Dimethyl cis- and trans-1,2-Dimethylcyclohexane-1,2-dicarboxylate

Abstract

1-Methoxy-3-trimethylsilyloxybuta-1,3-diene adds readily to dimethylmaleic anhydride, and the product can be converted into dimethyl cis-1,2-dimethylcyclohex-4-ene-1,2-dicarboxylate (3). The several steps required mean that this route compares unfavourably with direct addition of butadiene to the anhydride. The best route to the cis-diester (3) is cyclialkylation of the dianion from dimethyl 2,3-dimethylbutanedioate with (Z)-1,4-dichlorobut-2-ene. A bismethylation reaction is used to prepare imide (II), which can be converted into the cis-diester (1) only with considerable difficulty, and bismethylation of the dianion formed from dimethyl cis-cyclohex-4-ene-1,2- dicarboxylate gives mainly the trans-diester (8). 13C n.m.r. spectra were useful in distinguishing the isomeric pairs (1) and (2), and (3) and (8).