From bergamot oil (Citrus hcrgamia KISSO), (-)-(4S, 8R)-S-epi-a-bisaboloI (2) and (-)-(4R, 8S)-4-epi-p-bisa-bolo1 (3) were isolated. The absolute configuration of their stereoisomers 4 and 5 was established by an enantioselective synthesis starting from (-)-(S)-p-mentha-l,8-dien-4-ol.
The bisabolols 1-3 are encountered relatively frequently in the sesquiterpene fraction of essential oils. (-)-(4s. SS)-a-Bisabolol (1) in a concentration of up to 45 %, is contained in the oil or German camomile (Matricaria charnornilla L.) [l] where it belongs to the antiphlogistic principle of the blossoms [2]. (+)-(4R, 8R)-a-Bisabolol 1 4s,as 2 45,817 3 4R,as (Pnr-I) was isolated from Aflanfia rnonophylln DC. [3]. Both diastereoisomcrs 1 and 2 are round in Brazilian cabreuva oil (Myrocarpus frondosus; M.,fastigiutus FK. ALLEM) [4]. The racemic equivalent of the diastereoisomers 112 has been commercialized and is used in a large number of cosmetic products. The absolute configuration of the diastereoisomers 1 and 2 has recently been determined [3]. (+)-b-Bisabolol of unknown configuration has an apple-blossom-like odor and was first isolated from the essential oil ofcotton buds; (Gossypiurn hirsutum L.); it turned out to be moderately attractive to the boll weevil [5]. Later, b-hisabolol has been identified in camphor oil [ 6 ] , corn oil [7], vanilla [XI, and olibanum oil [9].
In this paper, we describe the isolation of (-)-(4S, 8R)-X-epi-cr-bisabolol (2) and (-)-(4R, 8S)-4-epi-,9-bisaboloI (3) from bergamot oil and the absolute configuration of the stereoisomeric alcohols 4 and 5. (-)-(S)-p-Mentha-1,8-dien-4-01(6) [lo] served as key compound in the enantioselective synthesis of (+)-(4S, 8R)-8-epi-p-bisabolol (4) and (+)-(4S, 8S)-p-bisabolol (5). In the first step, 6 was converted into the 1 :1 mixture of the diastereoisomeric diols 7 and 8 by hydroformylation followed by treatment with LiAIH,. After acetylation and silylation, the diastereoisomers 9 and 10 were separated by flash chromatography [ll]. The less polar diastereoisomer 9 was converted into (-)-(R)-3-(p-toly1)-1-butanol (13) (see Exper. Part) which has been linked with (+)-ar-turmerone ( = (S)-2-methyl-6-(p-tolyl)hept-2-en-4-one) [ 121. (+)-(S)-Alcohol 14 arose from the more polar derivative 10 (see Exper. Part).
After the clear establishment of the chiral centers at C(4) and C(8) of 9 and 10, the corresponding monosilylated alcohols 11 and 12 were converted stepwise in separate experiments to (+)-(4S, 8R)-8-epi-,!l-bisabolol (4) and (+)-(4S, 8S)-/?-bisabolol (3, respectively. The spectroscopic properties of the naturally occurring alcohol 3 agree with those of its enantiomer 4.