Synthesis of aryl-substituted 2-methoxyphenol derivatives from maltol-derived oxidopyrylium cycloadducts through an acid-mediated ring contraction cascade

Carbocycloaddition Strategies for Troponoid Synthesis

Tropone is the prototypical aromatic 7-membered ring, and can be found in virtually any undergraduate textbook as a key example of non-benzenoid aromaticity. Aside from this important historical role, tropone is also of high interest as a uniquely reactive synthon in complex chemical synthesis as well as a valuable chemotype in drug design. More recently, there has been growing interest in the utility of tropones for catalysis and material science. Thus, synthetic strategies capable of synthesizing functional tropones are key to fully exploiting the potential of this aromatic ring system. Cycloaddition reactions are particularly powerful methods for constructing carbocycles, and these strategies in turn have proven to be powerful for generating troponoids. The following review article provides an overview of strategies for troponoids wherein the 7-membered carbocycle is generated through a cycloaddition reaction. Representative examples of each strategy are also provided.

Investigations into a Stoichiometrically Equivalent Intermolecular Oxidopyrylium [5 + 2] Cycloaddition Reaction Leveraging 3-Hydroxy-4-pyrone-Based Oxidopyrylium Dimers

Oxidopyrylium [5 + 2] cycloaddition reactions are powerful strategies for constructing complex bicyclic architectures. However, intermolecular cycloadditions of oxidopyrylium ylides are limited due to competing dimerization processes; consequently, high equivalents of dipolarophiles are often used to help intercept the ylide prior to dimerization. Recent studies by our lab have revealed that oxidopyrylium dimers derived from 3-hydroxy-4-pyrones are capable of reverting back to ylides in situ and as a result can be used as clean oxidopyrylium ylide sources. The following manuscript investigates intermolecular cycloaddition reactions between 3-hydroxy-4-pyrone-derived oxidopyrylium dimers and stoichiometrically equivalent ratios of alkyne dipolarophiles under thermal conditions. With certain reactive alkynes, pure cycloadducts can be obtained following a simple evaporation of the solvent, which is a benefit of the completely atom-economical reaction conditions. However, when less reactive alkynes are used the yields suffer due to a competing dimer rearrangement. Finally, when reactive-yet-volatile alkynes are used, such as methyl propiolate, competing 2:1 ylide/alkyne cycloadducts are observed. Intriguingly, these complex cycloadducts, which can be obtained in good yields from the pure cycloadducts, form with high regio- and stereoselectivities; however, both the regio-and stereoselectivities differ remarkably based on the source of the oxidopyrylium ylide.