Synthesis of Chiral Polyhydroxylated Benzimidazoles by a Tandem Radical Fragmentation/Cyclization Reaction: A Straight Avenue to Fused Aromatic-Carbohydrate Hybrids

Article abstract of DOI:10.1021/acs.joc.8b01988

The synthesis of benzimidazole-fused iminosugars through a tandem β-fragmentation-intramolecular cyclization reaction is described. The use of the benzimidazole ring as the internal nucleophile and the use of phenyliodosophthalate (PhI(Phth)), a new metal-free and low toxic hypervalent iodine reagent, are the most remarkable novelties of this synthetic strategy. With this approach, we have demonstrated the usefulness of the fragmentation of anomeric alkoxyl radicals promoted by the PhI(Phth)/I₂ system for the preparation of new compounds with potential interest for both medicinal and synthetic chemists.

Full text of DOI:10.1021/acs.joc.8b01988

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Article
Synthesis of Chiral Polyhydroxylated Benzimidazoles by
a Tandem Radical Fragmentation/Cyclization Reaction: a
Straight Avenue to Fused Aromatic-Carbohydrate Hybrids
Emy André-Joyaux, Andrés González Santana, and Concepción C. González
J. Org. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.joc.8b01988 • Publication Date (Web): 27 Dec 2018
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Synthesis of Chiral Polyhydroxylated Benzimidazoles by a Tandem Radical
Fragmentation/Cyclization Reaction: a Straight Avenue to Fused Aromatic-
Carbohydrate Hybrids
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Emy André-Joyaux, Andrés G. Santana, Concepción C. González*
Instituto de Productos Naturales y Agrobiología del C.S.I.C., Avenida Astrofísico
Francisco Sánchez 3, 38206 La Laguna, Tenerife, Spain.
ccgm@ipna.csic.es
Abstract
The synthesis of benzimidazole-fused iminosugars through a tandem β-fragmentation-
intramolecular cyclization reaction is described. The use of the benzimidazole ring as
the internal nucleophile as well as the use of phenyliodosophthalate (PhI(Phth)), a new
metal-free and low toxic hypervalent iodine reagent, are the most remarkable novelties
of this synthetic strategy. With this approach, we have demonstrated the usefulness of
the fragmentation of anomeric alkoxyl radicals (ARF) promoted by the PhI(Phth)/I2
system for the preparation of new compounds with potential interest for both medicinal
and synthetic chemists.
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1. INTRODUCTION
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The benzimidazole ring is an aromatic bicyclic heterocycle increasingly present in
numerous drugs due to its ability as hydrogen-bond donor and acceptor, as well as its
electron-rich aromatic platform, all of which makes it a privileged structure for
supramolecular recognition and therefore, the development of new therapeutic agents.1
N-Ribosyldimethyl benzimidazole (I) is the most prominent natural benzimidazole and
acts as an axial ligand for cobalt in B12 vitamin.2 Furthermore, different bioactive hybrid
heterocycles containing the benzimidazole ring have been described in the literature as
shown in Figure 1.3 Strikingly, scarce examples of benzimidazoles fused to
carbohydrates have been reported, with applications ranging from nucleoside surrogates
for kinase inhibition to chiral ligands in metal-mediated C-C bond formation.4a-b
Nevertheless, we believe the scope of activities of benzimidazole-containing derivatives
could be further extended into the inhibition of glycosidases, thus leading to potential
drugs targeting these enzymes. By virtue of the resulting structure (the position of the
basic center, the hydroxylation pattern, the ring size as well as its flexibility, the sp2-
flattening of the pseudo-anomeric region of the ring and the interactions with the
aglycon binding site), hybrid benzimidazole-fused iminosugars display every
requirement for potential selective inhibition of glycoside-processing enzymes.4c-j
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Figure 1. Bioactive benzimidazole-containing hybrid heterocycles
Recently, our research group has been involved in the development of new sustainable
syntheses of potentially active compounds. In this regard, synthetic strategies based on
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alkoxyl radicals using metal-free promoters such as hypervalent iodine (III) reagents
have been developed, featuring among them the tandem -fragmentation of anomeric
alkoxyl radicals/intramolecular cyclization.5 This methodology relies upon the initial
formation of the alkoxyl radical generated by the homolysis of a transient hypoiodite
intermediate, triggering a β-fragmentation of the C1-C2 bond that generates an acyclic
C-2 radical. From this point on, the electronic nature at C-2 has a strong influence on
the outcome of this reaction intermediate. In fact, when the substituent at C-2 is an
electron-donating group, the radical is rapidly oxidized to provide an oxocarbenium ion
that can be trapped either by external or internal nucleophiles (Scheme 1).6
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Scheme 1.- Anomeric Alkoxyl Radical Fragmentation (ARF): Mechanism and
Applications.
Probably the most interesting application of this reaction is towards the synthesis of
polyhydroxylated heterocycles. Given the potential in this field, the main goal of our
research has been focused on synthetizing new classes of iminosugar derivatives
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starting from carbohydrates. In order to do so, a nitrogen-containing moiety must be
suitably positioned to react as the internal nucleophilic species. Following this strategy,
a variety of novel polyhydroxylated heterocyclic compounds has been recently
prepared.7 However, to the best of our knowledge, the benzimidazole ring has never
been engaged as the nucleophilic counterpart in this kind of reaction (Scheme 1).
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2. RESULTS AND DISCUSSION
The retrosynthesis of the benzimidazole-sugars is depicted below (scheme 2), where
four steps are required from the corresponding starting materials. Apart from the -
fragmentation/cyclization reaction, the iodine-catalyzed condensation of the
corresponding aldehydes with ortho-phenylenediamine is the key step of this strategy.8
In turn, these aldehydes would come either from the oxidation of primary alcohols or
the reduction of the corresponding nitriles.
Scheme 2.- Retrosynthetic analysis.
D-Lyxose derivative 4 was prepared starting from 1 (1-O-benzoyl-2,3-O-
isopropylidene--D-mannofuranose, readily available in two steps from commercial D-
mannose diacetonide).9 Oxidative cleavage of the vicinal diol with sodium periodate in
a mixture of acetone/water provided the corresponding aldehyde 2, which was
subsequently engaged in the condensation step to obtain compound 3 in good yield.
