Sesquiterpenes from Dictyopteris divaricata. I

Abstract

In 1951, Takaoka and Ando1) isolated a sesquiterpene alcohol from Dictyopteris divari-cata, Okamura, and they suggested that it was a new type of cadinol. Recently acquired knowledge about the constitution and stereo-chemistry of sesquiterpenes2) has prompted us to reexamine the sesquiterpenes of the brown alga. The methanol extracts of the air-dried seaweed , Dictyopteris divaricata3) (92 kg.), were concentrated to a small volume in vacuo, and the residual light brown oil was then distilled with steam. The ether extracts of the distillate were washed with 5% aqueous potassium hydroxide and then by 1 N hydrochloric acid. The neutral essential oil (790g.)4) thus obtained was subjected to fractional distillation in vacuo ; each fraction was chromatographed on alumina or silica gel, the homogeneity was-tested by gas chromatography and thin-layer chromatography, and six components were isolated in a pure state. Four sesquiterpenes among them were identified as (-)-copaene,5) (-)-ƒÁ1-cadinene,6) cadalene and(-)-ƒÂ-cadinol7)by a comparison of their infrared absorption spectra and other physical constants with those the corresponding sesquiterpenes given by Sorm.8) The alcohol was also shown to be identical with that isolated by Takaoka and Ando.1 A new hydrocarbon (b. p.97•Ž/5 mmHg, [ƒ¿]23D •{12.8•‹)showed an infrared spectrum which was superimposable on that of(-)-ƒÀ-ele-mene.8)The NMR spectrum showed the presence of one quaternary methyl(ƒÑ=9.07, s),two methyls attached to the double bond(ƒÑ=8.38, ss), one vinyl group (ƒÑ=4.45, q;5.35, d;5.43, d) and four olefinic protons (ƒÑ=5.45•`5.65). These data have shown that the hydrocarbon is the optical antipode of(-)-ƒÀ-elemene. This