Pyridones tethered by a four-carbon chain at the 3- and 6'-positions c
an undergo [4 + 4] photocycloaddition to simultaneously form three, bu
t not four, tetrasubstituted carbons. In a study directed at the taxan
es, photocycloaddition of 28 was found to be fully controlled by a (te
rt-butyldimethylsilyl)oxy group on the tether, to give a photoproduct
with five stereogenic centers and both quaternary carbons found in tax
ol (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 cyc
looctene 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] ph
otodimerization, were found to undergo [4 + 4] photocycloaddition with
4-unsubstituted-2-pyridones both inter- and intramolecularly.