Intramolecular 1,5-hydrogen atom transfer reaction promoted by phosphoramidyl and carbamoyl radicals: synthesis of 2-amino-C-glycosides

Article abstract of DOI:10.1016/j.tetlet.2007.06.152

The preparation of 2-amino-C-glycosides of the hexahydro-2H-furo[3,2-b]pyrrole and octahydropyrano[3,2-b]pyrrole systems is described. A tandem 1,5-hydrogen atom transfer-radical oxidation-nucleophilic cyclisation mechanism is proposed for the intramolecu

Full text of DOI:10.1016/j.tetlet.2007.06.152

Tetrahedron Letters 48 (2007) 6384–6388
Intramolecular 1,5-hydrogen atom transfer reaction promoted by
phosphoramidyl and carbamoyl radicals: synthesis of
2-amino-C-glycosides
´
´
Cosme G. Francisco, Antonio J. Herrera, Angeles Martın,
*
´
´
´
´
Ines Perez-Martın and Ernesto Suarez
Instituto de Productos Naturales y Agrobiologı´a del CSIC, Carretera de La Esperanza 3, 38206 La Laguna, Tenerife, Spain
Received 25 May 2007; revised 25 June 2007; accepted 27 June 2007
Available online 4 July 2007
Abstract—The preparation of 2-amino-C-glycosides of the hexahydro-2H-furo[3,2-b]pyrrole and octahydropyrano[3,2-b]pyrrole
systems is described. A tandem 1,5-hydrogen atom transfer–radical oxidation–nucleophilic cyclisation mechanism is proposed for
the intramolecular hydrogen atom transfer reaction promoted by carbamoyl and phosphoramidyl radicals employing hipervalent
iodine oxidants.
Ó 2007 Elsevier Ltd. All rights reserved.
The intramolecular hydrogen atom transfer (IHAT)
reaction promoted by alkoxyl radicals has attracted
considerable interest in organic synthesis.1 In contrast,
comparatively, less effort has been devoted to the IHAT
reaction generated by N-centred radicals.1a,2 Since the
pioneering works on IHAT processes promoted by
aminyl or amidyl radicals developed by Hoffmann-
Lo¨ffler-Freytag3 and Barton,4 a number of procedures
have been reported for the generation of N-radicals.5
In previous papers from this laboratory, we have
reported that N-nitroamines, N-cyanamides, N-phos-
phoramidates and N-tert-butoxycarbonylamides react
with hypervalent iodine reagents in the presence of iodine
to generate N-centred radicals through homolytic frag-
mentation of a hypothetical iodoamide intermediate.6
Nitrogen thus-formed radicals may participate in an
intramolecular 1,5-hydrogen abstraction from remote
carbons to generate pyrrolidines. These results have
provided an excellent opportunity to demonstrate the
synthetic potential of this methodology for the prepara-
tion of 2-hemiaminals related to 2-amino-C-glycosides.
have been widely investigated as synthons for the prep-
aration of a variety of biologically active compounds, in
particular, glycopeptide mimics, glycosidase inhibitors
and analogues of tumour-associated carbohydrate anti-
gens.8 Unfortunately, some difficulties have been
encountered in the synthesis of these compounds. The
presence of the nitrogen-containing functionality at C-
2 does not seem to be fully compatible with standard
C-glycosidation methodologies. It is therefore not sur-
prising that current methods for their preparation are
somewhat limited.9
In this Letter, we disclose an efficient way to prepare
interesting oxa-azabicyclic systems such as hexahydro-
2H-furo[3,2-b]pyrrole and octahydropyrano[3,2-b]pyr-
role structures (Scheme 1). The electrophilic N-radical,
attached to a dimethylene tether extended from the C-
1 of a carbohydrate, abstracts a hydrogen atom at C-2
through a favourable six-membered transition state.
The nucleophilic C-radical thus formed is subsequently
oxidised to a carbenium ion which is intramolecularly
trapped by the nucleophilic amide to give pyrrolidines
fused to furanose or pyranose rings.10 Precedent for this
reaction comes from previous works from this labora-
tory, where a similar carbohydrate system with one
methylene tether abstract exclusively from C-4 (fura-
noses) or C-5 (pyranoses), thus the length of the side
chain constitutes a major determinant of the regioselec-
tivity of the reaction.6b
C-Glycosides have gained considerable importance in
recent years as carbohydrate mimetics and in view of
the increased detection of the C-glycosidic bond in
natural products.7 Among them, 2-amino-C-glycosides
*
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.2007.06.152

