SummaryThe core structure of the iridoid monoterpenes is formed by a unique cyclization reaction. The enzyme that catalyzes this reaction, iridoid synthase, is mechanistically distinct from other terpene cyclases. Here we describe the synthesis of two substrate analogs to probe the mechanism of iridoid synthase. Enzymatic assay of these substrate analogs along with clues from the product profile of the native substrate strongly suggest that iridoid synthase utilizes a Michael reaction to achieve cyclization. This improved mechanistic understanding will facilitate the exploitation of the potential of iridoid synthase to synthesize new cyclic compounds from nonnatural substrates.

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Herein, the synthesiso fh eteromerict etrahydroxanthone biaryls is described, aw idespread core structure of manyn atural products. Thed evelopment of both stannylation and Stille coupling procedureso fb ase-sensitivet etrahydroxanthones enabled their coupling with benzene derivatives as well as with xanthenes.T hese methodsp rovide access to structures that are analogous to parnafungins as well as to dimeric compounds similartos ecalonic acids or phomoxanthones. Lindner et al. Figure2.Successfully synthesized stannanes starting from the correspondingbromides or iodides.Scheme 2. Stannylation of bromide 13.Y ields are based on tetrahydroxanthone bromide 13.

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