Pyridone selective cycloadditions

Selective cycloadditions of 2-pyridone mixtures, as well as the analogous crosscycloadditions with furan, naphthalene, and 1,3-dienes, have been intensively investigated. 

SCHEME 6 c s-Selective cycloaddition with hydrogen-bonded pyridones. " ... 

A phase-selective photochemical reaction of 2-pyridones is observed. Irradiation of 225 in benzene gives mainly rearrangement products 226, whereas, in the solid state, [4+4] photocycloaddition to the photodimer 227 occurred in quantitative yield (Scheme 39) <20040L683>. The stereochemistry of the photodimer was exclusively the trans- /+configuration, as shown. This is presumably due to Jt-rt-stacking and dipole-dipole interactions between the pyridones. Intermolecular photocycloaddition of 2-pyridone mixtures can be selective and lead to useful quantities of [4+4] cycloaddition cross-products <1999JOC950>. 

Selective [4+4] cycloaddition with mixtures of 2-pyridones has been shown to be possible using 4-alkoxy-2-pyridone 144 as one component... 

Pyridones are accessible from formal [2 -i- 2 + 2] cycloaddition of two alkynes and an organic isocyanate. Two quite distinct catalyst systems have been developed for this reaction, with significant differences between them in mechanism and mechanistic implications on selectivity. 

Some degree of c -selectivity was initially anticipated on the basis of a possible steric interaction between the cyclopentane in 79 (and 81) and a solvated carbonyl of the approaching pyridone. Therefore, it was inexplicable that the cycloaddition is extraordinarily trans-se-lective (Figure 26) This trans-selectivity is solvent-independent for the A A -dimethyl (entries 1,2) and also for a monomethyl analog (entries 3, 4). Only when both IV-methyl groups are absent does the cis/trans-selectivity of the cycloaddition become solvent-dependent. This dramatic effect is illustrated with eleven solvents in reference 60. With benzene (or toluene) as solvent, the stereochemistry is completely reversed and yields the cis isomer 77 exclusively. The cis-... 

Sieburth, S. McN., Lin, C.-H., and Rucando, D., Selective intermolecular photo-[4 -I- 4]-cycloaddition with 2-pyridone mixtures. 2. Preparation of (la,2P,5P,6a)-3-butyl-9-methoxy-3,7-diaza-tricyclo[4.2.2.2 > ]dodeca-9,ll-diene-4,8-dione,/. Org. Chem., 64, 950-953,1999. 

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