[UV-dimerization of 1,3-dimethyluracil in ice-matrix].

UV irradiation of 1,3-dimethyluracil in ice-matrix yields four dimeric dimethyluracils (2 c) — (5 c) , which could be isolated by preparative thin-layer chromatography. Attemps for structure elucidation of these four dimers by UV, IR, NMR and mass-spectrometric methodes pointed out, that only NMR-spectra give structural evidences; by IR — not by mass-spectra — certain identification of dimers is possible. In accordance with dimeric thymines and dimeric uracils the syn-dimeric dimethyluracils (3 c) and (5 c) are stable in 6 N HCl, the anti-dimers are monomerisized. Investigations of stability in HCl gives informations about structures of cyclobutane pyrimidine dimers.