Synthesis of several bisabolane sesquiterpenoids from xanthorrhizol isolated from C. Xanthorrhiza

Abstract

Xanthorrhizol was isolated from the essential oil of fresh rhizomes of C. xanthorrhiza in 20.2% yield by fractionation using vacuum liquid chromatography. Several bisabolane-type sesquiterpenoids have been synthesised from this xanthorrhizol. Both diastereomers of 10,11-dihydro-10,11-dihydroxyxanthorrhizols, sesquiterpenoids isolated from the Mexican medicinal plant, Iostephane heterophylla, have been prepared in three steps from xanthorrhizol via Sharpless asymmetric dihydroxylation as the key steps. Fremy’s salt oxidation of xanthorrhizol gave curcuhydroquinone in 60% yield, which was successfully reduced with sodium dithionite to curcuhydroquinone in 100% yield. Sequential acetylation and Sharpless asymmetric dihydroxylation on curcuhydroquinone led to the diacetate derivative of helibisabonol A. Cleavage of the diacetate esters by reduction with lithium borohydride furnished helibisabonol A, an allelopathic agent isolated from Helianthus annuus (sunflowers). The unexpected difficulty in deprotection of helibisabonol A diacetate was due to acidic, basic and air-sensitive natures of helibisabonol A. An allylic alcohol derivative of O-methylxanthorrhizol, (3S,6R)-(3- methoxy-4-methylphenyl)-2-methylhept-1-en-3-ol, has been synthesised from xanthorrhizol in five steps via Sharpless asymmetric dihydroxylation as the key steps. Sharpless asymmetric dihydroxylation in all syntheses gave excellent enantioselectivity (ee > 98%). The enantiomeric excess and the absolute configuration of the diol was determined by the modified Mosher’s method.