Hydrogen atom transfer experiments provide chemical evidence for the conformational differences between C- and O-glycosides

Article abstract of DOI:10.1016/j.tetlet.2007.05.166

The regioselectivity of the hydrogen atom transfer (HAT) reaction promoted by alkoxyl radicals generated from 3-hydroxypropyl α-d-mannopyranoside derivative (O-glycoside) and 2,6-anhydro-d-glycero-d-manno-decitol derivative (C-glycoside) is studied. The O

Full text of DOI:10.1016/j.tetlet.2007.05.166

Tetrahedron Letters 48 (2007) 5507–5511
Hydrogen atom transfer experiments provide chemical evidence
for the conformational differences between C- and O-glycosides
*
´
´
Angeles Martın, Luis M. Quintanal and Ernesto Suarez
Instituto de Productos Naturales y Agrobiologı´a del C.S.I.C., Carretera de La Esperanza 3, 38206 La Laguna, Tenerife, Spain
Received 15 November 2006; revised 24 May 2007; accepted 30 May 2007
Available online 5 June 2007
Abstract—The regioselectivity of the hydrogen atom transfer (HAT) reaction promoted by alkoxyl radicals generated from 3-
hydroxypropyl a-D-mannopyranoside derivative (O-glycoside) and 2,6-anhydro-D-glycero-D-manno-decitol derivative (C-glycoside)
is studied. The O-glycoside model abstracts preferentially the hydrogen atom at C-5 (1,8-HAT) while the C-glycoside abstracts the
hydrogen atom at C-1 (1,6-HAT) but no abstraction at C-5 could be detected. These results are explained by the stereoelectronic
control exerted by the exo-anomeric effect in the O-glycoside.
Ó 2007 Elsevier Ltd. All rights reserved.
Intramolecular hydrogen atom transfer (HAT) is a
6
.
O
OMe
OMe
O
OMe
OMe
radical-mediated process, which has interesting applica-
tions in synthetic organic chemistry since it can be prof-
itably employed for the selective functionalization of
molecular positions considered to be unreactive under
other, more classical conditions.1 The reaction, when
promoted by alkoxyl radicals, occurs most frequently
through a chair-like six-membered transition state
(TS),2 although some exceptional cases of 1,6-HAT
reactions that proceed via a seven-membered TS are
known.3
O
HO
MeO
MeO
MeO
MeO
H
.
O
O
O
O
5'
OMe
OMe
OMe
OMe
MeO
MeO
A
B
PhtNO
H
O
MeO
MeO
5
1
X
OMe
MeO
In a recent paper from this laboratory, a rare 1,8-HAT,
promoted by alkoxyl radicals, between two glucopyra-
nose units in a a-D-Glcp-(1!4)-b-D-Glcp disaccharide
model was described (Scheme 1).4 The alkoxyl radical
A, generated from the alcohol at C-6 under oxidative
or reductive conditions, abstracts the hydrogen atom
at C-50 through a nine-membered transition state in a
highly efficient and regioselective manner to give C-radi-
cal B. The reaction is supposedly favored by the exo-
anomeric effect, and consequently, the glycosidic bond
adopts a preferred syn conformation (UH = ꢀ30.0°,
WH = ꢀ32.0°) with the alkoxyl radical positioned at a
1 X = O
2 X = CH2
Scheme 1. HAT between glucopyranose units in a disaccharide model.
On the other hand, the chemistry of C-glycosides has
recently attracted considerable interest from the point
of view of carbohydrate mimetics and in view of the
increased detection of the C-glycosidic bond in natural
products.6
˚
suitable distance (ꢁ3 A) from the hydrogen atom to be
Due to the numerous biochemical applications of C-gly-
cosides, especially as enzyme inhibitors, the comparison
of the conformational behavior of C-glycosides with the
naturally occurring O-counterparts is an interesting
research topic.7 The conformational studies have so
far been carried out by using physical methods, particu-
larly NMR spectroscopy (NOE analysis and coupling
abstracted.5
*
Corresponding author. Tel.: +34 922251004; fax: +34 922260135;
e-mail: esuarez@ipna.csic.es
0040-4039/$ - see front matter Ó 2007 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2007.05.166

