LS2 9JT1 -Ethynyl-2-methylcyclohex-l -ene, hitherto obtained only as a mixture with one of its double-bond isomers, is obtained pure by dehydrohalogenation of o-halogenodienes such as 1 -(2-chlorovinyl) -2-methylcyclohexene (6 : X = CI). The optically active 5-hydroxy-2-methylcyclohex-I -enecarbaldehyde (1 0) reacts with chloromethylenetriphenylphosphorane giving mixed cis-and trans-w-chloro-dienes [ c f (1 1 )], dehydrohalogenation of which gives the title compound (1 2). required for the synthesis of precalciferol,.FOR use in the synthesis3 of precalciferol, we required the title compound (l2),f which was destined to provide ring A and carbon atoms 6 and 7 of the previtamin. Experiments on its preparation are now reported.The parent compound 1-ethynyl-2-methylcyclohexl-ene (3) has been prepared several times in the past,4 but always in admixture with one of its double-bond isomers. Thus, by dehydration of the appropriate isomer rac-(1) t of 1-ethynyl-2-methylcyclohexanol with phosphoryl chloride and pyridine, a mixture is obtained containing ca. 75% of the desired compound (3) and 25% of the isomer mc-(2). We have separated these two isomers by preparative g.1.c. The isomer (3), A , , 230 nm. (E 11,500), in its n.m.r. spectrum, showed no vinyl proton signal but singlets corresponding to one ethynyl proton (7 7.0) and three allylic methyl protons ( 7 8-12>. In contrast, the isomer rac-( 2) showed signals for one vinyl proton (7 3.9 multiplet), one ethynyl proton (7 7.33, singlet), and three methyl protons (7 8-85, doublet, J 6.5 Hz).As this method is not suitable for preparing hydroxyderivatives of the enyne (3), other routes which might lead to the homogeneous parent compound were explored. The unconjugated isomer mc-(5) was obtained by treatment of 6-chloro-l-methylcyclohexene rac-(4) with ethynylmagnesium bromide and copper(1) chloride 6 in tetrahydrofuran. Treatment of compound rac-(5) with sodamide in liquid ammonia moved the double bond into conjugation with the ethynyl group, and gave in high yield the enyne (3); it was free (g.1.c.; n.m.r. spectrum) from the starting material. In further experiments of this kind, however, formation of conjugated material was incomplete, although extensive, and although we were later able to repeat the original successful experiment, the conditions necessary could not be defined.t Structural formulae in this paper represent absolute configurations. The prefix YUC denotes a racemate; thus rac-(1) means the racemate corresponding to the structure (1).