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IV. Three-Step Sequential Reactions

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    23927

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    Three-step sequential reactions often consist primarily of combinations of cyclization and β-frag­mentation steps.56,73–82 One group of such reactions involves ring opening of cyclic acetals.73–77 Since these reactions contain more frag­men­tation than cyclization, they tend to produce less compli­cated structures (e.g., compounds with fewer rings). In the reaction shown in Scheme 12, for example, internal radical addition to a cyano group creates a nitrogen-centered radical. This cyclization then is followed by suc­cess­ive β‑fragmentation steps. Together these elementary reac­tions cause acetal ring opening.73

    s12.png

    Balancing the advantage gained against additional effort needed to prepare the starting materials and adjust the reaction condi­tions is always a consideration when conducting a sequential reaction. Such consideration motivated the prepar­ation of the acetal 6 (Scheme 12),73–75 when the effort neces­sary to synthesize 7, the first acetal used in this type of reaction, signif­icantly offset the advantage derived from the sequential process.73,76,77

    The synthetic potential of three-step sequential reactions changes when there is more cycli­za­tion than fragmentation. When such a change occurs, reactions can be used to build more com­plex structures.78–81 Specifically, the process shown in Scheme 13, which contains only cyclization steps, generates three new rings in a single reaction.81

    s13.png


    This page titled IV. Three-Step Sequential Reactions 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.

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