Photocyclization reactions 2-pyridones

Photochemistry has also been a prominent theme this past year. Intermolecular photocyclization reactions involving pyridones have been reported . An unusual photocyclization between 1-cyanonaphthalene and substituted pyridines was also reported . In similar fashion the reaction between benzofurans and substituted pyridines was reported by Sakamoto <00CC1201>. 

Optically pure P-lactam derivatives can also be prepared by an enantioselective photocyclization reaction of 2-pyridone in its inclusion complex with a chiral host. For example, irradiation of the 1 1 inclusion complex of 1 and 4-methoxy-/V-inethyl-2-pyridone (33) in the solid state gave (—)-34 of 100% ee... 

Pyridines have also been used in cyclization reactions. Two noteworthy examples are shown in scheme 3. The reaction of substituted pyridine 32 with a nitrile affords imidazo[l -ajpyridine 33 in excellent yield <01JOC2862>. Oku and co-workers have reported the use of tetrahydroquinolizinium ylides in a 13-dipolar cycloaddition reaction (34- 35) <01JOC1638>. Sieburth has also published an account of the [4+4] photocyclization reaction of pyridones on route to fusicoccin <01S1185>. 

Mariano, P.S. and Leone, A.A., Excited-state chemistry of l-alkenyl-2-pyridones. exploratory and mechanistic investigations and kinetic analysis of photocyclization reactions involving conversion of the N-vinylamide to oxazoUum ylide function, /. Am. Chem. Soc., 101, 3607-3617,1979. 

Novel photocyclizations of l-vinyl-2-pyridones (74) in aqueous perchloric acid to oxazolo[3,2-a]pyridinium perchlorates (75) have been reported. A reaction pathway involving reversible electrocyclization of the singlet excited vinyl-pyridone to the highly resonance-stabilized azomethine ylide (76) is favoured. For example, photolysis of the pyridones in aqueous solution permits isolation of the alcohols (77 R = Me) and (77 R = H) in 87.9% and 81% yield respectively. 

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