Photocatalyst flips common sugars to rare ones

Rare sugars such as D-tagatose and D-allose are in demand as both chemical feedstocks and building blocks in the pharmaceutical industry. But they’re called rare because they don’t occur commonly in nature. Chemists would like to source these rare sugars from biomass, but it’s complicated to transform the common sugars in those natural sources into their rare counterparts.

Now, Alison Wendlandt and coworkers at the Massachusetts Institute of Technology have found a straightforward way to switch up the stereochemistries of common sugars, turning them into rare ones (Nature 2020, DOI: 10.1038/s41586-020-1937-1).

Converting between sugars usually involves isomerization, such as the aldose-ketose isomerization, but in some cases it requires epimerization, changing the stereochemistry of a single carbon center.

In the new work, Wendlandt and her team used a blue light–emitting diode, an amine, a photocatalyst, and a thiol to perform site-selective epimerization reactions at 63–88% yields, which are higher than those of enzymatic reactions or previous chemical methods.

The reaction occurs through two steps in one reaction pot. First an amine radical formed by the photocatalyst pulls off a specific hydrogen atom from the sugar molecule, and then the cysteine thiol transfers a new H atom to the same carbon on the sugar but in a different stereochemical position (shown). The researchers can break a specific C–H bond on common sugars with this method. The basis for this selectivity is not fully understood, Wendlandt says. Previous chemical methods required chemists to add protecting groups on the C–H bonds that they were not targeting for epimerization. The new method is more direct and saves time and energy in synthesizing these rare sugars.

Many chemists avoid using rare sugars because they’re hard to access, but this chemistry may change that, says Adriaan Minnaard, an organometallic chemist from the University of Groningen. This work also opens a robust new synthetic platform that will help steer further research, says Núria López, a catalytic chemist at the Institute of Chemical Research of Catalonia.