Glycals are unsaturated sugars having an enolic double bond present inside the ring and act as a versatile synthon for the synthesis of natural products and biologically important molecules. The lone pair of endocyclic ring oxygen which is in conjugation with the enolic double bond alters the reactivity of both unsaturated carbons of glycals and directs the selectivity of the incoming group at a particular center. Since the enolic double bond is very much prone to undergo various types of reactions such as epoxidation, addition, formylation, nitration halogenation, etc.; and such type of activated glycals or the derivatives of glycal have been extensively utilized as a precursor for the synthesis of branched sugars. 2‐haloglycals have emerged as an important building block for the functionalization at C‐2 position of glycals via metal‐catalyzed cross‐coupling reactions such as Heck, Suzuki, Sonogashira and carbonylative cross‐coupling reactions. In this review, we have summarized the recent progress on the utilization of 2‐haloglycals for the synthesis of 2‐C ‐branched sugars and further transformation of branched sugars into annulated sugars, heterocycles, and glycoconjugates.

中文翻译：

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2-卤代糖作为2-C-支链糖的“合成子”：有机合成的最新进展和应用

二醇是不饱和的糖，其在环内存在烯键式双键，并用作合成天然产物和生物学上重要分子的通用合成子。与烯醇双键共轭的环内环氧的孤对改变了糖基的两个不饱和碳的反应性，并将进入的基团的选择性引导在特定的中心。由于烯键式双键非常容易发生各种类型的反应，例如环氧化，加成，甲酰化，硝化卤化等；并且这种类型的活化的糖基或糖基的衍生物已被广泛用作合成支链糖的前体。2-卤代糖已成为通过金属催化的交叉偶联反应（例如Heck，Suzuki，Sonogashira和羰基交叉偶联反应）使糖在C-2位官能化的重要组成部分。在这篇综述中，我们总结了利用2-卤代糖合成2-羟基糖的最新进展。C分支糖，并将分支糖进一步转化为环状糖，杂环和糖缀合物。