Stereoselective synthesis of C-ketosides by sequential intramolecular hydrogen atom transfer-intermolecular allylation reaction

Article abstract of DOI:10.1016/j.tetlet.2008.06.070

A tandem 1,5 or 1,6 hydrogen atom transfer (HAT)-radical allylation using carbohydrate models is described. The HAT reaction generated a C-glycos-1-yl radical intermediate, which added to allyltri-n-butyltin with high diastereoselectivity, to give C-ketosides with the quaternary carbon carrying two differently functionalized tethers.

Full text of DOI:10.1016/j.tetlet.2008.06.070

Tetrahedron Letters 49 (2008) 5179–5181
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Tetrahedron Letters
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Stereoselective synthesis of C-ketosides by sequential intramolecular
hydrogen atom transfer–intermolecular allylation reaction
*
*
Angeles Martín , Inés Pérez-Martín, Luis M. Quintanal, Ernesto Suárez
Instituto de Productos Naturales y Agrobiología del C.S.I.C., Carretera de La Esperanza 3, 38206 La Laguna, Tenerife, Spain
a r t i c l e i n f o
a b s t r a c t
Article history:
A tandem 1,5 or 1,6 hydrogen atom transfer (HAT)–radical allylation using carbohydrate models is
described. The HAT reaction generated a C-glycos-1-yl radical intermediate, which added to allyltri-n-
butyltin with high diastereoselectivity, to give C-ketosides with the quaternary carbon carrying two dif-
ferently functionalized tethers.
Received 14 May 2008
Revised 9 June 2008
Accepted 16 June 2008
Available online 19 June 2008
Ó 2008 Elsevier Ltd. All rights reserved.
Keywords:
C-Ketoside
Radical reaction
Hydrogen transfer
Allylation
Tandem
C-Ketosides or bis-C,C-glycosides are a class of carbohydrate
analogs that have received increased synthetic attention recently
due to their relationship with C-glycosides of ulosonic acids1 and
the detection of this unit as a recognizable substructure in many
natural products with interesting biological properties.2
( )m
H
( )m
H
PO
PO
PO
PO
O
O
ONPht
ATBT
O
.
1
( )n
( )n
1,5- or 1,6-HAT
A number of synthetic procedures have been reported by sev-
eral groups,3 in many cases inspired by processes developed for
the preparation of the closely related and more extensively studied
C-glycosides.4
However, the use of radical reactions for the synthesis of
C-ketosides has, somewhat surprisingly, received very little
attention, apart from a few products prepared by the reductive
denitration of 1-nitro-C-glycosides with n-Bu3SnH and acrylo-
nitrile,5 the reaction of sugar dihalides with allyltri-n-butyltin,6
and recently, through the diastereocontrolled Norrish–Yang photo-
cyclization of 5,9-anhydro-1,4-dideoxy-deco-2,3-diuloses.2b Radi-
cal reactions have also been used for the synthesis of C-
glycosides of neuraminic acids.7
We envisaged a simple methodology for the preparation of C-
ketosides using an intramolecular hydrogen atom transfer (HAT)
reaction as the key step. A conveniently disposed alkoxyl radical
would trigger the HAT reaction and the C-radical intermediate
could be added to allylstannanes to give allylsubstituted products
as depicted in Scheme 1. The resulting product is a C-ketoside with
the quaternary carbon carrying two differently functionalized teth-
OH
( )m
PO
PO
PO
PO
( )m
O
O
OH
.
( )n
( )n
Scheme 1. n, m = 1, 2; ATBT = allyltri-n-butyltin; Pht = phthalimide.
ers. The diastereoselectivity at the pseudoanomeric center may be
controlled by two stereoelectronic effects: the radical anomeric
effect and the quasi-homo-anomeric effect.8
Remote free radical functionalization on inactivated carbons via
HAT reactions has attracted considerable interest among synthetic
organic chemists.9 The reaction, when promoted by alkoxyl radi-
cals, occurs most frequently through a six-membered transition
state (TS).10 Some exceptional cases are known of 1,6-HAT reac-
tions that proceed, generally in low yield, via a seven-membered
TS; only when the hydrogen atom to be removed is bonded to an
oxygen-substituted carbon atom can the yield be considered to
be of synthetic interest and in some cases even competes favorably
with the 1,5-process.11
* Corresponding authors. Tel.: +34 922 251004; fax: +34 922 260135 (E.S.).
E-mail addresses: angelesmartin@ipna.csic.es (A. Martín), esuarez@ipna.csic.es
(E. Suárez).
To the best of our knowledge, the tandem intramolecular HAT–
intermolecular allylation reaction proposed in Scheme 1 has never
0040-4039/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2008.06.070

