Stereoselective reduction,methylation, and phenylation of the 13-carbonyl group in chlorophyll derivatives

Regioselective reduction of the 13-carbonyl group on the five-membered exo-ring of methyl pyropheophorbide-a, one of the chlorophyll-a derivatives, with sodium borohydride gave an epimeric mixture of (13¹R/S)-hydroxy-chlorins. The stereoselectivity was controlled by the steric effect of the (17S)-methoxycarbonylethyl group to afford the (13¹S)-rich secondary alcohol (25% de). The use of sterically large lithium tri(sec-butyl)borohydride as the reductant enhanced the stereoselectivity to 55% de. The regio- and stereoselective methylation and phenylation of the 13-CO of pyropheophorbide-a were observed using methyl and phenyl lithium, respectively. The major diastereomer of the tertiary alcohols obtained had the same configuration at the 13¹-stereogenic center as in the reduced product. All of the anion species (H^?, CH₃^?, and C₆H₅^?) favorably attacked the 13¹-carbon atom from the reverse side of the 17-propionate residue, that is, the less sterically crowded face of the 13-CO plane.