5508
A. Martı´n et al. / Tetrahedron Letters 48 (2007) 5507–5511
constants data) in combination with molecular mechan-
ics and ab initio calculations.7a
The alkoxyl radicals were generated under reductive
conditions by treatment of the O-phthalimido deriva-
tives with n-Bu3SnH/AIBN and n-Bu3SnD/AIBN in
benzene solutions.9 In order to acquire additional
insight into the HAT reaction mechanism, the alkoxyl
radicals in the case of the C-glycosides were also pre-
pared under oxidative conditions, by visible light irradi-
ation of the alcohol with hypervalent iodine reagents in
the presence of iodine.
With our results on the 1,8-HAT reaction in mind, we
sought to develop a radical process, which could provide
chemical evidence for the conformational differences
between C- and O-glycosides. As far as we know, studies
of this type have not been reported to date.
To probe the entropic barrier resulting from flexibility
around glycosidic bonds in going from the more flexible
C-glycosides to O-glycosides it was necessary to synthe-
size a relatively simple model system. In designing 3-
hydroxypropyl a-D-mannopyranoside derivative 1 and
2,6-anhydro-D-glycero-D-manno-decitol derivative 2 as
models for the HAT reaction, we were guided by a num-
ber of considerations. Firstly, a a-D-manno configura-
tion of the sugar would ideally permit hydrogen
abstraction from almost all positions of the pyranose
ring (specifically from C-1, C-2, C-3, and C-5). Sec-
ondly, none of the three staggered conformations
The synthesis of O-phthalimido derivatives was readily
achieved from the corresponding alcohol and N-hydroxy-
phthalimide under Mitsunobu conditions in accordance
with a previously described protocol.10
The results of the reductive HAT reaction for the O-gly-
cosides are outlined in Scheme 2.11 Under reductive
conditions phthalimide 1 was transformed into three
compounds that were characterized after acetylation to
facilitate their chromatographic separation:12 b-L-gulo-
pyranoside derivative 3 formed by the hydrogen abstrac-
tion process and inversion of the configuration at C-5
(18%), b-D-mannopyranoside derivative 5 which, evi-
dently arises from hydrogen abstraction, inversion of
the radical at C-1, and axial quenching by the stannane
(13%),13 and compound 4 with the starting a-D-manno-
pyranoside configuration (29%). This last compound
may be formed by three different mechanisms: abstrac-
tion and retention of configuration at C-5 or at C-1 or
simply by a failed in the hydrogen abstraction process
and a reduction of the O-radical prior to the abstraction
reaction, or by a combination of these. Repetition of
the reduction of phthalimide 1 with n-Bu3SnD shed
more light on the mechanism. Analysis of the isotopic
distribution in compounds 6 and 8 showed a complete
substitution by deuterium at C-5 and C-1, respectively.14
On the other hand, only 30% of deuterium labeling was
4
around UH angle for the C1 chair (exo–syn, exo–anti,
and non-exo) should undergo substantial steric interac-
tion between the axial substituent at C-2 and aglycon.
The influence of this interaction on the conformational
equilibrium seems to be especially important in the
absence of additional stereoelectronic effects, as in the
case of C-glycosides.7a Thirdly, the aglycon should be
a simple n-alkyl alcohol tether of five atoms, with the
least possible conformational constraints, to avoid, as
much as possible, destabilizing steric interactions in
the HAT transition state.
In addition we have also synthesized tetraacetate 9; the
presence of such electron withdrawing groups (EWG)
should deactivate the molecular hydrogen atoms for
the abstraction by the electrophilic alkoxyl radicals.8
AcO
O
AcO
PhtNO
AcO
O
MeO
MeO
MeO
MeO
MeO
MeO
MeO
MeO
i)
i)
n
n
-Bu3SnH/AIBN
or
R
R
R
O
O
1'
5
1
O
O
O
O
+
-Bu3SnD/AIBN
+
OMe
OMe
OMe
OMe
ii) Ac2O/py
MeO
MeO
MeO
MeO
3 R = H 18%
4 R = H 29%
7 R = D(H) (3:7) 30%
5 R = H 13%
1
6
8
R = D 14%
R = D 13%
PhtNO
O
AcO
AcO
O
i)
i)
n
-Bu3SnH/AIBN
AcO
AcO
AcO
AcO
R
R
or
O
O
n-Bu3SnD/AIBN
O
O
+
OAc
AcO
OAc
AcO
OAc
ii) Ac2O/py
AcO
AcO
AcO
11 R = H 58%
13 R = D(H) (3:7) 60%
9
10 R = H 6%
12 R = D 8%
HO
PhtNO
O
HO
MeO
MeO
MeO
MeO
i)
i)
n
-Bu3SnH/AIBN
or
-Bu3SnD/AIBN
R
H
O
O
1'
5
1
+
n
R
OMe
MeO
OMe
MeO
OMe
MeO
MeO
MeO
2
14 R = H 40%
16 R = D 32%
15 R = H 59%
17 R = D(H) (4:1) 61%
Scheme 2. Hydrogen atom transfer in a-O- and C-glycosides under reductive conditions. PhtN = phthalimide.

