Abstract
Bacteriochlorophyll-a 5 present in Rb. sphaeroides was converted into bacteriopurpurin-18 methyl ester 10 by following three different approaches and was used as a substrate for the preparation of a series of stable bacteriochlorins with long wavelength absorptions ranging from 760 to 824 nm. Bacteriopurpurin p 6 17, obtained by the base hydrolysis of 10, on reacting with L-aspartyl-di-tert -butyl ester in presence of EDCI and DMAP afforded the corresponding N-aspartyl-bacteriopurpurinimide 21 in high yield, possibly by the base-catalyzed intramolecular cyclization via the isoimide intermediate(s) 19 and 20. A possible mechanism for the formation of 10 and bacteriochlorin 15-glyoxilic acid trimethyl ester 8 from 5 via hydroxylactone 11 is also discussed. Among the compounds synthesized, the bacteriochlorin 21 containing a fused cyclic imide ring system was found to be the most stable in various solvents at room temperature and exhibited the longest wavelength absorption at 824 nm in dichloromethane.
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