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Then, Zemplén deprotection of compound 3 afforded the desired substrate 4 in 79%
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yield (Scheme 3).
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Scheme 3.- Reagents and conditions: a) NaIO4, acetone/H2O (1:1), rt, 3.5 h, then b) o-
C6H4(NH2)2, CeCl3·7H2O, CuI, I2, acetonitrile, rt, 1 h, 87% (two steps); c) MeONa,
MeOH, rt, 3 h, 79%.
D-Ribose derivative 8 was synthetized starting from 5 (1-O-benzoyl-2,3-O-
isopropylidene--D-ribofuranose, readily available in four synthetic steps from D-
ribose)10 by using a similar protocol. In order to oxidize the primary alcohol into an
aldehyde, a battery of reagents was tested. Chromium (VI) reagents, such as pyridinium
dichromate (PDC) and pyridinium chlorochromate (PCC), afforded the corresponding
aldehyde in low yields. Disappointingly, we were not able to improve the yield when
Dess-Martin reagent was used. Finally, IBX11 was found to be the best oxidizing agent
and afforded, after condensation with the corresponding diamine, the desired
benzimidazole-containing derivative 7 in 48% yield over the two steps, which was then
deprotected to afford 8 following the same protocol (Scheme 4).
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Scheme 4.- Reagents and conditions: a) IBX, DMSO, rt, 3 h, then b) o-C6H4(NH2)2,
CeCl3·7H2O, CuI, I2, acetonitrile, rt, 4.5 h, 48% (two steps); c) MeONa, MeOH, rt, 1 h,
84%.
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In order to test the scope of our synthetic proposal, precursors of six-membered
benzimidazole-sugars were also synthetized. The synthetic scheme used was similar to
that applied before. Nitrile 9,7b derived from D-ribofuranose in four synthetic steps, was
first reduced with DIBAL-H to generate the corresponding aldehyde 10, which was then
engaged in the condensation step, without further purification, to afford the
benzimidazole-containing compound 11 in 53% yield over the two steps. The anomeric
benzyl protecting group was finally removed by catalytic hydrogenation to provide the
final precursor 12 (Scheme 5).
Scheme 5.- Reagents and conditions: a) DIBAL-H, toluene, 78 °C, 1.5 h, then b) o-
C6H4(NH2)2, CeCl3·7H2O, CuI, I2, acetonitrile, rt, 1.5 h, 53% (two steps); c) Pd(OH)2/C
(30%), H2 (1 atm), EtOAc, rt, 72 h, 81%.
Additionally, substrate 13 (available from D-mannose diacetonide in five steps),10 from
the hexose series in the furanose form, was used to investigate the influence of carrying
a substitution at position C-5. The benzimidazole-containing glycoside 15 was readily
prepared by oxidation of the primary alcohol 13 under Dess-Martin conditions and
subsequent condensation of the resulting aldehyde (72% yield). The anomeric PMB
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protecting group was finally removed by treatment with CAN, to quantitatively yield
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the hemiacetal 16 (Scheme 6).
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Scheme 6.- Reagents and conditions: a) DMP, DCM, rt, 1.5 h, then b) o-C6H4(NH2)2,
CeCl3·7H2O, CuI, I2, acetonitrile, rt, 2.5 h, 72% (two steps); c) CAN, acetonitrile/H2O
(10:1), rt, 2 h, quant.
The purpose of precursor 20 was to test the viability of the tandem β-
fragmentation-intramolecular cyclization onto a six-membered ring derivative; when
compound 17 (prepared in five steps from D-mannose)7b was oxidized and subsequently
treated with o-phenylenediamine, the corresponding benzimidazole-containing sugar 19
was obtained in 59% yield over the two steps. Then, deprotection of the benzyl ether
was performed via catalytic hydrogenation, affording the desired anomeric alcohol 20 in
good yield (Scheme 7).
Scheme 7.- Reagents and conditions: a) DMP, DCM, rt, 2 h; b) o-C6H4(NH2)2,
CeCl3·7H2O, CuI, I2, acetonitrile, rt, 1 h, 57% (two steps); c) Pd(OH)2/C (30%), H2 (1
atm), EtOAc, rt, 48 h, 83%.
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As an integral part of the research carried out in our group on homolytic
trasnformations in carbohydrate scaffolds, the development of new synthetic strategies
to generate alkoxyl radicals using hypervalent iodine, mainly with DIB/I2 or PhIO/I2
systems has been thoroughly explored. Herein, a novel phenyliodosophtalate
(PhI(Phth))/I2 system is also presented as an alternative to prevent the nucleophilicity
and acidity of DIB-derived acetic acid and to avoid the unstability of the PhIO. The
synthesis of the PhI(Phth) can be readily accomplished in a two-step procedure starting
from dibutyl phthalate. Saponification and cyclization with DIB afforded the desired
PhI(Phth) in 89% yield over two steps.12
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The first goal of this study was to identify the optimal conditions to carry out the β-
fragmentation/intramolecular cyclization of the benzimidazole-containing sugar-
hemiacetals described above. In this context, precursor 4 was first chosen to perform the
study, which started by testing the same conditions reported before for the synthesis of
structurally related tetrazolo-sugar.7b Thus, when compound 4 was treated with PhIO/I2
in EtOAc at 80 ºC under irradiation (entry 1, Table 1) a complex mixture of products
was obtained, from which the benzimidazole-derivative 21 could be isolated in 29%
yield. When PhIO/I2 system was changed to DIB/I2 (entry 2) under the same conditions,
a mixture of compounds 21 (47%) and 21b (14%) was generated, the latter one
originating from the trapping of the oxocarbenium ion intermediate by acetic acid
released during the formation of the hypoiodite intermediate. Replacement of EtOAc by
DCE minimized the formation of compound 21b to only a 4% yield. Alternatively,
when the mixture was stirred at room temperature, under irradiation with two 80W
visible light lamps, both compounds 21 and 21b were obtained in a similar proportion
(entries 3 and 4, respectively). To our delight, when precursor 4 was treated with the
less nucleophilic PhI(Phth)/I2 system under the original conditions (EtOAc at 80ºC
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under irradiation), the desired cyclic compound 21 was obtained in a better yield and as
a sole product (entry 5). Higher reaction temperatures (100 ºC, entry 6) accelerated the
overall process, albeit with a slight decrease in the yield of the cyclized product. Thus,
in view of the results obtained so far, the best conditions found involved EtOAc as
solvent and heating at 80 ºC under irradiation, either with DIB/I2 or PhI(Phth)/I2
reagents (entries 2 and 5, respectively). Out of the two oxidizing reagents, PhI(Phth)/I2
appears to be more appropriate in order to generate the cyclized compound 21.13
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entry Solvent
Reagentsa
T (°C) T (h)
21
21b
(yield %) (yield %)
EtOAc
EtOAc
DCE
PhIO/I2
DIB/I2
DIB/I2
DIB/I2
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rt
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EtOAc PhI(Phth)/I2
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100
EtOAc PhI(Phth)/I2
-
a
Table 1.- Optimization of the -ARF/cyclization reaction conditions. Reaction
conditions: All reactions were performed at 0.05 M in dry EtOAc under irradiation with
two 80 W tungsten-filament lamps at the stated temperature containing the oxidizing
hypervalent iodine reagent (2.2 mmol) and I2 (1.2 mmol) per mmol of substrate
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When compound 8 was treated with PhI(Phth)/I2, the desired all-syn cyclic
benzimidazole-sugar 22 was successfully obtained in a 53% yield. The homolysis
protocol was then applied to potential precursors of six-membered fused heterocycles.
