SYNTHESIS AND SPECTROSCOPIC PROPERTIES OF NOVEL BENZOCHLORINS DERIVEDFROM CHLOROPHYLL-A

Methylpheophorbide-alpha (a chlorophyll alpha analog) was converted in to methyl 13(1)-deoxypyropheophorbide-alpha 2 in excellent yield. The Ni(II) complex of 2 on hydrogenation and subsequent Vilsmeier reaction with phosphorus oxychloride and 3-(N,N-dimethylamino)acrolein produce d Ni(II) methyl -(2-formylvinyl)-13(1)-deoxypyrapheophorbide-alpha 4. In contrast to other synthetic and naturally occurring isobacteriochlo rins, the isobacteriochlorin obtained by acid-catalyzed cyclization of 4 was found to be unstable and oxidized to the corresponding benzochl orin. The free base benzochlorin 7, obtained by demetalation, up on DD Q/methanol treatment produced a mixture of 13(2)-oxo- and 13(1)-methox y-13(2)-oxabenzochlorins 8, 9. Surprisingly, under similar reaction co nditions, an ethanolic solution of DDQ afforded 13(1),13(2)-dioxobenzo chlorin 13 as a major product. Vilsmeier reaction (POCl3/DMF) of Ni(II ) benzochlorin 6 gave unexpected 5-formyl-13(2)-oxobenzochlorin 15. Zn (II) complexes of the newly synthesized benzochlorins showed long wave length absorptions in the range of 750-753 nm. Thus, compared to the r espective free base analogues bathochromic shifts of 40-42 nm were obs erved. Despite extensive studies in benzochlorins, these are the first examples which exhibit such remarkable long wavelength absorptions in their electronic absorption spectra. The structures of novel benzochl orins were confirmed by extensive NMR [2D NMR (ROESY), C-13 NMR] and X -ray crystallographic studies. On the basis of the crystal structure o f oxobenzochlorin 8, the chemical reactivity of other benzochlorin ana logues were examined by semiempirical molecular orbital theory. Our re sults are in agreement with the previous qualitative electron sextet h ypothesis proposed for chlorin systems by Woodward, The fluorescence q uantum yields and the singlet oxygen yields of the free base and Zn(II ) benzochlorins were measured relative to tetraphenylporphyrin (TPP) i n benzene.