The alkene-isocyanate cycloaddition method affords β-lactams from glycals with high regio- and stereoselectivity, but the factors that determine substrate reactivity are poorly understood. Thus, we synthesized a library of 17 electron-rich alkenes (glycals) with varied protecting groups to systematically elucidate the factors that influence their reactivity toward the electron-poor trichloroacetyl isocyanate. The experimentally determined reaction rates exponentially correlate with the computationally determined highest occupied molecular orbital–lowest unoccupied molecular orbital (HOMO–LUMO) gap and natural bond orbital (NBO) valence energies. The electron-withdrawing ability of the protecting groups, but not bulk, impacts the electron density of the glycal allyloxocarbenium system when oriented pseudo-axially (i.e., stereoelectronics). In this conformation, ring σ_C–O* orbitals oriented antiperiplanar to the allyloxocarbenium system decrease glycal reactivity via negative hyperconjugation as protecting group electron withdrawal increases. Transition-state calculations reveal that protecting group stereoelectronics direct the reaction to proceed via an asynchronous one-step mechanism through a zwitterionic species. The combined experimental and computational findings, along with experimental validation on an unknown glycal, provide insight on the reaction mechanism and the role of distant protecting groups in glycal reactivity. Together, these studies will aid in the synthesis of new β-lactam antibiotics, β-lactamase inhibitors, and bicyclic carbohydrate-β-lactam monomers prepared by the alkene-isocyanate method.

中文翻译：

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β-内酰胺合成指南：糖基保护基决定了立体电子学和[2 + 2]环加成动力学。

烯烃-异氰酸酯环加成法可从糖基中获得具有高区域选择性和立体选择性的β-内酰胺，但决定底物反应性的因素知之甚少。因此，我们合成了具有不同保护基的17个富电子烯（糖）的文库，以系统地阐明影响它们对贫电子三氯乙酰基异氰酸酯的反应性的因素。实验确定的反应速率与计算确定的最高占据分子轨道-最低未占据分子轨道（HOMO-LUMO）间隙和自然键轨道（NBO）价能成指数关系。当取向为假的时，保护基团的吸电子能力而不是本体的吸电子能力影响糖基烯丙基氧碳鎓系统的电子密度。-轴向（即立体声电子学）。按照这种构型，环σC _–O*随着保护基团电子吸收的增加，相对于烯丙基氧碳鎓系统反平面定向的轨道通过负超共轭作用降低了糖反应性。过渡态计算表明，保护基团立体电子引导反应通过两性离子物质通过异步一步机制进行。实验和计算结果的结合，以及对未知糖基的实验验证，提供了对反应机理以及远距离保护基在糖基反应性中的作用的深入了解。总之，这些研究将有助于合成新的β-内酰胺抗生素，β-内酰胺酶抑制剂和通过烯烃-异氰酸酯方法制备的双环碳水化合物-β-内酰胺单体。