A new 1,2‐trans‐selective glycosylation reaction is described. Glucosyl donors protected cyclically at the C‐2 and C‐3 hydroxy groups as six‐ (butane diacetal), seven‐ (tetraisopropyldisiloxanylidene), or eight‐ (2,3‐o‐xylylene) membered fused rings were synthesized in a straightforward manner. The glycosylation reactions of the glucosyl donors with various acceptors mainly generated β‐glycosides under conventional reaction conditions. The results show that the o‐xylylene group is a suitable 1,2‐trans‐directing group from the points of view of stereoselectivity and chemical stability. A conformational study of the oxocarbenium ion of an o‐xylylene‐protected glucose derivative by NMR spectroscopy and computational simulation was carried out. The results imply that the oxocarbenium ion mainly adopts a 4H3 conformation owing to the rigid trans‐fused ring at C‐2 and C‐3, while a noncyclically protected derivative might fluctuate between conformations. These results suggest that an eclipsing interaction between the pseudoequatorial xyloxy group at C‐2 and the incoming nucleophile hampers 1,2‐cis attack.

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Diglycosyl diacylglycerols (DGDGs) are major components of Gram-positive bacterial plasma membranes and are involved in the immune response systems. The chemical synthesis of DGDGs has been highly demanded, as it will allow the elucidation of their biological functions at the molecular level. In this study, we have developed a novel β-stereodirecting 2,3-naphthalenedimethyl (NapDM) protecting group that is orthogonal to protecting groups commonly used in oligosaccharide synthesis. The NapDM group can be easily cleaved under TFA-mediated acidic conditions. Futhermore, we demonstrated the application of this protecting group to an acyl protecting-group-free strategy by utilizing the NapDM group for the synthesis of DGDGs. This strategy features the use of the βstereodirecting NapDM group as an acid-cleavable permanent protecting group and late-stage glycosylation of monoglycosyl diacylglycerol acceptors, enabling the stereoselective synthesis of three different bacterial DGDGs with unsaturated fatty acid chain(s).

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