Hetero-4 + 2-photocycloaddition

Examples of the versatility of photochemical hetero Diels-Alder photocycloadditions have also been reported. Thus, the 2,4,6-triphenylpyrylium terafluoroborate (TPT) photocatalyzed reaction of arylimines with N-vinyl-2-pyrrolidinone and N-vinylcarbazole represents a convenient approach for the preparation of tetra-hydroquinoline derivatives under mild conditions (Scheme 9.31) [52],... 

The classic de Mayo reaction involves the [2 + 2] photocycloaddition of an alkene to the hydrogen-bonded enol tautomer of a P-dicarbonyl compound as exemplified by the formation of 1,5-diketone 9 from pentane-2,4-dione and cyclohexene (vide supra). In addition to alkenes, allenes are also used as the olefinic component. For example, irradiation of a mixture of dimedone and allene results in the formation of 3,3-dimethyl-7-methylenecycloocta-l,5-dione 12 via the cyclobutane intermediate 11, together with the corresponding head-to-tail product 13, which spontaneously dimerizes to the hetero Diels-Alder adduct 14. Diketone 12 is a versatile building block for the preparation of substituted cyclooctadienones and 8-valerolactones. 

Among heterophanes, some thiophenophanes and carbazolophanes were successfully synthesized. Some [2.2]phanes with relatively small frameworks are too strained to be synthesized by this method. However, the photocycloaddition can afford some [2.n] (2,5)thiophenophanes 42—43 (n > 3), [2.n] (3,9)carbazol-ophanes 44—45 (n > 4), with the hmitations shown in parentheses. These syntheses are also dependent on the large atomic radii of the hetero elements and on the electrostatic repulsive interaction between two co-facially arranged heteroatoms. 

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