[4+2]/HyBRedOx Approach to C-Naphthyl Glycosides: Failure in the Projuglone Series and Reinvestigation of the HyBRedOx Sequence
Résumé
C-Naphthyl glycosides displaying a 1,5-difunctionality on the naphthalene ring that can undergo oxidation to bromonaphthoquinone are key intermediates in the synthesis of natural C-aryl glycoside analogues. In this area, sugar-modified derivatives are of specific interest, but their synthesis is challenging. The de novo access to such compounds has been investigated through a [4+2] heterocycloaddition route, previously validated in a model series. For this purpose, two new dienophiles, conveniently protected at the phenolic positions, were synthesized. From an extensive study of their reactivity towards a range of 4-hetero-substituted (''prosugar'') heterodienes, the expected heteroadducts were stereoselectively obtained in acceptable yields. Application of the hydroboration/reduction/oxidation sequence did not afford the target C-glycosides from the reduced adducts. The negative effect of the conformational bias of the substrate on this tandem reaction is discussed. ((C) Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009)