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Stereoselective synthesis of new chlorophyll A related antioxidants isolated from marine organisms

TitleStereoselective synthesis of new chlorophyll A related antioxidants isolated from marine organisms
Publication TypeJournal Article
Year of Publication1996
AuthorsMa, LF, Dolphin, D
JournalJournal of Organic Chemistry
Volume61
Pagination2501-2510
Date PublishedApr
Type of ArticleArticle
ISBN Number0022-3263
Keywords2-SULFONYLOXAZIRIDINES, CHEMISTRY, COMPOUND, DERIVATIVES, enol, HYDROXYLATION, ORIGINS, RCI, RUDITAPES-PHILIPPINARUM
Abstract

A new class of natural antioxidants, chlorophyll a related chlorins 3, 4(S), 4(R), 5(R), 6, 7, 8, and 9, have been synthesized from a chlorophyll a degradation product, pheophorbide a methyl ester (1). Claisen-type intramolecular condensation of pyropheophorbide a methyl ester (2) afforded the common intermediate enol 3. Chlorin 1 and enol 3 have a propensity to undergo exocyclic ring opening by ionic bases. The organic base DBU was found to be an efficient reagent for promoting the asymmetric hydroxylation of these chlorins, using N-sulfonyloxaziridines, without cleavage of the exocyclic rings. Model studies for hydroxylactonization have shown that periodate oxidation of hydroxy ketone 10 stereoselectively and predominantly forms hydroxy lactone 17(S). Periodate oxidation of alpha-hydroxy 1,3-diketone 4(R) and/or 4(S) to furnish hydroxy lactone 5(R) and diketone 7 was found out to be regioselective, and the site of reaction depends on the appropriate choice of reaction media. H-1 NMR spectra have provided information on the absolute configuration of diastereomers at the C-13(2) or C-15(1) position.

URL<Go to ISI>://A1996UD65300039