Compound 12, which is also a D-ribose derivative, afforded the desired compound 23 in
48% yield, where this lower yield could be explained by the higher degree of flexibility
of the molecule, thereby making the cyclization step more challenging due to
unfavorable entropy. In order to study how the intramolecular cyclization is affected
when functional groups are introduced at position C-5, the D-mannofuranose derivative
16 was tested next. Surprisingly, after 1 h of reaction under the same conditions, the
desired cyclized compound 24 was isolated in an unsatisfactory 26% yield (entry 4,
Table 2). We believe this poor reactivity might result from a hydrogen bond stablished
between the benzimidazole amine and the neighboring methoxy group, that translates
into a deactivation of the nucleophile. Then, in order to evaluate the effectiveness of the
radical tandem β-fragmentation on six-membered ring derivatives, D-mannopyranose
derivative 20 was treated with PhI(Phth)/I2 as before (entry 5, Table 2). In this case,
despite the presence of a methoxy group at position C-4, compound 25 was obtained
with a good yield. This could stem from the fact that the six-membered ring derivative
20 possesses a more restricted flexibility, which prevents such hydrogen-bonding and
preorganizes the system into a conformation that allows the cyclization reaction to
proceed more easily.
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Table 2.- Synthesis of Benzimidazolo-sugarsa
Substrate
t(h)
Products (yield %)
Entry
Yield (%)
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a Reaction conditions: All reactions were performed at 0.05 M in dry EtOAc (20 mL)
under irradiation with two 80 W tungsten-filament lamps at reflux temperature
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containing PhI(Phth) (2.2 mmol) and I2 (1.2 mmol) per mmol of substrate; Isolated
yield.
Finally, in order to show the applicability of our methodology to produce new sugar
mimics with potential inhibitory properties, two selected examples were partially or
totally deprotected. In our experience, the formate group can be easily removed under
mild basic conditions, resulting in a compound with a free hydroxyl group ready for
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further derivatization if so desired. For example, when compound 21 was treated under
Zemplén conditions, an oil identified as the corresponding alcohol was obtained in 80 %
yield. Alternatively, the total elimination of all protecting groups can be carried out
under standard acidic conditions, i.e. with TFA in DCM or TFA in THF/H2O at room
temperature or reflux, depending on the model. As an example, compound 25 was
treated with TFA in DCM to yield an oil identified as 27 after column purification
(Scheme 8).
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Scheme 8.- Reagents and conditions: a) MeONa, MeOH, rt, 80%; b) TFA, DCM, rt,
40%.
3.- CONCLUSION
In summary, the first approach to the synthesis of benzimidazole-fused
iminosugars through a tandem β-fragmentation-intramolecular cyclization reaction has
been presented. This key radical step was newly promoted by phenyliodosophtalate
(PhI(Phth)), a metal-free hypervalent iodine (III) promoter, easily synthesized in a two-
step procedure. Due to its non-nucleophilic properties, PhI(Phth) has been found to be
more effective than DIB in generating the corresponding cyclic products. Furthermore,
PhI(Phth) has demonstrated a greater stability compared to the well-known PhIO
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reagent. By using this radical-polar crossover reaction, hybrid benzimidazole-sugars
were produced in moderate to good yields starting from their corresponding
hemiacetalic precursors. Arguably, the synthesis of these final compounds involves four
successive reaction steps: formation of the anomeric alkoxyl radical, β-fragmentation of
the alkoxyl radical, oxidation of the resulting C-radical to generate the corresponding
oxocarbenium ion, and trapping of this cationic intermediate by the nucleophilic
nitrogen of the benzimidazole. Thus, the overall yield for the synthesis of compounds
21 to 25 actually constitutes a combined yield of a four-step process, with up to 85%
yield for each one of these individual steps. We believe the straightforward
transformation of hemiacetals into the corresponding fused benzimidazole-sugars
described in this study, as well as the demonstrated possibility to remove all protecting
groups, can be extrapolated to other chiral substrates and will therefore be of ample
interest to the synthetic scientific community.
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4. EXPERIMENTAL SECTION
General Methods. Optical rotations were measured with a polarimeter at the sodium
line at ambient temperature in CHCl3 solutions. IR spectra were recorded CHCl3 unless
otherwise stated, with a FTIR instrument. NMR spectra were recorded with a 400 MHz
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spectrometer for H and 100.6 MHz for C in CDCl3 unless otherwise stated, in the
presence of TMS as internal standard. Structures were confirmed by COSY, DEPT,
HSQC and NOESY experiments when necessary. Mass spectra were recorded with a
spectrometer by using electrospray ionization (ESI+-TOF). Merck silica gel 60 PF
(0.063-0.2 mm) was used for column chromatography. Circular layers of 1 mm of
Merck silica gel 60 PF254 were used on a Chromatotron for centrifugally assisted
chromatography. Commercially available reagents and solvents were analytical grade or
were purified by standard procedures prior to use. All reactions involving air- or
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moisture-sensitive materials were carried out under a nitrogen atmosphere. The staining
reagents for TLC analysis were conducted with 0.5% vanillin in H2SO4-EtOH (1:4) and
further heating until development of color.
