Nucleophilic additions to α-haloacetals proceed via an oxocarbenium ion intermediate where the carbon-halogen bond can either adopt a pseudo-axial or a pseudo-equatorial position. trans-Selectivity is typically observed when a halogen atom is positioned at C-2 of an acetal because hyperconjugative interactions involving the pseudo-axial carbon-halogen bond stabilize the oxocarbenium ion intermediate.

中文翻译：

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卤素原子参与指导缩醛取代反应的立体化学结果

α-卤缩醛的亲核加成通过氧碳正离子中间体进行，其中碳-卤素键可以采用伪轴向或伪赤道位置。当卤素原子位于缩醛的 C-2 时，通常会观察到反式选择性，因为涉及假轴向碳-卤素键的超共轭相互作用稳定了氧碳正离子中间体。