C. G. Francisco et al. / Tetrahedron Letters 48 (2007) 6384–6388
RO
6385
followed by treatment of the resulting crude amine with
the corresponding protecting group reagent: with diphe-
nyl chlorophosphate and TEA to give phosphorami-
dates 3, 6, 11, 13, 15, 18 and 19 and with di-tert-butyl
dicarbonate to give N-Boc-protected derivatives 1 and
9. It is noteworthy that the use of carbamates is interest-
ing regarding further manipulation at the nitrogen cen-
tre because of its facile deprotection.12
R1HN
RO
O
O
4,5
I
I
2
(III)/
( )n
RO
N
R1
2
( )n
RO
O
R
OR
.
R1HN
R1N
R1HN
RO
RO
RO
O
O
O
.
The phosphoramidyl and carbamoyl radicals were gen-
erated by reaction of the corresponding phosphorami-
dates and carbamates with (diacetoxyiodo) benzene
(DIB) or iodoxyl benzene and iodine under the reaction
conditions mentioned in Table 1.
( )n
RO
( )n
RO
( )n
RO
OR
OR
OR
+
Scheme 1. Synthesis of 2-amino-C-glycosides. R = Protective group;
R1 = PO(OPh)2, Boc; n = 1,2.
The starting amines were prepared through a sequence
involving (a) Lewis acid-mediated C-glycosidation with
allyltrimethylsilane to afford the non-8-enitols, in gen-
eral with high stereoselectivity,11 (b) olefin ozonolysis
followed by reductive (NaBH4) opening of the ozonide
to give the octitols, (c) transformation of the primary
unprotected alcohols into the corresponding mesyl-
derivatives and subsequent nucleophilic substitution
with azide ion, (d) reduction (H2, Pd/C 10%, AcOEt)
In entry 1, we describe an IHAT reaction over a C-gly-
coside derived from D-mannose 1 possesing a three-
atoms side chain. In this case, with the pyranose ring
in a restricted 4C1 conformation, the amidyl radical
would preferentially abstract the hydrogen located at
either the C-4 or C-7 position.13a Taking into account
that the abstraction from C-7 evolves through a seven-
membered transition state (TS) and that from C-4
through in principle, a more favourable six-membered
Table 1. 1,5-HAT reaction promoted by N-radicalsa
Entry
Substrate
Reagent (mmol)
Time (h)
Products yield (%)
RHN
O
BocHN
1
O
MeO
MeO
MeO
7
4
OAc
OMe
MeO
OMe
MeO
MeO
1
1R = Boc
2.5
5
2 (61, S/R 2:1)
O
MeO
N
MeO
O
PO(OPh)2
MeO
Me
2b
3R = PO(OPh)2
2.2
3
4 (75)
O
MeO
N
MeO
O
H
PO(OPh)2
MeO
3c
3R = PO(OPh)2
4
5
5 (74)d
(PhO)2OPHN
O
O
N
MeO
O
MeO
PO(OPh)2
OMe
MeO
Me
MeO
4b
6
2.1
5.5
7 (66)
O
N
MeO
O
H
PO(OPh)2
MeO

6386
C. G. Francisco et al. / Tetrahedron Letters 48 (2007) 6384–6388
Table 1 (continued)
Entry
5c
Substrate
Reagent (mmol)
3.2
Time (h)
6
Products yield (%)
6
8 (91)d
RHN
O
O
O
O
O
O
N
R
O
O
O
O
6b
7b,e
9R = Boc
11R = PO(OPh)2
2.5
1.5
6
12
10R = Boc (55)
12R = PO(OPh)2 (64)
(PhO)2OPHN
(PhO)2OPHN
O
O
MOMO
MOMO
OAc
O
O
O
O
8f
13
3.7
7
14 (51)
a A solution of the amide (1 mmol) in dry DCM or CH3CN (20 mL) containing (diacetoxyiodo)benzene (DIB) and iodine (1 mmol) was irradiated
with two 80 W tungsten-filament lamps at room temperature.
b NaHCO3 was added.
c PhIO was used as an oxidant.
d The hemiaminal is in equilibrium with its open form.
e T = 40 °C.
f Reflux temperature.
TS, it was not surprising that compound 2 was formed
exclusively. The product was obtained as a mixture of
two diastereoisomers 4S/4R (2:1). A NOE enhancement
observed between the axial hydrogen at C-6 and the
methoxyl group at C-4 revealed that the major isomer
has a 4S stereochemistry. Apparently, a lower nucleo-
philicity of the carbamate group is responsible for the
competitive intermolecular attack of the acetate anion
coming from the reagent.
(PhO)2OPN
O
MeO
MeO
MeO
MeO
O
O
a
MeO
N
N
MeO
O
O
H
O
PO(OPh)2
PO(OPh)2
MeO
MeO
MeO
Me
4
5
Scheme 2. Reagents and conditions: (a) citric acid, DCM, rt, 33 h.
On the contrary, with phenyl phosphonate as the amine
protecting group (entry 2), the cyclic product 4 was
exclusively obtained in good yield. Curiously, the
absence or presence of solid NaHCO3 in the reaction
medium may either lead to methyl ether 4 or to the
hydroxylated alcohol 5 (entries 2 and 3).
anosyl derivatives 9 and 11 gave the expected cyclic
product in moderate yield. In both cases, it was neces-
sary to neutralise the medium with solid NaHCO3 to
prevent undesirable hydrolysis of the acid-sensitive 7,8-
isopropylidene group.
Phosphoramidate 13, derived from D-ribose, yielded
smoothly the acetate-derivative 14 showing that the
1,5-HAT reaction had occurred but no cyclisation
product was formed because of the competitive acetate
nucleophilic addition.
The slightly acidic condition of the reaction in entry 3
seems to be sufficient to catalyse hydrolysis of the ketal
group at C-4. Indeed, compound 5, which is at room
temperature in equilibrium with its open form, can be
obtained in quantitative yield by acid-catalysed hydroly-
sis of 4 under mild conditions (Scheme 2).
Alkoxyl radicals are able to undergo 1,6-HAT reactions
on pyranose and furanose systems to obtain bicyclic
compounds in good yields.13 However, 1,6-HAT pro-
moted by nitrogen centred radicals is not a general pro-
cess and only a few examples are known.6c,f,14 With the
aim to direct the IHAT reaction to form products de-
rived from the 1,6-abstraction, we prepared some pre-
cursors where the hydrogen atom at C-4 is deactivated
towards the 1,5-HAT (Table 2). In most cases, no reac-
tion was observed (entries 2 and 3), but surprisingly with
the L-fucose model 15, three 1,6-HAT products were
generated (entry 1).
This behaviour concerning NaHCO3 was reproduced in
other models in good yields as can be observed with the
L-rhamnose derivative 6 in entries 4 and 5. Analogously,
hemiaminal 8 was also quantitatively obtained from 7
after treatment with citric acid. Similar or even better
results were obtained using iodosyl benzene instead of
DIB as the oxidant (compare entries 2 vs 3 and 4 vs 5).
This IHAT-cyclisation sequence was further extended to
furanose models (entries 6, 7 and 8). The D-mannofur-

