3 research outputs found
1,2-<i>cis</i>-α-Stereoselective Glycosylation Utilizing a Glycosyl-Acceptor-Derived Borinic Ester and Its Application to the Total Synthesis of Natural Glycosphingolipids
1,2-<i>cis</i>-α-Stereoselective glycosylations
were conducted using a 1,2-anhydroglucose donor and mono-ol acceptors
in the presence of a glycosyl-acceptor-derived borinic ester. Reactions
proceeded smoothly under mild conditions to provide the corresponding
α-glycosides with high stereoselectivity in moderate to high
yields. In addition, the present method was applied successfully to
the direct glycosylation of a protected ceramide acceptor and the
total synthesis of natural glycosphingolipids (GSLs)
Glycosyl-Acceptor-Derived Borinic Ester-Promoted Direct and β‑Stereoselective Mannosylation with a 1,2-Anhydromannose Donor
β-Stereoselective
mannosylations were conducted using a 1,2-anhydromannose
donor and mono-ol acceptors in the presence of a glycosyl-acceptor-derived
borinic ester. Reactions proceeded smoothly under mild conditions
to provide the corresponding β-mannosides with high stereoselectivity
in moderate to high yields. In addition, the present glycosylation
method was applied successfully to the total synthesis of acremomannolipin
A
Boronic-Acid-Catalyzed Regioselective and 1,2-<i>cis</i>-Stereoselective Glycosylation of Unprotected Sugar Acceptors via S<sub>N</sub>i‑Type Mechanism
Regio- and 1,2-<i>cis</i>-stereoselective chemical glycosylation
of unprotected glycosyl acceptors has been in great demand for the
efficient synthesis of natural glycosides. However, simultaneously
regulating these selectivities has been a longstanding problem in
synthetic organic chemistry. In nature, glycosyl transferases catalyze
regioselective 1,2-<i>cis</i>-glycosylations via the S<sub>N</sub>i mechanism, yet no useful chemical glycosylations based on
this mechanism have been developed. In this paper, we report a highly
regio- and 1,2-<i>cis</i>-stereoselective S<sub>N</sub>i-type
glycosylation of 1,2-anhydro donors and unprotected sugar acceptors
using <i>p</i>-nitrophenylboronic acid (<b>10e</b>) as a catalyst in the presence of water under mild conditions. Highly
controlled regio- and 1,2-<i>cis</i>-stereoselectivities
were achieved via the combination of boron-mediated carbohydrate recognition
and the S<sub>N</sub>i-type mechanism. Mechanistic studies using the
KIEs and DFT calculations were consistent with a highly dissociative
concerted S<sub>N</sub>i mechanism. This glycosylation method was
applied successfully to the direct glycosylation of unprotected natural
glycosides and the efficient synthesis of a complex oligosaccharide
with minimal protecting groups