Two routes leading to the optically active title compound (9) are described. The first uses, as intermediates, lactonic acids, prepared from enantiomers of 1 -methylcyclohex-4-ene-l,2-dicarboxylic acids ; the absolute configurations of these enantiomers are thereby established. The second, preferred, route starts from 5-methoxy-Z-methylbenzoic acid and, proceeding via the intermediates (21), (5). and (8) affords the diol (9) in 14% overall yield. THE title compound (9),'f used for the synthesis2 of tachysterol,, was originally prepared from 2-ethoxybutadiene by way of l-methyl-4-oxocyclohexane-cis-1,Z-dicarboxylic acid, the lactonic acids (15) and (14), and the hydroxy-ester (8); the route was long, and the overall yield (ca. 0.25~0) low. In connection with recent work on the synthesis of precalciferol, considerable amounts of the diol were required, and we were obliged to seek more efficient ways of preparing it.5At first, in line with the earlier work,3 our ideas centred on the use of intermediates such as the lactonic acids (14) and (4), and methods for their preparation were explored. The readily available frauts-dicarboxylic acid rac-(1) 7 was resolved by the use of quinine, giving both the enantiomers in good yield. The (+)enantiomer which, as will become clear, has the absolute configuration (l), was converted via its anhydride into the crystalline monomethyl ester (2). The corresponding bromolactonic ester (3) reacted with triphenyltin hydride giving the methyl ester of the lactonic acid (4).The free lactonic acid was then obtained by hydrolysis with alkali followed by re-lactonisation of the product with acid. Treatment of the acid (4) with lead tetraacetate in hot benzene containing pyridine and copper(I1) acetate gave a crystalline tertiary acetate, probably (7), and an unsaturated lactone fraction which appeared to be mainly (5) mixed with up to 30% of the exocyclic double-bond isomer (6). Pyrolysis of the acetate (7) provided more of the lactone (5). Treatment of the mixture of unsaturated lactones with methanolic sodium methoxide gave the ester (8), which was then reduced to give the diol (9) in ca. 9% overall yield from the racemic dibasic acid rac-(1).When this route was planned, we anticipated the possibility of using, as well as the enantiomer (l), the ' unwanted ' enantiomer e f i f -( l ) for transformation into the required diol (9). The first step of the projected route, the epimerisation of the enantiomer e d -( l ) , via its anhydride, to give the cis-dibasic acid (lo), [a]= -4.8" (in A brief account of part of this work has appeared: T. M. Dawson, J. Dixon, B. Lythgoe, and I. A. Siddiqui, Chem. Comm., 1970, 992.
