Paterno-Biichi-type reaction

Another type of dimeric lesions are pyrimidine-pyrimidone (Pyr[6-4]Pyr) dimers formed by a Paterno-Biichi-type reaction at dipyrimidine sites between the C(5)=C(6) double bond of the first pyrimidine and the C(4)=0 carbonyl group of the second base. This kind of dimerization is demonstrated in Scheme 8.2 for the case of adjacent thymine moieties. 

A clear division of Paterno-Biichi reactions into several distinct categories is possible on the basis of the type of reacting carbonyl compound (alkyl or aromatic), the excited state responsible for reaction (n—71 or Ti—n, singlet or triplet), and the type of olefin (electron deficient or electron-rich). Some examples of these reactions are given in Eqs. 7—11, where only the oxetane products are shown. 

The photoinduced Paterno-Biichi reaction of 1-acetylisatin 130 with benzo [3]furan 82 was employed to make a structurally interesting spiro-type molecule 131 in good yield, as depicted in Equation (111) <2002J(P1)345>. 

The low quantum yield indicates that intermediate complexes and diradicals decay unproduc-tively. In general, Norrish type II photoreactions and other hydrogen abstraction processes must be overcome in order to achieve successful cycloaddition. Only in a few reported cases is intramolecular hydrogen abstraction a serious competitive reaction path. For example, the cycloheptenyl-substituted ketone 5 yields an oxetane 6 in addition to a cyclobutanol derivative 758, whereas the unsaturated cycloheptanone 8 only gives oxetane products 9 and 10 on irradiation59. The main product 10 was converted in two steps to azulene in 25 % overall yield. The reaction sequence 11 - 12 - 13 also demonstrates the high synthetic potential of the intramolecular Paterno-Biichi reaction61. 

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