Efficient and First Regio‐ and Stereoselective Direct C‐Glycosylation of a Flavanone Catalysed by Pr(OTf)3 Under Conventional Heating or Ultrasound Irradiation†
Dedicated to the centenary of the Portuguese Chemical Society on the occasion of the 6th Spanish Portuguese Japanese Organic Chemistry Symposium
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Abstract
A simple, regio‐ and stereoselective one‐step methodology for the mono‐C‐glycosylation of flavanone has been established. Naringenin was directly C‐glycosylated with D‐glucose by catalysis with rare‐earth metal triflates in acetonitrile/water. Different reaction conditions, namely the solvent system, reaction time, energy source and catalyst were studied. Commercially available lanthanide triflates were evaluated as catalysts and praseodymium triflate proved to be the most effective for this coupling reaction. The optimized reaction conditions were applied to other monosaccharidyl donors, such as D‐mannose, D‐galactose and L‐rhamnose, as well as disaccharides, such as lactose and maltose, leading regio‐ and stereoselectively to 8‐C‐glycosyl derivatives with equatorial glycosidic bonds in yields ranging from 28 to 38 %. Ultrasound irradiation was used to enhance the reaction outcome and a considerable improvement in reaction time and efficiency was achieved, allowing selective access to the target C‐glycosylflavanones in yields of 43–56 %.
