Highly Stereoselective -Mannopyranosylation via the 1--Glycosyloxy-isochromenylium-4-gold(I) Intermediates

While the gold(I)-catalyzed glycosylation reaction with 4,6-O-benzylidene tethered mannosyl ortho-alkynylbenzoates as donors falls squarely into the category of the Crich-type -selective mannosylation when Ph3PAuOTf is used as the catalyst, in that the mannosyl -triflates are invoked, replacement of the -OTf in the gold(I) complex with less nucleophilic counter anions (i.e., -NTf2, -SbF6, -BF4, and (-BAr4F)) leads to complete loss of -selectivity with the mannosyl ortho-alkynylbenzoate -donors. Nevertheless, with the -donors, the mannosylation reactions under the catalysis of Ph3PAuBAr4F (BAr4F=tetrakis[3,5-bis(trifluoromethyl)phenyl]borate) are especially highly -selective and accommodate a broad scope of substrates; these include glycosylation with mannosyl donors installed with a bulky TBS group at O3, donors bearing 4,6-di-O-benzoyl groups, and acceptors known as sterically unmatched or hindered. For the ortho-alkynylbenzoate -donors, an anomerization and glycosylation sequence can also ensure the highly -selective mannosylation. The 1--mannosyloxy-isochromenylium-4-gold(I) complex (C), readily generated upon activation of the -mannosyl ortho-alkynylbenzoate (1) with Ph3PAuBAr4F at -35 degrees C, was well characterized by NMR spectroscopy; the occurrence of this species accounts for the high -selectivity in the present mannosylation.