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(5R)-5-O-Benzoyl-3,4-O-isopropylidene-D-arabinodialdo-5,2-furanose (2):14 To a
solution of 1-O-benzoyl-2,3-O-isopropylidene-α-D-mannofuranose9 (1) (1620 mg, 5.00
mmol) in a (1:1) acetone/water mixture (50 mL, 10 mL/mmol) was added NaIO4 (1872
mg, 8.75 mmol). The reaction mixture was stirred for 3.5 h at room temperature. The
mixture was then diluted with DCM and poured into water, extracted several time with
DCM, dried over sodium sulfate and concentrated under vacuum. The crude was
directly engaged in the next step without further purification to prevent the potential
degradation of the compound. As a consequence, all the aldehyde derivatives
synthetized were directly engaged in the next step without further purifications.
1-O-Benzoyl-2,3-O-isopropylidene-β-D-ribodialdo-1,4-furanose (6): IBX (421 mg,
1.50 mmol) was dissolved in DMSO (4 mL). The dissolution requires 5-15 minutes. 1-
O-Benzoyl-2,3-O-isopropylidene-α-D-ribofuranose (5)10 (294 mg, 1.00 mmol) was then
added to the mixture. The solution was stirred at room temperature without any
particular precaution (such as inert atmosphere and dry solvent) during 3 h. Then the
mixture was diluted with water to give a white precipitate, filtrated, extracted with ether,
and finally concentrated under vacuum to afford a pink oil. The crude was directly
engaged in the next step.
Benzyl 5-deoxy-2,3-O-isopropylidene-β-D-ribo-hexodialdo-1,4-furanoside (10): A
solution of the corresponding nitrile 912 (504 mg, 1.74 mmol) in dry toluene (11 mL, 2
mL/mmol) was cooled to 78 °C before addition of DIBAL-H (1M in Toluene), and
stirred at this temperature for 1.5 h. Afterward, 1M HCl was added until the organic
layer became clear. The reaction mixture was extracted with ice-cold Et2O and the
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combined organic layer was dried over sodium sulfate, filtrated and concentrated under
vacuum to afford a yellow oil. The crude was directly engaged in the next step.
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4-Methoxybenzyl
(6S)-2-O-methyl-4,5-O-isopropylidene-D-manno-dialdo-6,3-
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furanoside (14): A solution of 4-methoxybenzyl 5-O-methyl-2,3-O-isopropylidene-α-
D-mannofuranoside (13)7b (500 mg, 1.41 mmol) in dry DCM (10 mL) containing Dess-
Martin periodinane (DMP) (721 mg, 1.70 mmol) was stirred at room temperature for 1.5
h. The reaction mixture was diluted with ether (30 mL) and 1.3M NaOH (10 mL) was
added, stirred for 15 minutes, and extracted. The organic layer was washed with water,
dried and concentrated under vacuum to afford a yellow pale oil. The crude obtained
was directly engaged in the next step.
Benzyl
(6S)-3-O-methyl-4,5-O-isopropylidene-α-D-mannodialdo-6,2-pyranoside
(18):7b A solution of benzyl 4-O-methyl-2,3-O-isopropylidene-α-D-glucopyranoside
(17)7b (487 mg, 1.50 mmol) in dry DCM (12 mL) containing DMP (1272 mg, 3.00
mmol) was stirred at room temperature for 2 h. The reaction mixture was diluted with
ether (30 mL) and 1.3M NaOH (12 mL) was added, stirred for 15 minutes and
extracted. The organic layer was washed with water, dried and concentrated under
vacuum. The crude was then purified by column chromatography on silica gel
(hexane/EtOAc 7:3) to afford the corresponding aldehyde (285 mg, 0.89 mmol, 59%) as
a pale yellow oil.
General procedure for the synthesis of benzimidazole8 derivatives: CeCl3 (0.3
mmol), CuI (0.3 mmol) and I2 (2.0 mmol) were dissolved in acetonitrile (15 mL). The
mixture was stirred at room temperature and under agitation during 30 minutes. Then
the corresponding aldehyde (1.0 mmol) was added. The mixture was stirred during 10
minutes before addition of the diamine (1.5 mmol). Then the solution was stirred at
room temperature for 1-5 h. The mixture was then extracted and washed with aqueous
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sodium thiosulfate and EtOAc. The organic phase was dried and concentrated under
vacuum. The residue was purified by column chromatography on silica gel
(hexane/EtOAc mixtures) to afford the desired benzimidazole derivative.
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(4R)-4-C-(1H-benzimidazol-2-yl)-1-O-benzoyl-2,3-O-isopropylidene--L-erythro
furanoside (3): Following the general procedure, compound 2 (1460 mg, 5.00 mmol)
afforded, after 1 h, the desired compound 3 (1653 mg, 4.35 mmol, 87% two steps) as an
off-white powder after silica gel chromatography (hexane/EtOAc 1:1): IR: 휈max 3458,
1
2995, 2952, 1732, 1602, 1455, 1260, 1090 cm-1; H NMR (400 MHz, CDCl3): δH 9.63
(bs, 1H, NH), 8.058.03 (m, 2H, Har), 7.76 (bs, 1H, Har), 7.637.60 (m, 1H, Har),
7.497.45 (m, 3H, Har), 7.297.27 (m, 2H, Har), 6.58 (s, 1H, H1), 5.63 (d, J = 3.4 Hz,
1H, H2), 5.26 (dd, J = 5.6, 3.4 Hz, 1H, H3), 5.05 (d, J = 5.6 Hz, 1H, H4), 1.53 (s, 3H,
CH3), 1.35 (s, 3H, CH3); 13C NMR (100.6 MHz, CDCl3): δC 164.9 (C, C=O), 148.2 (C,
C=N), 142.8 (C, Car), 133.6 (CH, Car), 133.4 (C, Car), 129.7 (2 x CH, Car) 129.0 (C, Car),
128.4 (2 x CH, Car), 123.0 (CH, Car), 122.1 (CH, Car), 119.6 (CH, Car), 113.6 (C, isop),
111.2 (CH, Car), 100.9 (CH, C1), 85.0 (CH, C4), 80.9 (CH, C3), 79.0 (CH, C2), 26.0
(CH3, isop), 24.4 (CH3, isop); MS (ESI+) m/z (rel intensity) 403 [(M+Na)+, 100]; HRMS
(ESI-TOF) m/z [M+Na]+ Calcd for C21H20N2O5Na 403.1270; Found 403.1266.
