Synthesis of Chlorophyll a Labeled at C(32) from Pheophorbide a Methyl Ester

[3²-¹⁴C]Chlorophyll a ( 10b ) was synthesized from pheophorbide a methyl ester ( 5a ) in a seven-step partial synthesis. The key intermediate pheophorbide d methyl ester ( 6 ) was obtained by ozonolysis of the vinyl group of 5a in 91% yield. Selective reduction of the CHO group of 6 gave the corresponding alcohol 7 , and conversion of the latter to the phosphonium bromide 8 yielded after Wittig reaction with [¹⁴C]paraformaldehyde, [3²-¹⁴C]pheophorbide a methyl ester ( 5b ). The final transformation to the title compound was achieved by acid hydrolysis of 5b , esterification with natural phytol to [3²-¹⁴C]pheophytin a ( 9b ), and eventual insertion of a Mg²⁺ ion.