5180
A. Martín et al. / Tetrahedron Letters 49 (2008) 5179–5181
been described. However, examples are known of sequential alk-
oxyl radical promoted HAT and subsequent addition of the C-
radical intermediate to electron-deficient olefins.12 Despite its
potential usefulness, this methodology has not been widely
employed, probably because of the difficulties encountered in the
preparation of alkoxyl radicals under non-oxidative conditions.13
We have used the fragmentation of N-hydroxyphthalimide
derivatives to generate the alkoxyl radicals and allyltri-n-butyltin
as a radical trap in the conditions summarized in Table 1.14
The alditol precursors of phthalimides 1, 3, 4, and 6 were ob-
The stereochemistry at the quaternary center was tentatively
assigned as R on the basis of the NOE interaction observed between
H-3 and the methoxy group at C-5.17 It will be noted that, as
expected, radical a-axial attack operates regardless of the stereo-
chemistry at the pseudoanomeric center in the starting phthali-
mide.18 Since electrophilic radicals abstract axial hydrogens very
much faster than the equatorial ones, it is not very surprising that
the reaction of the b-isomer 3 gave somewhat better results (entry
2).19
The reaction can also be extended to isomers 4 and 6 where a
four-carbon tether would require a HAT through a seven-mem-
bered TS. The reaction proceeded in a similar manner to that of
the previous case; once again only the R diastereomer 5 formed
tained by reaction of 2,3,4,6-tetra-O-methyl-D-mannopyranosyl
chloride with butenylmagnesium bromide. The resulting dec-1-
enitol was submitted to ozonolysis to give the three-carbon tethers
or to hydroboration to afford the four-carbon tether alcohols.
While the alditol precursors of phthalimides 7 and 9, possessing
a three-carbon tether, were synthesized by C-glycosidation of the
corresponding saccharide with allyltrimethylsilane/BF3ÁEt2O and
subsequent hydroboration of the terminal olefin.4j The stable N-
hydroxyphthalimido derivatives were prepared by reaction of the
respective alcohols with N-hydroxyphthalimide under Mitsunobu
conditions.15
The thermally initiated reaction of the isomeric phthalimides 1
and 3 with allyltri-n-butyltin/AIBN gave exclusively the C-ketoside
2 (entries 1 and 2).16 This compound was seemingly obtained as a
single diastereoisomer within the detection limits of 1H and 13C
NMR spectroscopy, by the analysis of the crude reaction mixtures.
by radical quenching along the
and also the b-isomer 6 gave the best yield (entries 3 and 4). The
low yield obtained with the -isomer deserves a brief comment.
a-axial direction could be detected
a
In a previous work from this laboratory, the reaction of phthali-
mide 4 with n-Bu3SnH/AIBN was studied in some detail, two prod-
ucts being obtained: the inverted alcohol 11 (40%) and alcohol 12
(59%) where the configuration at C-5 has apparently been retained
(Scheme 2).20 An experiment with n-Bu3SnD/AIBN gave com-
pounds 13 (32%) and 14 (61%); the incorporation of deuterium at
C-4 in the latter indicates that the 1,6-HAT reaction suffers from
competing 1,5-HAT with hydrogen abstraction at C-4. This is also
observed in the reaction with allyltri-n-butyltin where apart from
5 the C-4 allylated compound 15 is obtained, albeit in low yield
and poor diastereoselectivity.
Interestingly, in the b-isomer 6 with the C-5 abstractable hydro-
gen in axial position the intramolecular functionalization pro-
ceeded exclusively by an a priori less favorable 1,6-HAT reaction.
On the contrary, both 1,5- and 1,6-HAT processes compete in the
Table 1
Sequential intramolecular HAT–intermolecular allylation reactiona
Entry Substrate
Product
Yieldb
(%)
a
-isomer 4 since abstraction of the equatorial hydrogen at C-5 is
slower.
This methodology can also be extended to the synthesis of
PhtNO
OMe
OH
MeO
MeO
3
4
O
O
C-ketosides derived from furanosyl radicals where, contrary to
observations in the case of pyranosyl radicals, steric effects seem
to control the stereoselectivity.8c,d,21 The reaction of phthalimide
5
MeO
OMe
MeO
MeO
MeO
7, derived from D-ribose, afforded C-ketoside 8 (56%) with good
1
2
1
a
-isomer
2
2
42
54
diastereoselectivity (3.5:1) (entry 5). Apparently, the allyl radical
attack occurs preferentially by the less hindered b-face of the
tetrahydrofuran ring. The presence of NOE interactions between
3 b-isomer
PhtNO
MeO
MeO
MeO
MeO
4
5
OH
O
O
OMe
OMe
MeO
O
MeO
MeO
5
ONPht
3
4
4
a
-isomer
5
5
35
58
6 b-isomer
MeO
OMe
MeO
MeO
MeO
3
OH
4
O
O
O
4
7
9
ONPht
n-Bu3SnH(D)/AIBN
or
MeO
7
OMe
MeO
OMe
ATBT/AIBN
5
6
8 (4S/4R, 3:5:1)
56
61
HO
R
R
O
MeO
MeO
MeO
MeO
4
BnO
BnO
3
O
OH
O
HO
+
4
7
9
ONPht
OMe
OMe
BnO
BnO
10 (4R/4S, 2:1)
MeO
MeO
9
11 R = H 40%
13
12 R = H 59%
14 R = D(H) (4:1) 61%
15
a
A solution of the corresponding phthalimide-derivative (1 mmol) in dry ben-
R = D 32%
5 R = allyl 35%
zene (10 mL) was treated with allyltri-n-butyltin (10 mmol) and AIBN (0.01 mmol)
R = allyl (1.5:1) 4%
under reflux.
b
The reduced starting alcohol was always also obtained (5–28%), see Supple-
mentary data.
Scheme 2. ATBT = Allyltri-n-butyltin.

A. Martín et al. / Tetrahedron Letters 49 (2008) 5179–5181
5181
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Acknowledgments
This work was supported by the Investigation Programmes nos.
CTQ2007-67492 and CTQ2004-02367 of the Ministerio de Educa-
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