Changes in the concentration of reagents (0.009–0.2 M) have been shown to dramatically effect the yield and stereoselectivity of glycosylation with a sialic acid based glycosyl donor in a complex nonlinear manner that correlates with changes in the structures of the supramers of the reagents. The yield of disaccharide gradually increases with concentration and levels off at concentrations of glycosyl donor higher than 69 mM. The ratio of anomers is very high at some concentrations (α/β ≈ 20:1), moderate (α/β ≈ 8:1) or very low (α/β ≈ 4:1) at others. The formation of mixed supramers of glycosyl donor and glycosyl acceptor at concentrations exceeding 69 mM was detected by polarimetry and laser light scattering.
Ni(II)
complexes containing (S)-o-[N-(N-benzylprolyl)amino]benzophenone
as an auxiliary chiral moiety in the form of a Schiff base with α-amino
acids (α-amino acid = glycine, alanine, dehydroalanine; Gly-Ni, Ala-Ni, and Δ-Ala-Ni) were subjected
to various types of electrochemical activation (oxidation, reduction,
and a treatment with electrogenerated base), affording regio- and
diastereoselective synthesis of novel types of binuclear Ni(II) complexes
via C–C coupling. New compounds were fully characterized by
HRMS, MALDI-TOF, CD, and 1H and 13C NMR (including
two-dimensional techniques) spectroscopy; two complexes were characterized
by X-ray diffraction analysis. The structures of the novel complexes
obtained via electrosynthesis completely match the predictions (made
from preliminary voltammetric investigations of the starting complexes
as well as from DFT estimations of the energy and symmetry of their
frontier molecular orbitals) about the nature of chemical transformations
which may follow the electron transfer steps. Electrochemical oxidation
of Gly-Ni and Ala-Ni allows access to new
dimeric complexes linked via benzophenone moieties in the Ni(II) coordination
environment. These new binuclear Ni(II) complexes are of interest
as chiral redox mediators for both oxidative and reductive transformations,
since they exhibit quasi-reversible electrochemical behavior (their
reduced and oxidized forms are stable, at least on the time scale
of cyclic voltammetry). Three other binuclear Ni(II) complexes which
were obtained via reductive dimerization of the Δ-Ala-Ni complex, via nucleophilic addition of electrochemically deprotonated Gly-Ni to Δ-Ala-Ni, and via oxidative electrochemical
dimerization of deprotonated Gly-Ni are of interest as
convenient precursors for the stereoselective preparation of diamino
dicarboxylic acids HO(O)CCH(NH2)(CH2)
n
(NH2)CHC(O)OH (n = 2–0),
since the obtained binuclear Ni(II)–Schiff base complexes can
be easily disassembled using aqueous HCl in methanol.
A pyranose ring contraction
of ethyl 1-thio-β-d-galactopyranosides has been discovered
that proceeds with retention of aglycon under mildly acidic conditions
(aq TFA in CH2Cl2). Key factors for success
of this rearrangement are the presence of bulky silyl (TIPS or TBDPS)
substituents at both O-2 and O-3 and a free hydroxy
group at C-4 (derivatives with acid-labile protective groups at O-4
will also engage in this reaction). The rearrangement cleanly proceeds
for 2,3-di-O-TIPS derivatives with two hydroxy groups
at C-4 and C-6, acid-labile TES groups at O-4 and O-6, or one acyl
substituent (Bz, ClAc) at O-6. A possibility to switch the direction
of the debenzylidenation reaction in 4,6-O-benzylidene-2,3-di-O-TIPS/TBDPS derivatives by the choice of an acid (TFA,
which cleanly gives furanose, versus AcOH, which cleaves benzylidene
acetal only) may present an advantage in the divergent synthesis of
selectively protected glycosyl donors (either in furanose or pyranose
form) useful for the synthesis of biologically important oligosaccharides.
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