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Studies related to the synthesis of guaiane-type sesquiterpenes Cheng, Kin-Fai

Abstract

An efficient, 8-step synthesis of (+)-∝-cyperone 168 from (-)-∝-santonin 107 is described. Epimerization of 107 followed by hydrogenolysis of the resulting product 174 gave the acid 175, which was esterified by treatment with diazomethane. Conversion of the resulting keto ester 176 into the substituted octalone 184 was achieved by hydrogenation of the former in the presence of the homogeneous catalyst tris(triphenylphosphine)-chlororhodium. Lithium aluminum hydride reduction of 184, followed by oxidation of the product with 2,3-dichloro-5,6-dicyanobenzoquinone gave keto alcohol 189. Pyrolysis of the corresponding keto carbonate 191 afforded (+)-∝-cyperone 168 in good yield. Photochemical rearrangement of a number of cross-conjugated dienones (192, 193, 200, and 176) into hydroguaiazulene derivatives (194, 195, 195, and 221, respectively) by the irradiation of the former compounds in 45% aqueous acetic acid is also described. Conversion of 194 into 5-epi-∝-bulnesene 216 was achieved by the following sequence. Birch reduction of 194, followed by chromium trioxide-pyridine oxidation of the resulting product afforded ketone 201. The stereochemistry of the latter was established by means of optical rotatory dispersion (o.r.d.). Wolff- , Kishner reduction of 201 and subsequent dehydration gave 5-epi-∝-bulnesene 216. In a similar reaction sequence, 195 was converted into 4-epi-∝-bulnesene 218. Conversion of compound 221 into ∝-bulnesene 7 was accomplished, according to the following scheme. Stereoselective catalytic hydrogenation of the acetate derivative of 221 gave 223 as the only product. The stereochemistry of the latter was established by an o.r.d. study and by chemical evidence. Sodium borohydride reduction of 223, followed by tosylation of the resulting product, gave a mixture of the olefin 230 and the tosylate 229. Hydrogenation of the former in the presence of the homogeneous catalyst, tris(triphenylphosphine)chlororhodium, followed by lithium aluminum hydride reduction, gave the diol 235. The same diol was also obtained by lithium aluminum hydride reduction of the tosylate 229. Successive treatment of the diol 233 with methyl chloroformate and thionyl chloride in pyridine afforded the monocarbonate 235. Pyrolysis of 235 gave ∝-bulnesene 7. The described stereoselective synthesis of the sesquiterpene ∝-bulnesene 7 fully corroborates the structure and stereochemistry assigned to this compound and indicates a general approach to the synthesis of guaiane-type sesquiterpenes.

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