The syntheses and properties of the cycloadducts of p-benzoquinones with 1,3-dipoes

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1.Some of the 1,3-dipolar cycloadducts of aromatic nitrile oxides with 2,5- and 2,6-dialkyl-p-benzoquinones undrgo base-induced isomerization. We have proposed the structure of the isomerized products as isoxazole-fused derivatives which were formed by ring-opening and recyclization with a base from spectroscopic data. However the X-ray structure determination of the isomerized product from 2,5-di-tert-butyl-p-benzoquinone (25DBQ) with 2,6-dichlorobenzonitrile oxide (CNO) showed that one of the tert-butyl group migrated from the bridgehead poositon to neighboring carbonyl carbon atom. The cycloadducts which have a methyl group at the bridgehead position did not undergo isomerization. We investigate the mechanism of rearrangement in detail.Also we investigate the reaction of some novel 1,3-dipolar cycloadducts with base. The cycloadduct of 25DBQ with N-phenyl-alpha-p-nitrophenylnitrone yields no isomerized product by the reaction with base. The reaction of 25DBQ with trimethylsilyldiazo … More methane generates pyrazoline derivative instead of simple 1,3-dipolar cycloadduct. This compound reacts with NaOH and isomerize to pyrazole derivative with the migration of the bridgehead tert-butylgroup. Also in the case of the cycloadduct of 2-tet-butylnaphthoquinone with CNO,the bridgehead trt-butyl group moves to neighboring carbonyl carbon to form isoxazole derivative. It is seemed that such a very unusual alkylmigration results from the aromatization of heterocyclic moieties of cycloadducts.2.Treatment of the cycloadducts with acetic anhydride-sulkfuric acid caused elimination of the bridgehead t-Bugroup to afford the isoxazole-fused hydroquinone diacetate derivatives. The rearranged products were also susceptible to the similar acetylation, but the resulting products were isoxazole-fused catechol derivatives and/or hydroquinone diacetate derivatives.3.The reaction of 2-tert-butyl-5-methyl-p-benzoquinone with a large excess of the nitrile oxide gave the 1 : 3 cycloadduct instead of the 1 : 2 cycloadduct. The cycloaddition occurred at two C=C bonds and one C=O bond.4.The cycloaddition of trimethylenemethane with p-benzoquinones gave the 1 : 1 cycloadduct at the C=C bond. The other alkyl-substituted benzoquinones also gave the corresponding cycloadducts. Less