Skip to main content
Chemistry LibreTexts

VI. Summary

  1. Last updated
  2. Save as PDF
  • Page ID
    24623

    \( \newcommand{\vecs}[1]{\overset { \scriptstyle \rightharpoonup} {\mathbf{#1}} } \) \( \newcommand{\vecd}[1]{\overset{-\!-\!\rightharpoonup}{\vphantom{a}\smash {#1}}} \)\(\newcommand{\id}{\mathrm{id}}\) \( \newcommand{\Span}{\mathrm{span}}\) \( \newcommand{\kernel}{\mathrm{null}\,}\) \( \newcommand{\range}{\mathrm{range}\,}\) \( \newcommand{\RealPart}{\mathrm{Re}}\) \( \newcommand{\ImaginaryPart}{\mathrm{Im}}\) \( \newcommand{\Argument}{\mathrm{Arg}}\) \( \newcommand{\norm}[1]{\| #1 \|}\) \( \newcommand{\inner}[2]{\langle #1, #2 \rangle}\) \( \newcommand{\Span}{\mathrm{span}}\) \(\newcommand{\id}{\mathrm{id}}\) \( \newcommand{\Span}{\mathrm{span}}\) \( \newcommand{\kernel}{\mathrm{null}\,}\) \( \newcommand{\range}{\mathrm{range}\,}\) \( \newcommand{\RealPart}{\mathrm{Re}}\) \( \newcommand{\ImaginaryPart}{\mathrm{Im}}\) \( \newcommand{\Argument}{\mathrm{Arg}}\) \( \newcommand{\norm}[1]{\| #1 \|}\) \( \newcommand{\inner}[2]{\langle #1, #2 \rangle}\) \( \newcommand{\Span}{\mathrm{span}}\)\(\newcommand{\AA}{\unicode[.8,0]{x212B}}\)

    Forming a new ring by internal addition of a carbon-centered radical to a multiple bond is a powerful tool in carbohydrate synthesis. Regio­selec­tivity and stereoselectivity are vital aspects of this type of reaction. Being able to predict regioselectivity is critical because a cyclization reaction potentially can form rings of two sizes. Since the newly formed ring nearly always has an addi­tional chiral center (sometimes two), understanding stereoselectivity is essential in predicting stereo­chemistry in the cyclic product.

    Compounds with five-membered rings are the ones most often produced by radical cycli­zation. Reactions that form five-membered rings are capable of generating six-membered rings also, but rarely do so because the tran­sition state leading to the larger ring has greater ring strain. Compounds with six-membered rings are the major products when cyclization is capable of forming either six- or seven-membered rings consisting only of second row elements. Larger rings (seven or more members) are created when a radical center and a distant multiple bond are linked by a tether, usually one con­taining a silicon–oxygen bond.

    The stereoselectivity of reactions that produce five- and six-membered rings usually can be rationalized by assuming that the reaction passes through a chair-like transition state. The lowest energy transition state for such a reac­tion has as many substituents as possible in pseudoequatorial positions. A variety of factors (pseudo-1,3-diaxial interaction, allylic strain, hydrogen bonding, conformation of an existing ring) affect transition-state energy and can, on occasion, cause a boat-like transition state to be more stable than a chair-like one.

    Various types of unsaturated carbohydrates, often α,β-unsaturated esters, undergo radical cyclization. Also prominent among reactive com­pounds are those in which the radical-forming part of the molecule and the portion containing the multiple bond are connected by a silicon–oxygen tether. A third group of compounds that cyclize readily includes allyl and propargyl ethers and related compounds.

    This page titled VI. Summary is shared under a All Rights Reserved (used with permission) license and was authored, remixed, and/or curated by Roger W. Binkley and Edith R. Binkley.