C. G. Francisco et al. / Tetrahedron Letters 48 (2007) 6384–6388
Table 2. 1,6-HAT reaction promoted by N-radicalsa
6387
Entry
Substrate
(PhO)2OPHN
O
Reagent (mmol)
Time (h)
Products yield (%)
(PhO)2OPHN
1
AcO
O
4
MeO
OMe
MeO
OMe
MeO
16
MeO
(47)
+
(PhO)2OP
OH
N
1
15
2.4
3.5
OMe
OMe
(20)
(PhO)2OPHN
O
MeO
17
MeO
MeO
R
MeO
2
3
18R = OMe
19R = H
3.1
2.0
9
6
No reaction
No reaction
a A solution of the amide (1 mmol) in dry CH3CN (20 mL) containing (diacetoxyiodo)benzene (DIB), iodine (1 mmol) and NaHCO3 was irradiated
with two 80 W tungsten-filament lamps at room temperature.
Product 16 is an inseparable mixture of epimers, by
conventional column chromatography, resulting from
the 1,6-HAT and consecutive nucleophilic addition of
acetate. However, compound 17 to which an eight-
membered enamine structure was tentatively assigned
may proceed from the nucleophilic addition of the phos-
phoramidate group and subsequent opening of the pyra-
nose ring under acid conditions (Scheme 3).15 The high
complexity observed in the NMR spectra may be indic-
ative of the existence of a sluggish conformational equi-
librium, feasible in this sort of eight-membered rings.
Nevertheless, it was impossible to carry out variable
temperature NMR experiments due to the instability
of the product.
yields are comparable to those previously obtained from
the O-centred radical studies. We are conscious that the
1,6-HAT reaction promoted by N-radicals is not easy to
accomplish, probably due to the bulkiness of the N-pro-
tecting group but the results obtained from L-fucose
model are encouraging.
Acknowledgments
This work was supported by the research programs
´
BQU2000-0650 and BQU2001-1665 of the Direccion
´
´
´
General de Investigacion Cientıfica y Tecnica, Spain.
´
A.J.H. and I.P.-M. thank the Ministerio de Educacion
y Ciencia, Spain, and the Program I3P-CSIC, respec-
tively, for fellowships.
In summary, we have now demonstrated that 1,5-HAT
reactions can be useful to synthesise hexahydro-2H-
furo[3,2-b]pyrrole and octahydropyrano[3,2-b]pyrrole
structures, which are not readily accessible by other
methodologies. The virtues of hypervalent iodine
reagents are the mildness of the conditions and the ob-
served high protecting group tolerance. The achieved
Supplementary data
Full characterisation of compounds and experimental
details. Supplementary data associated with this article
can be found, in the online version, at doi:10.1016/
j.tetlet.2007.06.152.
PO(OPh)2
N
PO(OPh)2
HN
PO(OPh)2
HN
+
O
O
O
OMe
OMe
OMe
References and notes
OMe
OMe
OMe
MeO
MeO
MeO
15
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+
(PhO)2OP
(PhO)2OP
OH
OH
N
N
-H+
OMe
OMe
OMe
MeO
MeO
OMe
17
Scheme 3.

6388
C. G. Francisco et al. / Tetrahedron Letters 48 (2007) 6384–6388
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