(4R)-4-C-(1H-benzimidazol-2-yl)-1-O-benzoyl-2,3-O-isopropylidene--D-erythro
furanoside (7): Following the general procedure, compound 6 (292 mg, 1.00 mmol)
afforded, after 5 h, the desired compound 7 (182 mg, 0.48 mmol, 48% two steps) as a
20
yellow oil after silica gel chromatography (hexane/EtOAc 80:20): [α]D +81.9 (c 0.16,
1
CHCl3); IR: 휈max 2985, 2962, 1728, 1602, 1454, 1270, 1094 cm-1; H NMR (400 MHz,
CDCl3): δH 10.11 (s, 1H, NH), 7.73 (d, J = 7.9 Hz, 1H, Har), 7.30 (t, J = 7.4 Hz, 1H,
Har), 7.247.19 (m, 3H, Har), 7.10 (bs, 2H, Har), 6.94 (t, J = 7.8 Hz, 2H, Har), 6.58 (s,
1H, H1), 6.07 (d, J = 5.8 Hz, 1H, H2), 5.66 (s, 1H, H4), 4.99 (d, J = 5.8 Hz, 1H, H3), 1.55
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(s, 3H, isop), 1.42 (s, 3H, isop); C NMR (100.6 MHz, CDCl3): δC 164.9 (C, C=O),
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151.6 (C, C=N), 142.6 (C, Car), 134.1 (C, Car), 133.4 (CH, Car), 129.2 (2 x CH, Car),
128.2 (C, Car), 128.0 (2 x CH, Car), 123.4 (CH, Car), 122.2 (CH, Car), 119.4 (CH, Car),
113.2 (C, isop), 110.8 (CH, Car), 102.6 (CH, C1), 84.8 (CH, C3), 82.8 (CH, C4), 82.3
(CH, C2), 26.2 (CH3, isop), 24.7 (CH3, isop); MS (ESI+) m/z (rel intensity) 403
[(M+Na)+, 100]; HRMS (ESI-TOF) m/z [M+Na]+ Calcd for C21H20N2O5Na 403.1270;
Found 403.1266.
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Benzyl 5-(1H-benzimidazol-2-yl)-5-deoxy-2,3-O-isopropylidene-β-D-ribofuranoside
(11): Following the general procedure, compound 10 (508 mg, 1.74 mmol) afforded,
after 1.5 h, the desired compound 11 (352 mg, 0.93 mmol, 53% two steps) after silica
gel chromatography (gradient hexane/EtOAc from 50:50 to 0:100) as a yellow oil: IR:
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휈max 3436, 3034, 2958, 1614, 1515, 1456, 1376, 1252, 1083 cm-1; H NMR (400 MHz,
CDCl3): δH 9.69 (bs, 1H, NH), 7.617.22 (m, 9H, Har), 5.31 (s, 1H, H1), 4.80 (d, J = 6.1
Hz, 1H, H2), 4.77 (d, J = 6.1 Hz, 1H, H3), 4.72 (t, J = 7.4 Hz, 1H, H4), 4.71 (d, J = 11.9
Hz, 1H, ABsyst), 4.60 (d, J = 11.9 Hz, 1H, ABsyst), 3.28 (d, J = 7.3 Hz, 2H, H5), 1.49 (s,
3H, isop), 1.32 (s, 3H, isop); 13C NMR (100.6 MHz, CDCl3): δC 151.3 (C, C=N), 136.9
(C, Car), 128.7 (2 x CH, Car), 128.5 (C, Car), 128.1 (CH, Car), 127.9 (C, Car), 127.8 (2 x
CH, Car), 122.3 (2 x CH, Car), 119.1 (CH, Car), 112.9 (C, isop), 110.5 (CH, Car), 108.8
(CH, C1), 85.4 (CH, C2), 85.2 (CH, C4), 83.8 (CH, C3), 70.3 (CH2, C[Bn]), 34.7 (CH2,
C5), 26.5 (CH3, isop), 25.0 (CH3, isop); MS (ESI+) m/z (rel intensity) 381 [(M+H)+,
100]; HRMS (ESI-TOF) m/z [M+Na]+ Calcd for C22H25N2O4 381.1814; Found
381.1811.
4-Methoxybenzyl (5R)-5-C-(1H-benzimidazol-2-yl)-5-O-methyl-2,3-O-isopropylide
ne-β-D-lyxofuranoside (15): Following the general procedure, compound 14 (496 mg,
1.41 mmol) afforded, after 2.5 h, the desired compound 15 (445 mg, 1.01 mmol, 72%
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two steps) after silica gel chromatography (gradient hexane/EtOAc from 50:50 to 30:70)
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as a yellow oil: [α]D 7.14 (c 0.28, CHCl3); IR: 휈max 3692, 3449, 3000, 2940, 1613,
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1515, 1249, 1084 cm-1; H NMR (500 MHz, CDCl3): δH 9.70 (bs, 1H, NH), 7.847.82
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(m, 1H, Har), 7.487.47 (m, 1H, Har), 7.307.26 (m, 1H, Har), 7.12 (d, J = 8.5 Hz, 2H,
Har), 6.79 (d, J = 8.5 Hz, 2H, Har), 6.736.71 (m, 1H, Har), 5.18 (s, 1H, H1), 4.90 (d, J =
6.6 Hz, 1H, H5), 4.88 (dd, J = 6.0, 3.8 Hz, 1H, H3), 4.64 (d, J = 6.0 Hz, 1H, H2), 4.46 (d,
J = 11.3 Hz, 1H, CH2), 4.36 (d, J = 11.3 Hz, 1H, CH2), 4.35 (dd, J= 7.0, 4.0 Hz, 1H,
H4), 3.78 (s, 3H, OCH3), 3.42 (s, 3H, OCH3[PMB]), 1.28 (s, 3H, isop), 1.21 (s, 3H,
isop); 13C NMR (125.7 MHz, CDCl3): δC 159.2 (C, Car), 152.2 (C, C=N), 142.8 (C, Car),
133.7 (C, Car), 129.5 (2 x CH, Car), 129.3 (C, Car), 123.1 (CH, Car), 122.0 (CH, Car),
120.3 (CH, Car), 113.8 (2 x CH, Car), 112.6 (C, isop), 110.8 (CH, Car), 105.5 (CH, C1),
84.6 (CH, C5), 80.9 (CH, C3), 79.5 (CH, C4), 75.4 (CH, C2), 69.0 (CH2, PMB), 58.0
(CH3, OMe), 55.2 (CH3, OMe), 25.5 (CH3, isop), 24.5 (CH3, isop); MS (ESI+) m/z (rel
intensity) 463 [(M+Na)+, 100]; HRMS (ESI-TOF) m/z [M+Na]+ Calcd for
C24H28N2O6Na 463.1845; Found 463.1833.
