Date of Award

1983

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Chemistry

Abstract

The isolation and structure determination of seven new cis-1(10), cis-4-germacranolides, longicornin A-D, from Melampodium longicorne, 9-desacetoxymelcanthin F from Polymnia maculata, and 9-desacetoxymelcanthin A and 9-desacetylmelcanthin B from M. leucanthum is described. The structure, configuration and conformation of the new compounds were determined by chemical transformations, and spectral correlation using NMR, MS and CD. The single crystal X-ray diffraction of longicornin A represented the first crystal structure of a germacra-cis-1(10), cis-4-dien-trans-6,12-olide. Sodium borohydride reduction of 2(beta)-hydroxy-cis,cis-germacranolides formed rearrangement products under loss of the ester function at C-9 and double bond migration from 1(10) to the 9(10) position, via S(,N)2' reactions involving neighboring group participation of the C-2-OH. Reduction of 9-desacetylmelcanthin B (70) with sodium borohydride afforded the 1(10), 11(13)-tetrahydro derivative. 9-Desacetylmelcanthin B does not give the reduction rearrangement due to the poor leaving ability of the hydroxyl group at C-9 in 70. The reaction of 9-desacetoxymelcanthin F (52) with sodium methoxide produced a 13-alkoxyleucantholide (56) and a 13-alkoxygermacranolide (55) which were also obtained from the reaction of melampodin D with sodium borohydride followed by sodium methoxide treatment. The formation of the 13-alkoxygermacranolide from melampodin D represents the first conversion of a leucantholide into a cis,cis-germacranolide. The conversion of melcanthin A, a cis,cis-germacranolide, to a leucantholide is described. In the rearrangement reactions of leucantholides involving the 9(10)-double bond two mechanisms are postulated, the carbanionic and the S(,N)2', depending on the leaving group present at C-1. For those cases where a poor leaving group is present, for example the ethoxy group in cinerenin, the carbanionic mechanism seems to be more favored. The S(,N)2' route predominates with compounds containing acyloxy leaving groups. The structure determination of four new melampolides, melampodinin B, melampodinin C, 9-desacetylmelampodinin A, and 11,13-dihydromelampodin A-9{2-methylbutanoate}, and the germacrolide 15-desacetoxymelfusin which were isolated from Melampodium americanum, are reported. The configuration and conformation of the chiral centers in melampodinin B were obtained by correlation with the known melampodinin A. . . . (Author's abstract exceeds stipulated maximum length. Discontinued here with permission of author.) UMI.

Pages

236

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