The combined action of boronic acid, photoredox catalyst and hydrogen atom transfer mediator enables the transformation of furanosides to 2-keto-3-deoxyfuranosides, a synthetic analog of the process catalyzed by the ribonucleotide reductase enzymes.

The first total syntheses of punicafolin (1) and macaranganin (2) were achieved in seven steps, respectively, from commercial α-d-glucose. The characteristic features of the synthesis are (1) sequential site-selective introduction of the adequate galloyl groups into unprotected d-glucose by a catalyst-controlled manner and (2) stereodivergent construction of the 3,6-HHDP bridge by oxidative phenol coupling of a common intermediate via a ring-flipping process of the glucose core. Because no protective groups were used for glucose throughout the process, extremely short-step total syntheses of natural glycosides 1 and 2 (MW 938) were performed.

An S N 2 mechanism was proposed for highly stereoselective glycosylation of benzoic acid with unprotected α-Dglucose under Mitsunobu conditions in dioxane, while an S N 1 mechanism was indicated for nonstereoselective glycosylation in DMF. The S N 2-type stereoselective Mitsunobu glycosylation is generally applicable to various unprotected pyranoses as glycosyl donors in combination with a wide range of acidic glycosyl acceptors such as carboxylic acids, phenols, and imides, retaining its high stereoselectivity (33 examples). Glycosylation of a carboxylic acid with unprotected α-D-mannose proceeded also in an S N 2 manner to directly afford a usually less accessible 1,2-cis-mannoside. One-or two-step total syntheses of five simple natural glycosides were performed using the glycosylation strategy presented here using unprotected α-D-glucose.

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