Anti Regiospecificity in the Photosensitized Cycloaddition of 4-Tetrazolouracil Nucleoside

The [2 + 2] photocycloaddition of natural pyrimidine nucleobases is devoid of regioselectivity. Although modified pyrimidines have been developed to selectively obtain syn-cyclobutane isomers, the targeted formation of anti-cyclobutane isomers has not been addressed yet. Herein, using NMR analyses and DFT calculations, we demonstrate that the acetone photosensitized excitation of the 4-tetrazolouracil motif in the nucleoside series specifically provides anti-cyclobutane photoproducts in 51% yield. In addition, the cis stereomer formation is preferred over the trans-cyclobutane formation (71:29).