Xanthone, cycloaddition reactions

E)-2-Styrylchromones undergo photooxidative cyclization to afford 12//-benzopz]xanthen-12-ones in moderate yield (Equation 337) <1998EJO2031, 1996F1C0251>. 2-Vinyl chromones undergo [4+2]-cycloaddition reactions with enamines to form the intermediates 812, which afford xanthones upon elimination of pyrrolidine (Scheme 228) <2000J(P1)3732>. 

Dimethylamino)vinyl)chromones 813 undergo a [2+2] cycloaddition reaction with DMAD to form the intermediate 814, which rearrange to afford xanthones in modest yield (Scheme 229) <1997J(P1)2167, 1999J(P1)3005>. ( )-2-(2-(Dimethylamino)vinyl)chromones 813 can also react with iV-phenylmaleimide 815 and chromone-3-carboxylic acid 816 to afford xanthones in modest yield (Scheme 230) <1997J(P1)2167, 1999J(P1)3005>. 

Cyanochromone exhibits high stereoselectivity in [4+2]-cycloaddition reactions with electron-rich dienes which lead to partially reduced xanthones <97JOC7904>. The dienamine (44) affords xanthones on reaction with N-phenylmaleimide and with chromone-3-carboxylic acid through loss of dimethylamine from the [4+2]-cycloadduct. However, more reactive dienophiles such as DMAD undergo a [2+2] cycloaddition followed by an intramolecular [4+2] reaction to give the xanthone (Scheme 13) <97JCS(P1)2167>. 

A photosensitized dimerization of an isolated olefin, norbomene, has been reported by Scharf and Korte.<3) Irradiation in acetone or in the presence of acetophenone (Et = 74 kcal/mole) produced dimers (5) and (6) as major products. However, benzophenone (Et = 69 kcal/mole) failed to sensitize the reaction to (5) and (6), but in ether solution led to the quantitative formation of benzpinacol and in benzene to the oxetane (7) in 80% yield. Sensitizers of intermediate energy, such as xanthone (Et — 72 kcal/mole), demonstrated a competition between energy transfer to form triplet norbomene and cycloaddition to form the oxetane ... 

Vinylchromones undergo a [4+2]-cycloaddition with pyrrolidine enamines, generated in situ, leading to xanthones substituted in the C-ring. The reaction proceeds by way of a methylidene tetrahydroxanthone 76 (Scheme 52) <00JCS(P1)3732>. 

Loyola University of Chicago), found that Hammond s estimate is accurate, and the fluorescent lifetime of 2 and 40 ate 180 ps. Triplet-sensitized reactions of 2-pyridones have not been extensively studied. However Sharp and Hammond found no evidence for 2-pyri-done photoproducts when xanthone is used as a photosensitizer. Using tethered pyridones, Nakamura found [2+2] and [4+2] cycloaddition products with benzophenone sensitization (see Figure 14). This is consistent with 2-pyrone photochemistry,but the generality of these results await further experimentation. 

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