A variety of chiral nine‐membered N‐heterocycles were prepared in moderate to good yields with high diastereoselectivity through a silver(I)‐catalyzed [3+2] cycloaddition and [3,3]‐rearrangement of N‐vinyl‐α, β‐unsaturated nitrones and chiral 3‐propioloyloxazolidin‐2‐ones. Experimental studies showed that silver catalyst promoted the cycloaddition and rearrangement process, and the stereochemistry of the nine‐membered N‐heterocycles was controlled via [3,3]‐rearrangement by chiral oxazolidinone‐auxiliary through a boat‐like transition state. Moreover, the obtained nine‐membered N‐heterocycle diastereomers were converted to chiral pyrrolizines with high diastereoselectivity and pyrrolizine carboxylate was obtained in 54% yield with 90% ee by the removal of chiral auxiliary.
Nitrones, served as one of the most important 1,3-dipoles in organic synthetic chemistry, can participate in various organic synthetic reactions to synthesize various heterocyclic compounds, such as nucleophilic addition, cycloaddition reaction, rearrangement reaction, C-H bond activation, etc. They are very important organic building blocks for the further construction of complex molecules. In recent years, α,β-unsaturated nitrone has attracted widespread attention from synthetic chemists because of its α,β-unsaturated bond containing rich chemical transformations, and can take part in various new chemical conversions to construct new structural heterocyclic compounds. The new strategies for the preparation of α,β-unsaturated nitrones in the past ten years, and their applications to construct nitrogen heterocyclic compounds, including O-transfer reaction, addition of nucleophiles and radical reagents, and various cycloaddition reactions, are reviewed.
Nine-membered N-heterocycle scaffolds are widely found in natural products and biologically active molecules. The rigid structure of the medium-sized ring plays important roles in the regulation of their biological and pharmacological activities. Therefore, nine-membered N-heterocycle is one of the most attractive medium-sized ring compounds. Because of the increasing entropy and ring extension of nine-membered ring skeleton, the synthesis of nine-membered N-heterocycles is still difficult; especially the effective control of substituent stereoselectivity for the nine-membered ring is one of the greatest challenging topics This review summarizes the novel synthetic strategies of nine-membered N-heterocycles and their applications in the synthesis of natural products and pharmaceutically active molecules containing nine-membered ring scaffold in recent years. Keywords nine-membered N-heterocycles; medium-sized ring compounds; cyclization; cycloaddition; ring expansion 氮杂环结构广泛存在天然产物和药物分子中, 是新 药设计和分子药理活性研究中的优势骨架 [1] . 中环骨架 (8~12 元环)的氮杂环化合物在自然界中广泛存在, 由 于环张力和熵增不利的影响, 中环骨架化合物的构建一 直是有机合成中极富挑战的难点之一 [2] . 其中, 九元氮 杂环化合物是最被关注的中环化合物之一 [3] . 如生物碱 antaroide [4] , (±)-cleavamine [5] , melohenine B [6] , (+)-lycoposerramine B [7] , (-)-rhazinilam [8] , palhinine A [9] 等(图 1).
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.