Furans intermolecular dimerization

Addition.—Intermolecular [ 2 + w2] cycloadditions in nitrogen-containing heterocycles continue to arouse interest, particularly in systems which are related to constituents of nucleic acids. Cyclobutane dimers are readily obtained on direct irradiation of furo[3,2-Z>]pyridin-2(4 )-ones and the pyrrolo[3,2-Z>]-pyridin-2-one (151), although it is not possible on the basis of the evidence available to distinguish between the head-to-head (152) and the head-to-tail (153) structures.118 Cyclobutane dimers have also been obtained from alkyl 4-stilbazole salts on irradiation in solution and in the solid state.118 Examples of analogous intramolecular [ 2 + 2] cycloadditions have also been reported.120-121 The [ 2 + 2] cycloaddition of quinol-2-one to furan has been described,122 but... 

---