Vitamin Boosts Cascade Synthesis

For Jan B. Metternich and Ryan Gilmour of the University of Münster, using vitamins is improving the health of their chemical reactions. Last year, the team was looking for a better way to prepare Z olefins when they found that (–)-riboflavin, also known as vitamin B-2, is a versatile photocatalyst for irreversible E-to-Z isomerization of conjugated alkenes (C&EN, Sept. 14, 2015, page 33). The researchers have now found that riboflavin has another catalytic trick up its sleeve—it can mediate C–O bond formation in ring-closing reactions. Taken together, riboflavin’s dual capability has enabled the Münster team to develop a “one photocatalyst, n activation modes” strategy for cascade reactions (J. Am. Chem. Soc. 2015, DOI: 10.1021/jacs.5b12081). ­Generating molecular complexity via tandem reactions in which one step opens up a new avenue for the catalyst to act in a subsequent step without changing reaction conditions is becoming a powerful approach in organic synthesis, the researchers note. To demonstrate with riboflavin, they devised a one-pot sequential reaction to isomerize (E)-cinnamic acids through an energy-transfer process followed by ring-closing of the Z isomer through a single-electron transfer process to form pharmaceutically important coumarin derivatives.