Although 2(1H)-pyridones are classified as aromatic heterocycles, there are many reports of Diels-Alder (DA) reactions in which 2(1H)-pyridones act as dienes. 1) In our laboratory, we have developed novel DA reactions involving 2(1H)-pyridones which contain an electron-withdrawing group that acts as a dienophile (Chart 1). 2-4) Thermal, uncatalyzed reactions of 1-methyl-2(1H)-pyridones with methoxycarbonyl, cyano, acetyl, benzoyl, and formyl groups at the 4-position produced only hydrogenated isoquinolones, in good to excellent yields.2) The reaction of 6-acetyl-1-methyl-2(1H)-pyridone yielded a phenanthridone at 11% yield, 3) but, the DA reactions of 3-or 5-substituted 1-methyl-2(1H)-pyridones were unsuccessful. Recently, we reported that DA reactions of 1-sulfonyl-2(1H)-pyridones containing an electron-withdrawing group at the 5-position produced only hydrogenated quinolones, in moderate yields. 4) Since the nitro group is a relatively strong electron-withdrawing group, 1-substituted 3-and/or 5-nitro-2(1H)-pyridones are highly electron-deficient heteroaromatic compounds. Therefore, it is to be expected that DA reactions of 3-nitropyridones would yield isoquinolones, that those of 5-nitropyridones would yield quinolones, and that reactions of 3,5-dinitropyridones with two moieties acting as dienophiles would produce double DA adducts (phenanthridones). The isoquinolone, 5) quinolone, 6) and phenanthridone 7) skeletons found in many alkaloids can be expected to be potentially valuable synthetic intermediates, and to possess interesting biological properties. In the present paper, we wish to report the first synthesis of isoquinolones, quinolones, and phenanthridones using DA reactions with 1-unsubstituted and 1-substituted nitro-2(1H)-pyridones and nitro-2(1H)-pyridones containing a methoxycarbonyl group.DA Reactions First, thermal DA reactions of 5-nitro-2(1H)-pyridones (1a, b, 8) 2a, 9) 3, 9) 4 9) ) with 2,3-dimethyl-1,3-butadiene (5) were examined (Chart 2, Table 1). The reactions of 1a, b with 5 at 160 or 180°C for 5 or 3 d yielded only quinolones [6a (30%), 6b (26%), entries 1, 2] aromatized by release of hydrogen and nitrogen dioxide (HNO 2 ), followed by dehydrogenation. Performing the same reaction with 5-nitropyridone (2a) containing a methoxycarbonyl group at the 3-position chemoselectively yielded a functionalized quinolone [7a (22%), entry 3], with recovery of the starting material. DA reactions of the 3-and/or 4-substituted 5-nitropyridones (3, 4), with two moieties acting as dienophiles under the conditions shown in Table 1, stereoselectively produced the same functionalized cis-adducts; i.e., a phenanthridone [8, double DA adduct (27, 33%)] and a hydrogenated isoquinolone [9 (10, 15%), entries 4, 5], respectively. Based on the fact that the reactions of 3 and 4 with 5 both yielded the same DA adducts (8, 9), we hypothesized the existence of two pathways. It is well known that the nitro group functions as a leaving group and a strong electronwithdrawing group. In one pathway, 3 first arises from 4 ...
