Synthetic and mechanistic study of the acid-promoted aromatisation of the Diels–Alder adduct: 6,7-dimethoxy-1-(4-pyridyl)-1,4-epoxytetrahydronaphthalene-2,3-dicarboxylate
作者:Masakatsu Sugahara、Yasunori Moritani、Tooru Kuroda、Kazuhiko Kondo、Tatsuzo Ukita
DOI:10.1039/b110265f
日期:2002.3.8
A synthetic study of a 1-arylnaphthalene lignan having a heteroaryl group, such as the pyridyl group, at the C(1) position was conducted in detail. The DielsâAlder reaction of 1-pyridylisobenzofuran precursor 1 with dimethyl maleate or dimethyl fumarate gave desired 1,4-epoxides 2â5 in good yields as mixtures of exo and endo diastereoisomers. Intensive investigation of the following acid-promoted aromatisation using a Lewis acid or a Brønsted acid as a reaction promoter revealed that BF3·Et2O was the most suitable acid for this step. Examination of the reaction rates in aromatisation of 1,4-epoxides 2â5 provided information for the elaboration of reaction mechanisms in these reactions. Both the neighbouring-group effect of the 2-methoxycarbonyl
substituent stabilising the benzyl cation intermediate and the longer length of the C(1)âO bond in 1,4-epoxide in 2-endo form 3 (or 5) than that of the C(4)âO bond presumably contribute to enhancing the reaction rate, and aromatisation of the 2-endo form would be greatly favoured over that of the 2-exo form.
我们对一种在 C(1)位具有杂芳基(如吡啶基)的 1-芳基萘木脂素进行了详细的合成研究。1-pyridylisobenzofuran precursor 1 与马来酸二甲酯或富马酸二甲酯进行 DielsâAlder 反应后,得到了所需的 1,4-环氧化物 2â5,产率很高,为外向和内向非对映异构体的混合物。使用路易斯酸或布氏酸作为反应促进剂,对随后的酸促进芳香化反应进行深入研究后发现,BF3Â-Et2O 是最适合这一步骤的酸。对1,4-环氧化物2â5芳香化反应速率的研究为阐明这些反应的反应机理提供了信息。2-甲氧基羰基取代基的邻位基团效应稳定了苄基阳离子中间体,2-内向形式的1,4-环氧化物3(或5)中C(1)-âO键的长度比C(4)-âO键的长度长,这两点可能都有助于提高反应速率,而且2-内向形式的芳香化比2-外向形式的芳香化更有利。