Benzyl (5R)-5-(1H-benzimidazol-2-yl)-4-O-methyl-2,3-O-isopropylidene--L-ribo
pyranoside (19): Following the general procedure, compound 18 (285 mg, 0.89 mmol)
afforded, after 2.5 h, the desired compound 19 (379 mg, 0.86 mmol, 97%) after silica
gel chromatography (gradient hexane/EtOAc from 70:30 to 30:70) as a pale yellow
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powder: H NMR (500 MHz, CDCl3): δH 9.47 (bs, 1H, NH), 7.857.83 (m, 1H, Har),
7.467.45 (m, 1H, Har), 7.327.27 (m, 7H, Har), 5.29 (s, 1H, H1), 4.89 (d, J = 9.8 Hz,
1H, H2), 4.74 (d, J = 12.0 Hz, 1H, ABsyst), 4.60 (d, J = 12.0 Hz, 1H, ABsyst), 4.40 (t, J =
6.3 Hz, 1H, H4), 4.29 (d, J = 6.3 Hz, 1H, H5), 3.60 (dd, J = 9.8, 6.3 Hz, 1H, H3), 3.36 (s,
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3H, OCH3), 1.56 (s, 3H, isop), 1.39 (s, 3H, isop); C NMR (125.7 MHz, CDCl3): δC
151.2 (C, C=N), 143.1 (C, Car), 136.8 (C, Car), 133.3 (C, Car), 128.5 (2 x CH, Car), 128.2
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(2 x CH, Car), 128.0 (CH, Car), 123.2 (CH, Car), 122.2 (CH, Car), 120.1 (CH, Car), 110.7
(CH, Car), 109.6 (C, isop), 97.1 (CH, C1), 80.8 (CH, C3), 77.8 (CH, C4), 75.5 (CH, C5),
69.9 (CH2, Bn), 66.0 (CH, C2), 59.3 (CH3, OCH3), 27.9 (CH3, isop), 26.1 (CH3, isop);
MS (ESI+) m/z (rel intensity) 433 [(M+Na)+, 100]; HRMS (ESI-TOF) m/z [M+Na]+
Calcd for C23H26N2O5Na 433.1739; Found 433.1739.
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General procedure for the hydrolysis of anomeric benzoate: To a solution of the
corresponding protected compound (1.0 mmol) in dry methanol (5 mL) was added
sodium methoxide (2.0 mmol). After 1–3 h of stirring, the reaction mixture was
concentrated under vacuum. The crude was then purified by column chromatography on
silica gel (hexane/EtOAc mixtures) to afford the desired anomeric alcohol.
(4R)-4-C-(1H-benzimidazol-2-yl)-2,3-O-isopropylidene-L-erythrofuranoside
(4):
Following the general procedure, compound 3 (1210 mg, 3.21 mmol) afforded, after 3
h, the desired compound 4 (693 mg, 2.51 mmol, 79%) after silica gel chromatography
(gradient hexane/EtOAc from 80:20 to 30:70) as a pale brown powder crystalline
compound: m.p. (n-hexane/Ethyl acetate) 193195 ºC; IR: 휈max 3691, 3602, 3408, 2990,
2934, 1602, 1457, 1380, 1231, 1165, 1069 cm-1; NMR showed a mixture of anomers in
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a 20:1 / ratio. Only major compound is described. H NMR (400 MHz, CDCl3): δH
7.49 (d, J = 7.9 Hz, 1H, Har), 7.42 (d, J = 7.9 Hz, 1H, Har), 7.25 (td, J = 7.9, 1.3 Hz, 1H,
Har), 7.20 (td, J = 7.9, 1.3 Hz, 1H, Har), 5.87 (d, J = 1.3 Hz, 1H, H1), 5.46 (d, J = 2.6 Hz,
1H, H4), 5.11 (dd, J = 6.9, 2.6 Hz, 1H, H3), 5.01 (dd, J = 6.9, 1.4 Hz, 1H, H2), 1.37 (s,
3H, CH3), 0.95 (s, 3H, CH3); 13C NMR (100.6 MHz, CDCl3): δC 151.2 (C, C=N), 140.9
(C, Car), 133.6 (C, Car), 124.0 (CH, Car), 123.2 (CH, Car), 119.1 (CH, Car), 109.8 (C and
CH, isop and Car), 77.4 (CH, C1), 77.1 (CH, C3), 76.9 (CH, C2), 64.6 (CH, C4), 26.2
(CH3, isop), 24.3 (CH3, isop); MS (ESI+) m/z (rel intensity) 299 [(M+Na)+, 100]; HRMS
(ESI-TOF) m/z [M+Na]+ Calcd for C14H16N2O4Na 299.1008; Found 299.1008.
