Synthetic and Structural Studies on Natural Coumarins

In the first part of the thesis, synthetic approaches to a number of natural coumarins are described. Oxidative cyclisation reactions of the prenylated phenolic coumarins osthenol and 7-demethylsuberosin are discussed. It has been found that, by varying the reaction conditions, cyclisation con be specifically directed to give either furanocoumarins or pyranocoumarins. In particular, the natural coumarins (+/-)-marmesin, (+/-)-columbianstin, (+/-)-decursinol and (+/-)-Iomatin have each been synthesised in high yield. The regioselective dehydration of (+/-)-columbianetin to give the naturally occurring isopropenyldihydrofurano-coumarin, masquin, has been investigated. Conditions for the regioselective dehydration of the linear hydroxyiso-propyldihydrofuranocoumarin, (+/-)-marmesin, to the previously unknown isopropenyldihydrofuranocoumarin have been determined. The corresponding reaction with the angular hydroxy-isopropyldihydrofuranocoumarin, (+/-)-columbianatin, has given synthetic (+/-)-masquin, which could not be separated from the isomeric isopropylfurocoumarin. Selenium dioxide oxidation of osthenol acetate has been shown to result in exclusive oxidation of the allylic methyl group. In the second section of the thesis, chemical evidence is presented which unequivocally establishes the structure of tomentin, the major aglycone of the wood of Prunus tomentosa, as 5-hydroxy-6,7-dimethoxycoumarin. Attempts were made to introduce a 1,1-dimethylalIyl group at C-8 of tomentin with a view to obtaining a direct correlation with nieshoutol, the sternutatory constituent of Ptaeroxylon obliquum. The key synthetic precursor, O-1,1-dimethyl-allyltomentin, was found to undergo a novel charge-induoed ortho-Claisen rearrangement, at R. T., specifically to C-6. This blocked ortho-dienone failed to undergo a para-Claisen rearrangement on heating, while on reduction followed by aromatisation, 6,7-dimethoxycoumarin was obtained. The complex mixture resulting from 1,1-dimethylpropargylation of tomentin has been separated and structures assigned to all of the principle components. The structures of two novel, isomeric, blocked ortho-dienones, each containing an alpha-3,3-dimethylallenyl group, have been rigorously secured on the basis of spectroscopic evidence and the reduction and aromatisation of one of them to the known natural coumarin, alloxanthoxyletin