1(2H)-Isoquinolone derivatives are classified as aromatic heterocycles. Substitution reactions 1,2) of 1(2H)-isoquinolone derivatives have been reported, however, little attention has been focused on addition reactions. Dyke et al. reported a Diels-Alder (DA) reaction of 2-methyl-1(2H)-isoquinolone derivatives as the diene. 3) To the best of our knowledge, however, there have been no reports of a DA reaction of 1(2H)isoquinolones acting as dienophiles. DA reaction of 1(2H)isoquinolone

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... s with dienes afforded phenanthridones. A number of Amaryllidaceae alkaloids contain phenanthridine skeletons, and are thought to be potentially valuable synthetic intermediates. Moreover, these alkaloids may possess novel pharmacological activities. 4-7) Recently, Weltin et al. verified the ability of 5(6H)-phenanthridones to inhibit PARP [poly(ADP-ribose)polymerase] activity in lymphoma cells. 8) Herein, we report a novel Diels-Alder reaction of 4-nitro-1(2H)-isoquinolones acting as the dienophile. This Diels-Alder reaction is a novel synthetic methodology employing the phenanthridine skeleton. Examination of the regioselectivity of 4-nitro-1(2H)-isoquinolones 9) with 1methoxy-3-silyloxy-1,3-butadienes using molecular orbital (MO) calculations is also discussed in the current work. DA Reaction of 4-Nitro-1(2H)-isoquinolones First, DA reactions of 2-methyl-(1a) 9) and 2-unsubstituted-1(2H)-isoquinolones (1b) 9) bearing a nitro group at the 4-position with 1-methoxy-1,3-butadienes (2a-c) were examined under atmospheric pressure conditions, as shown in Table A novel Diels-Alder (DA) reaction with 4-nitro-1(2H)-isoquinolones acting as the dienophile afforded 5(6H)phenanthridone derivatives. The DA reaction of 4-nitro-1(2H)-isoquinolone with 1-methoxy-1,3-butadiene gave biologically active 5(6H)-phenanthridone possessing in a high yield. Regioselectivity of 4-nitro-1(2H)-isoquinolones with 1-methoxy-3-silyloxy-1,3-butadiene was calculated using molecular orbital (MO) calculations.