A full account of the Brønsted acid catalyzed, enantioselective synthesis of 4H-chromenes and 1H-xanthen-1-ones from o-hydroxybenzyl alcohols and β-dicarbonyl compounds is provided. The central step of our strategy is the BINOL–phosphoric acid catalyzed, enantioselective cycloaddition of β-diketones, β-keto nitriles, and β-keto esters to in situ generated, hydrogen-bonded o-quinone methides. Upon acid-promoted dehydration, the desired products were obtained with generally excellent yields and enantioselectivity. Detailed mechanistic studies including online-NMR and ESI-MS measurements were conducted to identify relevant synthetic intermediates. A reversible formation of a dimer from the starting alcohol and the reactive o-quinone methide in an off-cycle equilibrium was observed, providing a reservoir from which the o-quinone methide can be regenerated and introduced into the catalytic cycle again. Reaction progress kinetic analysis was utilized to determine kinetic profiles and rate constants of the reaction uncovering o-quinone methide formation as the rate-limiting step. In combination with Hammett plots, these studies document the relationship between o-quinone methide stabilization by electronic effects provided by the substituents and the reaction rate of the described process. In addition, DFT calculations reveal a concerted yet highly asynchronous [4 + 2]-cycloaddition pathway and an attractive CH−π interaction between the catalyst’s tBu group and the o-quinone methide as an important stereochemical control element.

中文翻译：

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磷酸催化氢键合的邻醌甲基苯甲酸酯的形成。β-二羰基化合物对苯环氧杂环的对映选择性环加成反应。

提供了由邻羟基苄醇和β-二羰基化合物催化的布朗斯台德酸催化的4 H-色烯和1 H-黄嘌呤-1-酮的对映选择性的完整描述。我们策略的中心步骤是将BINOL-磷酸催化的β-二酮，β-酮腈和β-酮酯的对映选择性环加成反应，使其原位生成氢键合的邻苯二甲酰甲基甲烷。经酸促进的脱水后，通常以优异的收率和对映选择性获得所需产物。进行了详细的机理研究，包括在线NMR和ESI-MS测量，以鉴定相关的合成中间体。由起始醇和反应物可逆形成二聚体观察到邻-苯醌甲基化物处于非循环平衡状态，从而提供了一个储存器，可从该储器中再生邻-苯醌甲基化物并将其再次引入催化循环中。利用反应进程动力学分析确定反应速率的动力学曲线和反应速率常数，该反应速率速率常数揭示了邻醌甲基化物的形成。结合哈米特图，这些研究记录了由取代基提供的电子效应引起的邻醌甲基化物稳定与所述方法的反应速率之间的关系。此外，DFT计算还揭示了一个协调一致但高度异步的[4 + 2]-环加成路径以及催化剂的t之间有吸引力的CH-π相互作用group基和邻醌甲基化物为重要的立体化学控制元素。