The title natural products (1 and 2, respectively) have been synthesized by Au(I)-catalyzed intramolecular hydroarylation (IMHA) of the relevant aryl propiolate esters (e.g., 13), which were themselves formed by reaction of the corresponding phenols with either 3-(trimethylsilyl)propiolic acid or propiolic acid and N-(3-dimethylaminopropyl)-N'-ethylcarbodiimide hydrochloride or dicyclohexylcarbodiimide. (±)-Purpurasol (3) was readily derived from fraxetin (2) by established procedures.
Methods are reported for the efficient assembly of a series of phenol-derived propiolates, including the parent system 56, and their Au(I)-catalyzed cyclization (intramolecular hydroarylation) to give the corresponding coumarins (e.g., 1). Simple syntheses of natural products such as ayapin (144) and scoparone (145) have been realized by such means, and the first of these subject to single-crystal X-ray analysis. A related process is described for the conversion of propargyl ethers such as 156 into the isomeric 2H-chromene precocene I (159), a naturally occurring inhibitor of juvenile hormone biosynthesis.
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