Paternd-Biichi reaction

Paternd-Biichi reactions [152] this competition has been investigated for electron-rich alkene substrates for several combinations of carbonyl compounds and electron-donors, e.g. a-diketones and ketene acetals [153], aromatic aldehydes and silyl ketene acetals, and enol ethers. In polar solvents, the assumption of a 1,4-zwitterion as decisive intermediate is reasonable. This situation then resembles the sequence observed for ET-induced thermal [2 -I- 2]-cycloaddition reactions [154]. Both regio- and diastereoselectivity are influenced by this mechanistic scenario. The regioselectivity is now a consequence of maximum charge stabilization and no longer a consequence of the primary interaction between excited carbonyl compound and alkene. Whereas 3-alkoxyoxetanes are preferentially formed from triplet excited aldehydes and enolethers, 2-alkoxyoxetanes result from the reaction of triplet excited ketones or aldehydes and highly electron-rich ketene silylacetals (Scheme 40) [155]. 

Yang, N. C., Kimura, M., and Eisenhardt, W., Paternd-Biichi reaction of aromatic aldehydes with 2-butenes and their implication on the rate of intersystem crossing of aromatic aldehydes, /. Am. Chem. Soc., 95, 5058, 1973. 

Another main photoreaction that occurs upon UV-light absorption of pyrimidine bases is the Paternd-Biichi reaction between the C(5) = C(6) double bond of the first pyrimidine and the C(4) = O carbonyl group of the second base. This reaction furnishes initially an oxetane intermediate at TT dipyrimidine sites." The cytosine can react out of its (E)-imino tautomeric form with its C(4) = NH. The formed reaction product is an azetidine intermediate. 

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