Light in synthesis

Low quantum yield is just one of many troubles with conventional photochemical reactions such as isomerization of double bonds. In addition, scalability has been quoted as an obstacle that precludes many classic photochemical reactions from entering the mainstream of synthesis. However, when they work, these reactions can be marvelous. I tip my hat off to Mulzer and co-workers who used photochemistry at a late stage of their synthesis of 17-deoxyprovidencin. The key to success here is that the absorption maximum of the Z isomer (below) lies at 306nm, whereas the E isomer absorbs below 300nm. Irradiation with UV-B light resulted in the desired Z/E isomerization. This example highlights another point, namely the need to use the right gear in these kinds of reactions: Pyrex glass is absolutely critical as it cuts off UV wavelengths below 300 nm. Therefore, once the desired E isomer is formed, it has no chance to go back to the Z form. This is very crafty. In more general terms, I think it would be interesting to look at photochemistry in constrained macrocyclic environments offered by cyclic peptides. There’s got to be a lot of unusual reactions that will pop up there.



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