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Synthesis

Vitamin Boosts Cascade Synthesis

Tandem Reactions: Riboflavin plays a dual photocatalyst role in sequential olefin isomerization and cyclization reactions

by Stephen K. Ritter
January 18, 2016 | APPEARED IN VOLUME 94, ISSUE 3

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.

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