Department of Chemistry, Stanford University, Stanford, 94305 California, United States
ACS Catal., 2016, 6, pp 4653–4659
DOI: 10.1021/acscatal.6b01501
Publication Date (Web): June 06, 2016
Copyright © 2016 American Chemical Society
*E-mail: waymouth@stanford.edu.
Abstract
The
development of new strategies for the direct catalytic
functionalization of unprotected carbohydrates would be an enabling
advance for glycoscience. Herein we report that the catalytic oxidation
of unprotected carbohydrates can be carried out selectively with
[(neocuproine)Pd(OAc)]2(OTf)2 (1) to
generate the 3-ketoses. Catalytic aerobic oxidation can be carried out
with Pd loadings as low as 1% in the presence of phenolic additives.
Catalytic oxidation of a variety of unprotected pyranosides in
acetonitrile or acetonitrile/water with Pd catalyst 1 with either
oxygen or benzoquinone selectively generates the 3-ketoses. Minor
amounts of the 4-ketoses are formed competitively, particularly in the
case of pyranosides bearing axial substituents at C4 of the pyranoside.
Catalytic oxidations can also be carried out in trifluorethanol, but for
pyranosides bearing axial substituents at C2 or C4, selective oxidation
to the 3-ketose is accompanied by epimerization to afford the
equatorial 3-ketoses. Catalytic oxidation of unprotected hexafuranosides
or sialic acid derivatives occurs selectively at the exocyclic diol or
triol in trifluoroethanol to generate exocyclic hydroxyketones.
development of new strategies for the direct catalytic
functionalization of unprotected carbohydrates would be an enabling
advance for glycoscience. Herein we report that the catalytic oxidation
of unprotected carbohydrates can be carried out selectively with
[(neocuproine)Pd(OAc)]2(OTf)2 (1) to
generate the 3-ketoses. Catalytic aerobic oxidation can be carried out
with Pd loadings as low as 1% in the presence of phenolic additives.
Catalytic oxidation of a variety of unprotected pyranosides in
acetonitrile or acetonitrile/water with Pd catalyst 1 with either
oxygen or benzoquinone selectively generates the 3-ketoses. Minor
amounts of the 4-ketoses are formed competitively, particularly in the
case of pyranosides bearing axial substituents at C4 of the pyranoside.
Catalytic oxidations can also be carried out in trifluorethanol, but for
pyranosides bearing axial substituents at C2 or C4, selective oxidation
to the 3-ketose is accompanied by epimerization to afford the
equatorial 3-ketoses. Catalytic oxidation of unprotected hexafuranosides
or sialic acid derivatives occurs selectively at the exocyclic diol or
triol in trifluoroethanol to generate exocyclic hydroxyketones.
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