Synthesis of 3-arylated chlorophyll-a derivatives via Diels–Alder reaction and their atropisomerism

Methyl pyropheophorbides-a possessing an o/m-(methoxycarbonyl)phenyl group at the 3-position were prepared by Diels–Alder reaction of 3-(trans-1,3-butadienyl)chlorin with methyl propiolate and successive didehydrogenation of the resulting 1,4-cyclohexadienes. The 3-arylated chlorin bearing the sterically demanding o-COOMe group as the major product was a 1:1 mixture of high-performance liquid chromatography-separable rotational isomers around the C3–C3¹ bond, while the minor product with the m-COOMe was less sterically hindered to exhibit rapid atropisomerization at room temperature. The 3-aryl group of the major product was nearly perpendicular to the chlorin π-system and less conjugated with the chlorin moiety than that of the minor product wherein more π-conjugation occurred to give slightly red-shifted Qy bands. Although both the atropisomers of the o-substitute in dichloromethane showed similar visible absorption bands and fluorescence emission data, their circular dichroism bands in the ultraviolet-C light region were dependent on the stereochemistry.