Photochemical [2 + 2] Cycloaddition of Alkenyl Boronic Derivatives: An Entry into 3‑Azabicyclo[3.2.0]heptane Scaffold

The synthesis of
3-azabicyclo[3.2.0]­heptyl boropinacolates and
trifluoroborates via the [2 + 2] photocycloaddition
of the corresponding alkenyl boronic derivatives and maleimides or
maleic anhydride is described. Optimization of the reaction conditions
(i.e., wavelength, concentration of the reagents,
photosensitizer) was carried out, and the scope and limitations of
the method were studied. Alkenyl boronic acid pinacolates were found
to be more suitable for the [2 + 2] cycloaddition, providing better
reaction outcomes compared to the trifluoroborates. The utility of
this approach was shown by the preparation of bi- and trifunctional
building blocks (21 examples), which could be easily synthesized on
up to 60 g scale. These cycloadducts provide a convenient entry into
the 3-azabicyclo[3.2.0]­heptane scaffold through the C–C coupling
or oxidative deborylation reactions.