2-pyridones photocyclization

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... 

In the case of the formation of the [3-lactam, it was shown that the ability of a compound to crystallize in a chiral space group was increased by using compounds with meta-substituted aryl groups such as 50a-d instead of their ortho- and para-substituted analogues the absolute asymmetric photocyclization of the pyridones resulted in the formation of 51a-d with good ees.[31]... 

Highly enantioselective photocyclization of l-alkyl-2-pyridones, in the solid state, to P-lactams 29 has been achieved in inclusion crystals with optically active host compounds <02OL3255>. 

Scheme 8 Enantioselective [2 + 2] photocyclization of pyridones in their inclusion crystals.

The photocyclization of the enone (7) to yield (8) has been used as an approach to the synthesis of the spiro(4,5)decane system. The tetracyclic product (9a) is obtained from the photo-induced intramolecular cyclization of the pyridone (10a) Increase in the chain length involving (10b) resulted in the formation of... 

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>. 

Photosubstrate 69 was an intriguing and optimistic approach with all but two carbons of the tetracyclic taxol target. Unfortunately, this bis-2-pyridone did not photocyclize to 70 but only slowly decomposed photochemically. We speculated that the obstacle to this cycloaddition was the requirement that four fiilly substituted sp carbons be created in a single step. Molecular modeling reinforced this view. ... 

Photocyclization of an enamide bearing an o-methoxyl group takes place at the site occupied by the methoxyl to afford a substituted pyridone after work-up. ... 

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. 

Beak, P. and Zeigler, J.M., Molecular organization by hydrogen bonding juxtaposition of remote double bonds for photocyclization in a 2-pyridone dimer, /. Org. Chem., 46, 619,1981. 

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. 

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