohexadiene. Attempted dehydrogenation of dimethyl 6-morpholinobicyclo [4,2,0]octa-2,7-diene-7,8-dicarboxylate ( 2 ; R = H) led surprisingly to dimethyl 3-morpholinophthalate.THE reaction of an enamine with an acetylene usually proceeds through an initial cyclobutene which may then rearrange, often spontaneously, to form products whose structures depend on the starting enamine and the reaction conditions.2 Adducts from an enamine derived from a cyclic ketone often rearrange by ring expansion to the dienamine having two additional carbon atoms in the ring.3-5 Bis-homo-steroids have been prepared in this way.6 An example of the reaction with an activated enol ether has also been noted.'It was anticipated therefore that the cyclohexadienamines now readily available would react with dimethyl acetylenedicarboxylate in like manner, to give, after ring expansion, cyclo-octatriene derivatives. The extra double bond should then facilitate dehydrogenation of such compounds to cyclo-octatetraenes with a variety of substituents around the ring.Reaction of N-(4-methylcyclohexa-1,3-dienyl)morpholine with dimethyl acetylenedicarboxylate gave dimethyl 5-methyl-8-morpholinocyclo-octa-2,4,8-triene-l,2-dicarboxylate (1; R = morpholino), in high yield. 1.r. absorptions at 1724-1710 and 1645 cm-l, indicated the presence of an unsaturated ester function, which was confirmed by U.V. absorption at 226.5 nm. In the lH n.m.r. spectrum the vinyl methyl protons resonated at 6 1.82, and the methyl ester signals were at 6 3.64 and 3.73 p.p.m. (the upfield signal was assigned to the C-1 ester group, by analogy with similar compounds 3). The two olefinic protons resonated as doublets, J 5 Hz, at