In the present method, 1-Benzyl-3,4-dihydroxy-1H-pyrrole-2,5-dicarboxylic acid dimethyl ester, the pyrrole derivative labeled A4 in FIG. 4 (hereinafter referred to as compound A4) is first synthesized from commercially available chemicals according to prior art techniques (see A. Merz, R. Schropp, E. Dotterl, “3,4-Dialkoxypyrroles and 2,3,7,8,12,13,17,18-Octaalkoxyporphyrins”, Synthesis, 7, pp. 795-800, (1995)). Then, a solution of compound A4, 2,2-diethyl-1,3-propanediol (labeled B4 in FIG. 4), triphenylphosphine (PPh3; labeled C4 in FIG. 4), and diethyl azodicarboxylate (DEAD; labeled D4 in FIG. 4) is refluxed in tetrahydrofuran (THF) for about 10-20 hours (preferably under an inert atmosphere). The reaction mixture is then quenched using a small amount of water, mixed with silica gel, and evaporated to dryness. The resulting material is loaded into a silica gel column and eluded by hexane and dichloromethane (1:1 volume ratio) to yield 2-Benzyl-6,6-diethyl-6,7-dihydro-2H,5H-4,8-dioxa-2-aza-azulene-1,3-dicarboxylic acid dimethyl ester, the pyrrole derivative labeled E4 in FIG. 4 (hereinafter referred to as compound E4).
