Isomerism is a fundamental chemical concept, reflecting the fact that the arrangement of atoms in a molecular entity has a profound influence on its chemical and physical properties. Here we describe a previously unclassified fundamental form of conformational isomerism through four resolved stereoisomers of a transoid (BF)O(BF)-quinoxalinoporphyrin. These comprise two pairs of enantiomers that manifest structural relationships not describable within existing IUPAC nomenclature and terminology. They undergo thermal diastereomeric interconversion over a barrier of 104 ± 2 kJ mol⁻¹, which we term ‘akamptisomerization’. Feasible interconversion processes between conceivable synthesis products and reaction intermediates were mapped out by density functional theory calculations, identifying bond-angle inversion (BAI) at a singly bonded atom as the reaction mechanism. We also introduce the necessary BAI stereodescriptors parvo and amplo. Based on an extended polytope formalism of molecular structure and stereoisomerization, BAI-driven akamptisomerization is shown to be the final fundamental type of conformational isomerization.

异构是一个基本的化学概念，反映了一个事实，即原子在分子实体中的排列对其化学和物理性质有深远的影响。在这里，我们描述了通过四个类固醇的反式（BF）O（BF）-喹喔啉卟啉的四个已解析立体异构体，构象异构的先前未分类的基本形式。它们包括两对对映异构体，它们表现出现有IUPAC命名法和术语中无法描述的结构关系。它们在104±2 kJ mol ^-1的势垒上经历热非对映异构体互变，我们称之为“ akamptisomerization”。通过密度泛函理论计算，得出了可能的合成产物与反应中间体之间可行的相互转化过程，并将单键原子上的键角倒置（BAI）确定为反应机理。我们还介绍了必需的BAI立体声描述符parvo和amplo。基于分子结构和立体异构化的扩展多表位形式化，BAI驱动的akamptisomerization被证明是构象异构化的最终基本类型。