When the reaction was performed without a photocatalyst (Fig. ), no desired product 3a was observed under the irradiation at λmax = 450 nm; however, a little product formation was noticed under the irradiation at λmax = 405 nm (18% yield) and 365 nm (7% yield). Further, evaluation of the crossrelationship between triplet energy of the different photocatalysts and product yield reveals no detectable influence of the triplet energy of the photosensitizer (Fig. ), implying that the Dexter-type triplet-triplet energy transfer (TTEnT) process is unlikely to be operative. The product formation was significantly suppressed when the reaction was performed in the presence of different radical scavengers such as TEMPO (10% yield) and BHT (39% yield) (Fig. ). In both the cases, two key intermediates 17 & 18 and 19 & 20 were detected by HRMS which suggest the involvement of amidyl radical and cyclobutyl radical 16 in the reaction. Subsequently, when the reaction was performed in the presence of excess methanol (10 equiv), the methanol-trapped product 21 was isolated in 38% yield (dr = 2:1) (Fig. ) which indicates the formation of cyclobutyl cation 22, potentially originating from the corresponding radical 21.