The Diels-Alder Reactions of Protoporphyrin IX Dimethyl Ester with Electron Deficient Alkynes
Scherrer Pangka, V., Morgan, A.R., and David Dolphin
J. Org. Chem.
The vinyl and cross-conjugated porphyrin β,β' double bonds of either ring A or B of protoporphyrin IX di-Me ester react in a [4 + 2] cycloaddition with electron-deficient acetylenes to give the corresponding chlorins; thus, cycloadditions with ring B gave I (R = CO2R1, SO2Ph; R1 = Me, Et; R2 = Me). The ring A and B isomers are readily separable by chromatographically I and the ring A adducts are rearranged by treatment with Et3N or 1,5-diazabicyclo[5.4.0]undec-5-ene to give diastereomers, e.g., II and III from I. The Diels-Alder reaction with PhSO2C?CCO2R1 is both regio- and stereospecific.