Maltol- and Allomaltol-Derived Oxidopyrylium Ylides: Methyl Substitution Pattern Kinetically Influences [5 + 3] Dimerization versus [5 + 2] Cycloaddition Reactions

Oxidopyrylium
ylides are useful intermediates in synthetic organic
chemistry because of their capability of forming structurally complex
cycloadducts. They can also self-dimerize via [5 + 3] cycloaddition,
which is an oft-reported side reaction that can negatively impact
[5 + 2] cycloadduct yields and efficiency. In select instances, these
dimers can be synthesized and used as the source of oxidopyrylium
ylide, although the generality of this process remains unclear. Thus,
how the substitution pattern governs both dimerization and cycloaddition
reactions is of fundamental interest to probe factors to regulate
them. The following manuscript details our findings that maltol-derived
oxidopyrylium ylides (i.e., with ortho methyl substitution relative
to oxide) can be trapped prior to dimerization more efficiently than
the regioisomeric allomaltol-derived ylide (i.e., with a para methyl
substitution relative to oxide). Density functional theory studies
provide evidence in support of a sterically (kinetically) controlled
mechanism, whereby gauche interactions between appendages of the approaching
maltol-derived ylides are privileged by higher barriers for dimerization
and thus are readily intercepted by dipolarophiles via [5 + 2] cycloadditions.