a-MrltratnatinI~1~~~~~_)-2,4di~yl-5sthyla,8dioxabicyclo I3.2.lloctanal was syntheszzed from @)-2-butene-1,4-diol monobenzyl ether by employing the Sharpless aeymmetrlc epoxldatlon and the regloeelective epoxide cleavage with Me3Al as the key-steps.
The smaller European elm bark beetle, Scolytus multfstriatus Marsham is the principal vector of Dutch elm disease in Europe and in the United States. Xts aggregation pheromone was characterized as a mixture of (12,2&4g,5$)-(-)-a-multistriatin A(=l),
(32,4g)-(-)-4methyl-3-heptanol B and (-)-a-cubebene C by Silverstein and his co-workers.' The structure of (-)-a-multistriatin as depicted in 1 was confirmed by a number of syntheses of its racemate2r3 as well as the correct enantiomer. The existing eight different syntheses4-" of optically active a-multistriatin 1 started from D-glyceraldehyde acetonide(Wori5), (+citronelloL (Kocienski6), D-glucose(Weiler' and Fraser-Reid8), (S)malic acid (Larcheveque" ) and optically active intermediates obtained either by resolution4c9 or by asymmetric epoxidation.1°
Although the chiral syntheses of 1 served to establish the absolute configuration of the natural a-multistriatin, only very few of them were successful in affording the enantfomerically pure (-)-1. The syntheses reported by Weiler7 and Larchevegue" were successful enough to furnish the pure enantiomer of 1. Herein we report a new synthesis of the enantiomerically pure (-)-1 by employing the Sharpless asymmetric epoxidation12 of 2 to 3a and the regioselective ring-cleavage of the resulting epoxy alcohol 3a with Me3A113-75 to give 4a as the major product. This epoxidation-methylation technique was previously employed by us in the synthesis of (3g,4g)-(-)-B16 and also in the synthesis of brassinolide, a steroidal plant-growth promotor. @)-2-Butene-1,4-dial monobenzyl ether 2 was chosen as our starting material. Asymmetric epoxidation of 2, using (-)-diisopropyl D-tartrate, &-BuOOH and Ti(k-Pr0)4, gave in 86% yield the optically active epoxy alcohol (2!&3S)-3a, which was previously prepared by a multi-step synthesis from D-tartaric acid.18 As the specific rotation of our 3a revealed it to be of ca. 89% e.e., we converted it to the corresponding 3,5dinftrobenzoate 3b. Recrystallization of 3b afforded pure 3b in 73% yield from the crude 3a. Alkaline hydrolysis of the purified 3b gave pure 3a, whose enantiomeric purity was estimated to be ca. 100% by the 'H NMRanalysis at 400 MHz of the corresponding (II)-and (~)-u-methoxy-a-trifluoromethylphenylacetate(MTPA ester).19