Synthesis and spectral studies of natural chlorins having antioxidative activity

Starting from pheophorbide a methyl ester (l), a degradation product of chlorophyll a, novel natural antioxidants of chlorophyll a related chlorins 3, 5(S), S(R), 8(R), 9-13 have been synthesized in high yields. Key steps involve formation of 5(R) and 5(S) via a DBU-promoted asymmetric hydroxylation of 3, and regioselective periodate oxidations of 5 to 8(R) and 9. Since photosynthetic organisms began releasing oxygen into the primitive atmosphere 3.5 Won years ago, al l living things faced the challenges of a new chemical process, oxidation! The appearance of oxygen, while affording the possibility of synthesizing highly energetic compounds, also generated molecules capable of damaging constituents of living structures, including proteins, nucleic acids and lipids (ref. 1). It is therefore no surprise that all biological organisms have developed multiple and sophisticated ways to reduce the detrimental effects of oxygen. Thus, antioxidants were evolved in many organisms and microorganisms as a defense against these oxidations. Most antioxidants found in nature are hydroxylated and polyhydroxylated aromatic and heterocyclic compounds, such as vitamin A (retinol), vitamin C (ascorbic acid) and vitamin E (a-tocopherol) (ref. 2). Recently, a new class of antioxidants, metal-free chlorins, have been isolated from marine organisms (ref. 3) and it is interesting and typical that nature should modify the very molecule that was producing oxygen (chlorophyll) in order to protect against over oxidation. Of these chlorins, 132,173cyclopheophorbide a enol (3) was the first to be isolated from the marine sponge Darwinella oxeafu (ref. 4). The structural difference between 3 and chlorophyll a is in the formation of an additional exocylic ring VI. The rest of the antioxidative chlorins, which can be considered as the further oxidized products of 3, 132S-hydroxychlorophyllone a [5 (S) ] , 132R-hydroxychlorophyllone a [S(R)], 15lRhydroxychlorophyllonelactone a [8(R)], 132-oxypyropheophorbide a (10). purpurin 18 (ll), purpurin 18 methyl ester (12), and chlorophyllonic acid a methyl ester (13), were recently found in marine animals such as the short-necked clam, Ruditapes philippinurum, the viscera of the scallop Pacrinopecten yessoensis and attached diatoms and wafting diatoms (ref. 3). These novel antioxidative chlorins share a similar structural framework and molecular substitution pattern to chlorophyll a . They are most likely to be biogenetically related compounds evolved from chlorophyll a (refs. 3,5). The rich stereosmctural diversity of these antioxidative chlorins and their important biological activity prompted us to develop synthetic routes to them and thus be able to produce sufficient amounts of material for subsequent antioxidative and biomedical investigations. 765 766 L. MA AND D. DOLPHIN Antioxidants as therapeutic agents have shown various potentials in the treatment of inflammation, cancer, aging and neurodegenerative diseases (ref. 6). This contribution exemplifies our most recent progress on the syntheses of these antioxidative chlorins. A more comprehensive description of our earlier work in this area has been published previously (ref. 7). Synthesis of Enol3 and Asymmetric Hydroxylation Our basic strategy for the synthesis of hydroxydiketones 5 (S) and 5 (R) is shown in Fig. 1. A degradation product of chlorophyll a, pheophorbide u methyl ester (l), extracted from SpiruZina muximu alga was used as starting material (ref. 8). One key step was the Claisen-type intramolecular condensation of 2 to obtain 3 by modification of Eschenmoser's method (refs. 7, 9). ChloM 3 is a common intermediate to all the antioxidative chlorins and exists only in the enolic form in solution and in the crystalline state (ref. 4). A number of methods were investigated, which included aerial and iodine oxidation, to bring about the conversion of 3 to hydroxychlorophyllone u (5). None of them were successful since the two exocyclic rings of 3 are so labile that they can be readily cleaved, particularly by ionic bases. For example, treatment of 3 with (TMS)2NNa or LDA followed by oxidation with 1-phenyl-N(phenylsulfony1)oxaziridine (ref. 10) at -78°C for 15 min, failed to give the desired product 5, but gave a very polar yellowish-green material (24% yield), which was found, after treatment with diazomethane, to be chlorin p6 trimethyl ester (14). 4(s) (16% de) L ( C H C 1 3 ) 670(& 63.500) 414(& 141.200) L z ( C H C I 3 ) 874(& 48,300) 41 6( & 1 13.000)