The alkene-isocyanate cycloaddition
method affords β-lactams
from glycals with high regio- and stereoselectivity, but the factors
that determine substrate reactivity are poorly understood. Thus, we
synthesized a library of 17 electron-rich alkenes (glycals) with varied
protecting groups to systematically elucidate the factors that influence
their reactivity toward the electron-poor trichloroacetyl isocyanate.
The experimentally determined reaction rates exponentially correlate
with the computationally determined highest occupied molecular orbital–lowest
unoccupied molecular orbital (HOMO–LUMO) gap and natural bond
orbital (NBO) valence energies. The electron-withdrawing ability of
the protecting groups, but not bulk, impacts the electron density
of the glycal allyloxocarbenium system when oriented pseudo-axially (i.e., stereoelectronics). In this conformation, ring σC–O* orbitals oriented antiperiplanar to the allyloxocarbenium
system decrease glycal reactivity via negative hyperconjugation as
protecting group electron withdrawal increases. Transition-state calculations
reveal that protecting group stereoelectronics direct the reaction
to proceed via an asynchronous one-step mechanism through a zwitterionic
species. The combined experimental and computational findings, along
with experimental validation on an unknown glycal, provide insight
on the reaction mechanism and the role of distant protecting groups
in glycal reactivity. Together, these studies will aid in the synthesis
of new β-lactam antibiotics, β-lactamase inhibitors, and
bicyclic carbohydrate-β-lactam monomers prepared by the alkene-isocyanate
method.