1- Methyl-2-pyridone, photodimerization product

The photodimerization of 2-pyridones is an efficient, regiospecific, and stereoselective [4+ 4]-cycloaddition [56] that converts two achiral aromatics into a highly functionalized tricyclic cyclooctadiene with four stereogenic centers (Scheme 9.34). For tethered pyridones, the trans isomer is usually the major product when one or both pyridine nitrogens are methylated. By contrast, in the unsubstituted systems,... 

Photodimers have been reported as the principal products from irradiation of numerous arene substrates. While there is little selectivity to be considered in most of these reactions, it has been reported that almost all photolyses of 9-substituted anthracenes give head-to-head dimers as depicted in U10). ° An intramolecular version of these photodimerizations has also been reported. Irradiation of di(a-2-naphthylmethyl) ether gave a mixture of the endo cyclomer (111) and the exo cyclomer (112). ° The photodimerization of 2-pyridones has been the subject of extensive investigation for a number of years." "" In most instances the head-to-tail or trans-anti isomer was the major or exclusive product in these reactions. Irradiation of A -methyl-2-pyridone for 15 h provided the trans-anti dimer (113) in 51% yield. This material was accompanied by much smaller quantities of other dimeric species. Vari-... 

An analogous photo-isomer of A -benzyloxycarbonylmethyl-2(l//)-pyridone has been prepared and various transformations of its cyclobutene ring have been explored. The photodimerization of A -methyl-2(l/ )-pyridone in water gives exclusively the products of [4 + 4] addition the yields of trans-anti- (31), trans-syn-, cis-anti-, and cw-jjfn-dimers were 51, 0.6, 11.2, and 6.8%, respectively. In non-aqueous solvents (ethanol or benzene), only a t/-dimers were formed. The thermal cycloaddition of dimethyl acetylenedicarboxylate to 2(l/f)-pyridones also occurs across positions 3 and 6. Reaction is favoured where steric buttressing of the methyl groups in the starting pyridone occurs for example, the cyclo-adduct (32) is... 

Both the [4+4] photodimerization reaction (4 — 6) and the isomerization to give Dewar-pyridone products (4 —> 5) tolerates substitution at every position (Table 1), but certain combinations of substituent and position are notable exceptions. The parent 2-pyridone 4a and N-methyl 4b are typical, yielding either 5 or 6 depending on the starting concentration. - A single additional allyl substituent, represented by l,n-dimethyl examples 4c-4f, all yield at least the [4+4] dimer. Even a /-butyl substituent at C4 (4g) which presents a possible steric impediment, is compatible with the dimerization reaction. Fusion of a cyclohexyl at C5/C6 (4k) or a cyclohexadienyl ring at C4/C5 (41) leads to acceptable yields of the photodimer 6. Chlorine substitution... 

---