1,3-Dipolar Cycloaddition Reactions of Benzonitrile Oxide to 2(1H )-Pyrazinone and Its N - and O - Methyl Derivatives

2(1H)-Pyrazinone, which is in equilibrium with 2-pyrazinol, reacts with benzonitrile oxide (BNO) affording the N 1 -adduct with a 67% yield, while 2-methoxypyrazine gives two methoxypyrazinones (ca.3%) and a biscycloadduct together with its degradation product, which derive from the two unisolable monocycloadducts to the C=N 4 double bond.N-Methylpyrazinone gives only the degradation product (3.4%) of the initial monocycloadduct of BNO to C=N 4 double bond.Recently, we investigated the reactions of benzonitrile oxide (BNO) with aromatic diazines 1 within a study aimed at analysing the effect of the aza-substitution on the reactivity, regio-and stereochemistry of pyridine nucleus, the dipolarophilic reactivity of which has been object of previous researches by us 2-4 and others. 5-8 Among the three diazines, we found that pyrazine (1) behaves like pyridine affording an unisolable intermediate monocycloadduct (2), deriving from the addition of BNO in a concerted pseudopericyclic cycloaddition, 9 which in its turn originates the two stable cycloadducts (3) and ( 4), where a second molecule of BNO is regio-and anti-stereospecifically added to the C=N or C=C double bond of 2 (Scheme 1).The aza-substitution in the γ-position of the dienaminic system of monocycloadduct (2) increases the reactivity of the γ,δ-double bond and reverses the regiochemistry of the γ,δ-attack with respect to pyridine, 2 too.