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The use of camphor in natural product synthesis

University of British Columbia

(+)-9,10-Dibromocamphor 3̲7̲, prepared in three steps from (+)-3-e̲n̲d̲o̲-bromocamphor 1̲5̲a̲, was found to undergo facile ring cleavage to provide the cyclopentanoid ring systems 1̲5̲8̲, 1̲5̲9̲ and 1̲6̲1̲. The bromoacid 1̲5̲9̲ was readily lactonised to provide 1̲6̲0̲ in high yield. The hydroxyacid 1̲6̲1̲ was converted into the hydrindenone 1̲9̲0̲ in three steps and a further six steps were required to complete the total enantiospecific synthesis of (-)-estrone e̲n̲t̲4̲1̲. Studies directed toward the synthesis of vitamin D₃ (2̲1̲3̲) and metabolites have shown that diastereoselective alkylation of lactone 1̲6̲0̲ and ester 2̲8̲3̲ (derived from 1̲6̲1̲) can be accomplished in high yield and with almost complete stereoselectivity. As a result, diol 3̲2̲2̲, representing the structural sub-unit of ring D and side chain of vitamin D₃, has been synthesised. Ring cleavage of the bromoketone 3̲5̲0̲ (derived from 1̲5̲9̲) gave 3̲5̲2̲ which was transformed into the aldehyde 3̲3̲2̲ and the trienols 3̲4̲0̲a̲ and 3̲4̲0̲b̲ to complete a formal synthesis of 3̲2̲7̲a̲, one of the components of the California Red Scale pheromone. Methylation of camphor 1̲0̲ yielded the 3-e̲x̲o̲-methyI derivative 3̲6̲2̲b̲ as the major product. The thermodynamically most stable epimer was found to be 3-e̲n̲d̲o̲-methylcamphor 3̲6̲2̲a̲. In contrast, 3-methylcamphor 3̲6̲2̲a.̲b̲ undergoes preferential endo alkylation. The factors governing these results are discussed. (+)-3- e̲n̲d̲o̲-Bromocamphor 1̲5̲a̲ and (+)-3- e̲n̲d̲o̲-9-dibromocamphor 1̲8̲a̲ were found to rearrange to provide (-)-6- e̲n̲d̲o̲-bromocamphor 1̲7̲2̲ and (-)-6- e̲n̲d̲o̲-9-dibromocamphor 2̲6̲. Dehalogenation of 1̲7̲2̲ provided optically pure (-)-camphor e̲n̲t̲1̲0̲ while dehydrohalogenation gave (+)-5,6-dehydrocamphor 1̲7̲3̲.[formula omitted]

Text

2010-06-17

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http://hdl.handle.net/2429/25830

Chemistry

University of British Columbia

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