Pyridones tethered by a four-carbon chain at the 3-
and 6’-positions can undergo [4 + 4]
photocycloaddition
to simultaneously form three, but not four, tetrasubstituted carbons.
In a study directed at the taxanes,
photocycloaddition of 28 was found to be fully controlled by
a (tert-butyldimethylsilyl)oxy group on the tether,
to
give a photoproduct with five stereogenic centers and both quaternary
carbons found in taxol (paclitaxel). This
photoproduct proved to be unstable to silica gel, but saturation of one
alkene gave a stable product (30).
Epoxidation
of the enol ether of 30 proceeds exclusively from one face
of the cyclooctene ring, demonstrating the facial bias of
the polycyclic system and correctly introducing the C-2 stereogenic
center of taxol. As part of this study, 4-methoxy-2-pyridones, known to be inert to [4 + 4] photodimerization, were
found to undergo [4 + 4] photocycloaddition
with 4-unsubstituted-2-pyridones both inter- and
intramolecularly.
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