Investigations into a Stoichiometrically Equivalent Intermolecular Oxidopyrylium [5 + 2] Cycloaddition Reaction...

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.