In more polar solvents, such as DMF, the selectivity for the monosubstituted
product decreased considerably. After 30 min, almost 20% of disubstituted 3a was obtained (entry 8). When using a 10 fold excess of piperidine, the yield increased to 34% (entry 10). In DMSO the reaction (4 equiv of piperidine) afforded after 1 h over
40% of 3a. In all cases, longer irradiation (several hours) did not significantly
change the product ratios but was leading to partial degradation
of the products and thus to lower yields