Light-induced Dehydrodimerization of 3-Hydroxypyrroles

Irradiation (λ > 280 nm) of 3‐hydroxy‐1H‐pyrrole‐2‐carboxylates 1 in CH3CN gives the [2.2′‐bi(3‐oxo‐2,3‐dihydro‐1H‐pyrrole)]‐2‐,2′‐dicarboxylates 2 in reasonable to good yields. The corresponding N‐methylpyrroles 3 only undergo slow photodecomposition under similar conditions. Several 2‐methyl‐3‐oxo‐2,3‐dihydro‐1H‐pyrrole‐2‐carboxylates 4 and 5 were synthesized to compare their spectral data with those of the dehydrodimers 2. A X‐ray structure analysis was performed for diethyl [2,2′‐bi(4,5‐dimethyl‐3‐oxo‐2,3‐dihydro‐1 H‐pyrrole)]‐2,2′‐dicarboxylate (2b). The originally proposed [3,3′‐bi(3H‐pyroole)] structure for compounds 2a‐e proves incorrect.