Stereoselective Synthesis of New Chlorophyll a Related Antioxidants Isolated from Marine Metabolites
Ma, L. and David Dolphin
J. Org. Chem.
A new class of natural antioxidants, chlorophyll a related chlorins, have been synthesized from a chlorophyll a degradation product, pheophorbide a methyl ester. Claisen-type intramolecular condensation of pyropheophorbide a methyl ester afforded the common intermediate enol. Chlorin and enol 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 stereoselectively and predominantly forms hydroxy lactone (S). Periodate oxidation of the α-hydroxy 1,3-diketone (R) and/or (S) to furnish hydroxy lactone (R) and diketone was found out to be regioselective, and the site of reaction depends on the appropriate choice of reaction media. 1H NMR spectra have provided information on the absolute configuration of diastereomers at the C-132 or C-151 position.