A. Martı´n et al. / Tetrahedron Letters 48 (2007) 5507–5511
5509
found at C-5 in compound 7; no appreciable labeling
HO
at C-1 could be observed under the detection limits of
the 1H and 13C NMR spectroscopy. We therefore
conclude that in compound 7 all the deuterium comes
only from abstraction–retention at C-5 and that
the reduction of the O-radical is responsible for the
unlabeled molecules.
O
1'
MeO
MeO
5
1
OMe
MeO
15
Ι
Ι
2
Ph (OAc)2/
R1
Several other conclusions can be drawn from the results
presented above. For this O-glycoside the abstraction at
C-5, through an apparently less stable nine-membered
TS competes favorably with the abstraction at C-1
through a seven-membered TS (C-5:C-1 abstraction
ratio of 6:4). Interestingly, the abstraction at C-5
proceeded with an inversion–retention ratio of 6:4, while
a complete inversion of configuration was observed dur-
ing the abstraction at C-1.
O
O
O
O
MeO
MeO
+
MeO
MeO
R2
OMe
OMe
OMe
31%
OMe
18β
1
2
H+
β
α
Ι 18%
19 R = H; R =
18α 24%
1
2
Ι 14%
20 R = R =
Scheme 3. HAT in a-C-glycosides under oxidative conditions.
As expected, the HAT reaction was less effective with
tetraacetyl phthalimide 9 (26% yield), while 42% of
undeuterated compound 13 was obtained as the result
of failure of the hydrogen abstraction process. On the
other hand, the reaction was much more regioselective,
the abstraction now occurring exclusively at C-5 with
an inversion–retention ratio of approx. 1:2 (Scheme 2).
We should point out that no abstraction at C-1 could
be detected by NMR analysis of the crude reaction
mixture.
bly formed by a single or double abstraction at C-10
through a six-membered TS and subsequent radical
quenching by iodine atoms from the reaction medium
before the abstraction and spirocyclization at C-1 could
take place. In reality, the qualitative results of the reduc-
tive and oxidative experiments are similar, in both cases
the abstraction occurs only at C-1 or C-10 but not at C-
5. The differences appear to center around the possibility
of multiple abstractions by the alkoxyl radical in the
oxidative process, which ultimately leads to more com-
plex final products.
The results obtained for the reductive HAT reaction of
C-glycoside 2 are illustrated in Scheme 2. NMR spectral
analysis of compounds 14 and 15 and of their deuterated
counterparts 16 and 17 revealed that the reaction pro-
ceeded by hydrogen abstraction at C-1 and C-10, but
no abstraction at C-5 could be detected. As may be
expected, the abstraction at C-10 is, in this C-glycoside,
a competitive reaction since it presumably proceeds via
a six-membered TS. The inversion of configuration at
C-1 observed in compound 14 is in good agreement with
the previously reported stabilization of the a-radical by
the so-called radical anomeric effect.15
From the experimental results shown above it seems to
indicate that the stereoelectronic control exerted by the
exo-anomeric effect in the O-glycosides gives rise to
two minimum energy conformations, which can abstract
hydrogen from C-5 and C-1. The C-glycosides, in the
absence of this effect, adopt a more flexible conforma-
tion. The HAT reaction is now controlled by the energy
of the TS and the abstraction occurs exclusively at C-1.
In this work we have also developed a simple methodol-
ogy to generate the 5-glycopyranosyl radicals with mod-
erate efficiency, which may prove to be highly suitable
for reactivity studies. While numerous studies related
to 1-glycopyranosyl radicals have been reported, the
preparation and reactivity of its homologous 5-glyco-
pyranosyl radicals have received comparatively very lit-
tle attention.16 In the few cases studied, most of them
related with the D-glucuronic to L-iduronic acid isomer-
ization, the additions are notably non-stereoselective in
contrast with 1-glycopyranosyl radicals. Therefore, our
ongoing work in this area is aimed at further studying
the reactivity of 5-glycopyranosyl radicals generated
by using this method.
We have also studied the oxidative HAT reaction of a-
alcohol 15 in order to clarify the 1,5- versus 1,6-compet-
itive abstraction observed in the reductive process of
the C-a-glycoside. The alkoxyl radical was generated
by reaction of alcohol 15 with (diacetoxyiodo)benz-
ene and iodine under irradiation with visible light
(Scheme 3).
Four compounds were isolated and characterized from
the reaction mixture: the diastereoisomeric spiroacetals
18a and 18b and two iodinated compounds 19b and
20a. The stereochemistries at C-1 were determined by
NOE experiments and by acid-catalyzed isomerization
of kinetic 18b spiro to thermodynamic 18a, which may
exist in a minimum energy conformation with two stabi-
lizing anomeric effects. Reductive removals of the iodine
atoms with n-Bu3SnH/AIBN confirmed the proposed C-
1 stereochemistry of 19b and 20a. In the 1H NMR spec-
trum of 19b, the coupling pattern of H-10 (dd, 12.9 and
4.5 Hz) was consistent with the equatorial configuration
of the iodine atom. The iodine compounds were plausi-
Acknowledgements
This work was supported by the Investigation Pro-
grammes Nos. CTQ2004-06381/BQU and CTQ2004-
´
02367/BQU of the Ministerio de Educacion y Ciencia,
Spain, cofinanced by the Fondo Europeo de Desarrollo
Regional (FEDER). L.M.Q. thanks the Program I3P-
CSIC, for a fellowship.

5510
A. Martı´n et al. / Tetrahedron Letters 48 (2007) 5507–5511
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compounds are provided. Supplementary data associ-
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