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(4R)-4-C-(1H-benzimidazol-2-yl)-2,3-O-isopropylidene-D-erythrofuranoside
(8):
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Following the general procedure, compound 7 (364 mg, 0.96 mmol) afforded, after 1 h,
the desired compound 8 (222 mg, 0.80 mmol, 84%) after silica gel chromatography
(gradient hexane/EtOAc from 70:30 to 30:70) as a white powder: IR: 휈max 3691, 3602,
3455, 2990, 2938, 1602, 1456, 1385, 1160, 1073 cm-1; NMR showed a mixture of
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anomers in a 1:20  ratio. Only major compound is described. H NMR (400 MHz,
CD3OD): δH 7.67 (dd, J = 7.2, 1.1 Hz, 1H, Har), 7.53 (d, J = 7.2 Hz, 1H, Har), 7.317.22
(m, 2H, Har), 5.96 (d, J = 1.3 Hz, 1H, H1), 5.28 (d, J = 3.4 Hz, 1H, H4), 4.98 (dd, J =
7.2, 3.4 Hz, 1H, H3), 4.80 (dd, J = 7.2, 1.3 Hz, 1H, H2), 1.33 (s, 3H, isop), 0.87 (s, 3H,
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isop); C NMR (100.6 MHz, CD3OD): δC 154.3 (C, C=N), 143.6 (C, Car), 135.8 (C,
Car), 124.1 (CH, Car), 123.5 (CH, Car), 119.9 (CH, Car), 111.4 (C, isop), 110.5 (CH, Car),
79.0 (CH, C1), 78.4 (CH, C3), 77.1 (CH, C2), 66.5 (CH, C4), 26.7 (CH3, isop), 24.6
(CH3, isop); MS (ESI+) m/z (rel intensity) 277 [(M+H)+, 100]; HRMS (ESI-TOF) m/z
[M+Na]+ Calcd for C14H17N2O4 277.1188; Found 277.1181.
5-(1H-benzimidazol-2-yl)-5-deoxy-2,3-O-isopropylidene-D-ribofuranose (12): To a
solution of 11 (34 mg, 0.09 mmol) in EtOAc (2 mL) was added 30% Pd(OH)2/C (10
mg, 20% w/w). Three vacuum/hydrogen cycles were performed, and the mixture was
stirred at rt and under hydrogen atmosphere for 72 h. The reaction mixture was then
filtered over a Celite pad, washed with EtOAc, and the filtrate was evaporated. The
crude was purified by column chromatography on silica gel (hexane/EtOAc 1:1) to yield
compound 12 (21 mg, 0.07 mmol, 81%) as a pale yellow oil: IR: 휈max 3452, 3427, 2995,
2952, 1537, 1456, 1385, 1271, 1072 cm-1; NMR showed a mixture of anomers in a 1:6
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 ratio. Only major compound is described. H NMR (500 MHz, CDCl3): δH
7.367.34 (m, 2H, Har), 7.147.12 (m, 2H, Har), 5.64 (s, 1H, H1), 4.73 (d, J = 5.7 Hz,
1H, H2), 4.69 (d, J = 6.0 Hz, 1H, H3), 4.604.57 (m, 1H, H4), 3.33 (dd, J = 15.1, 10.4
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Hz, 1H, H5), 3.24 (dd, J = 15.1, 4.1 Hz, 1H, H5), 1.46 (s, 3H, isop), 1.30 (s, 3H, isop);
13C NMR (125.7 MHz, CDCl3): δC 151.6 (C, C=N), 143.5 (C, Car), 137.5 (C, Car), 122.6
(2 x CH, Car), 115.5 (CH, Car), 114.7 (CH, Car), 112.7 (C, isop), 103.9 (CH, C1), 86.7
(CH, C2), 84.7 (CH, C4), 84.0 (CH, C3), 34.6 (CH2, C5), 26.5 (CH3, isop), 25.0 (CH3,
isop); MS (ESI+) m/z (rel intensity) 313 [(M+Na)+, 100]; HRMS (ESI-TOF) m/z
[M+Na]+ Calcd for C15H18N2O4Na 313.1164; Found 313.1158.
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(5R)-5-C-(1H-benzimidazol-2-yl)-5-O-methyl-2,3-O-isopropylidene-D-lyxofuranose
(16): To a stirred solution of 15 (43 mg, 0.10 mmol) in acetonitrile/water mixture (1
mL, 10:1 v/v) at 0 °C was added cerium (IV) ammonium nitrate (CAN) (164 mg, 0.30
mmol) portion-wise. The reaction mixture was stirred at rt for 2 h, then concentrated
under vacuum, and the crude was purified by column chromatography on silica gel
(hexane/EtOAc 1:1) to afford the compound 16 (32 mg, 0.10 mmol, quant.) as a yellow
oil: IR: 휈max 3691, 3607, 2924, 2853, 1725, 1264, 1602, 1098 cm-1; NMR showed a
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mixture of anomers in a 20:1  ratio. Only major compound is described. H NMR
(500 MHz, CD3OD): δH 7.89 (bs, 1H, NH), 7.617.59 (m, 2H, Har), 7.317.29 (m, 2H,
Har), 5.17 (s, 1H, H1), 4.96 (dd, J = 5.7, 3.5 Hz, 1H, H3), 4.82 (d, J = 8.8 Hz, 1H, H5),
4.58 (d, J = 5.7 Hz, 1H, H2), 4.46 (dd, J = 8.8, 3.5 Hz, 1H, H4), 3.36 (s, 3H, OCH3),
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1.45 (s, 3H, isop), 1.36 (s, 3H, isop); C NMR (125.7 MHz, CD3OD): δC 154.4 (C,
C=N), 143.5 (C, Car), 138.1 (C, Car), 124.6 (2 x CH, Car), 116.0 (CH, Car), 114.1 (C,
isop), 113.9 (CH, Car), 102.4 (CH, C1), 87.2 (CH, C2), 81.7 (CH, C4), 81.2 (CH, C3),
76.5 (CH, C5), 58.4 (CH3, OCH3), 26.5 (CH3, isop), 25.1(CH3, isop); MS (ESI+) m/z (rel
intensity) 321 [(M+H)+, 100]; HRMS (ESI-TOF) m/z [M+Na]+ Calcd for C16H21N2O5
321.1450; Found 321.1448.
