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Biosynthesis of Vitamin B2: A Unique Way to Assemble a Xylene Ring

The biosynthesis of one riboflavin (vitamin B2) molecule requires one molecule of GTP and two molecules of ribulose 5‐phosphate as substrates. In the final step, the tricyclic isoalloxazine chromophore, which is the hallmark of flavocoenzymes, arises from a highly unusual dismutation of bicyclic 6,7‐dimethyl‐8‐ribityllumazine that is catalyzed by riboflavin synthase but can also proceed without catalysis. The reaction proceeds via a pentacyclic adduct of two 6,7‐dimethyl‐8‐ribityllumazine molecules, whose cleavage into riboflavin and a pyrimidine derivative (by a sequence of two elimination steps) is mechanistically straightforward. Recently, the formation of the pentacyclic adduct has been proposed to involve a hydride transfer step followed by a [4+2] cycloaddition. Surprisingly, two different classes of riboflavin synthases utilize different diastereomers of the pentacyclic adduct, but the newly generated chiral centers are lost upon the intermediates' subsequent fragmentation.