(5R)-5-(1H-benzimidazol-2-yl)-4-O-methyl-2,3-O-isopropylidene-L-ribopyranose
(20): To a solution of 19 (340 mg, 0.77 mmol) in EtOAc (15 mL) was added 30%
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Pd(OH)2/C (102 mg, 20% w/w). Three vacuum/hydrogen cycles were performed, and
the mixture was stirred at rt and under hydrogen atmosphere for 48 h. The reaction
mixture was then filtered over a Celite pad, washed with EtOAc, and the filtrate was
evaporated. The crude was purified by column chromatography on silica gel
(hexane/EtOAc 1:1) to yield compound 20 (205 mg, 0.64 mmol, 83%) as a pale yellow
powder. IR: 휈max 3448, 2992, 2940, 1623, 1456, 1384, 1244, 1072 cm-1; NMR showed a
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mixture of isomers in 5:1 / ratio. Only the major one is described. H NMR (500
MHz, CDCl3): δH 7.307.20 (m, 4H, Har), 5.61 (s, 1H, H1), 5.29 (d, J = 8.5 Hz, 1H, H2),
4.49 (t, J = 6.1 Hz, 1H, H4), 4.34 (d, J = 6.1 Hz, 1H, H5), 3.78 (dd, J = 8.5, 6.1 Hz, 1H,
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H3), 3.50 (s, 3H, OCH3), 1.47 (s, 3H, isop), 1.39 (s, 3H, isop); C NMR (100.6 MHz,
CDCl3): δC 152.4 (C, C=N), 141.5 (C, Car), 136.2 (C, Car), 123.5 (CH, Car), 123.0 (CH,
Car), 119.2 (CH, Car), 110.1 (C, isop), 109.5 (CH, Car), 92.2 (CH, C1), 80.2 (CH, C3),
76.4 (CH, C5), 76.0 (CH, C4), 65.6 (CH, C2), 59.0 (CH3, OCH3), 27.4 (CH3, isop), 25.7
(CH3, isop); MS (ESI+) m/z (rel intensity) 343 [(M+Na)+, 100]; HRMS (ESI-TOF) m/z
[M+Na]+ Calcd for C16H20N2O5Na 343.1270; Found 343.1283.
Phenyliodosophthalate synthesis:12 Dibutylphthalate (2000 mg, 7.20 mmol) was
dissolved in methanol (6 mL) before addition of potassium hydroxide (1613 mg, 28.80
mmol) and the reaction mixture was stirred for 1 h at room temperature. The mixture
was then poured into water and extracted with ether. The collected aqueous layer was
acidified with 6N HCl, extracted with ethyl acetate, and the organic phase was
concentrated under vacuum to afford the desired phthalic acid (1195 mg, 7.20 mmol,
quant.) as a white powder. Afterward, a solution of phthalic acid (1195 mg, 7.20 mmol)
and DIB (2318 mg, 7.20 mmol) in chlorobenzene (50 mL) was stirred for 1 h at 50°C,
and the reaction mixture was concentrated under vacuum. The solid obtained was
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washed several time with hexane, and finally concentrated under vacuum to afford the
desired PhI(Phth) (2358 mg, 6.41 mmol, 89%) as a white powder.
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General procedure for the synthesis of fused benzimidazole-sugars 21, 22, 23, 24
and 25 using PhI(Phth): A solution of the corresponding hemiacetal (1.0 mmol) in dry
EtOAc (20 mL) containing PhI(Phth) (2.2 mmol) and iodine (1.2 mmol) was irradiated
with 80 W tungsten-filament lamps at 80°C for 1 –1.7 h. The reaction mixture was then
poured into aqueous sodium thiosulfate and extracted. The organic layer was dried over
sodium sulfate and concentrated under vacuum. The crude was purified by column
chromatography on silica gel (hexane/EtOAc mixtures) to afford the desired product.
(1S,2S,3R)-3-O-formyl-1,2-O-isopropylidene-2,3-dihydro-1H-benzo[d]pyrrolo[1,2-
a]imidazolo-1,2,3-triol (21): Following the general procedure, compound 4 (342 mg,
1.23 mmol) afforded, after 1.7 h, the desired compound 21 (252 mg, 0.92 mmol, 75%)
after silica gel chromatography (hexane/EtOAc 70:30) as a white powder: [α]D20 +70 (c
0.14, CHCl3); IR: 휈max 2995, 2943, 1735, 1602, 1451, 1238, 1154, 1102 cm-1; 1H NMR
(400 MHz, CDCl3): δH 8.16 (s, 1H, OCOH), 7.847.82 (m, 1H, Har), 7.607.58 (m, 1H,
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Har), 7.377.34 (m, 2H, Har), 6.54 (d, J = 4.5 Hz, 1H, H1), 6.21 (s, 1H, H3), 5.26 (d, J =
13
4.5 Hz, 1H, H2), 1.51 (s, 3H, isop), 1.12 (s, 3H, isop); C NMR (100.6 MHz, CDCl3):
δC 159.3 (CH, OCOH), 153.1 (C, C=N), 148.7 (C, Car), 127.8 (C, Car), 124.0 (CH, Car),
123.6 (CH, Car), 120.9 (CH, Car), 115.3 (C, isop), 110.8 (CH, Car), 88.3 (CH, C1), 86.2
(CH, C2), 71.3 (CH, C3), 27.4 (CH3, isop), 27.2 (CH3, isop); MS (ESI+) m/z (rel
intensity) 297 [(M+Na)+, 100]; HRMS (ESI-TOF) m/z [M+Na]+ Calcd for
C14H14N2O4Na 297.0851; Found 297.0849.
(1S,2S,3R)-3-O-formyl-1,2-O-isopropylidene-2,3-dihydro-1H-benzo[d]pyrrolo[1,2-
a]imidazolo-1,2,3-triol (22): Following the general procedure, compound 8 (50 mg,
0.18 mmol) afforded, after 1 h, the desired compound 22 (26 mg, 0.09 mmol, 53%) after
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silica gel chromatography (hexane/EtOAc 70:30) as a white powder: [α]D 87.7 (c
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0.13, CHCl3); IR: 휈max 2995, 1735, 1602, 1451, 1232, 1162, 1084 cm-1; H NMR (400
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MHz, CDCl3): δH 8.26 (s, 1H, OCOH), 7.817.79 (m, 1H, Har), 7.597.57 (m, 1H, Har),
7.35 (dd, J = 6.1, 3.2 Hz, 2H, Har), 6.38 (d, J = 4.5 Hz, 1H, H1), 6.06 (d, J = 5.0 Hz, 1H,
H3), 5.60 (dd, J = 4.5, 5.0 Hz, 1H, H2), 1.51 (s, 3H, isop), 1.04 (s, 3H, isop); 13C NMR
(100.6 MHz, CDCl3): δC 159.6 (CH, OCOH), 153.2 (C, C=N), 148.8 (C, Car), 125.7 (C,
Car), 123.8 (CH, Car), 123.5 (CH, Car), 120.8 (CH, Car), 115.8 (C, isop), 110.8 (CH, Car),
85.6 (CH, C1), 81.7 (CH, C2), 66.5 (CH, C3), 27.5 (CH3, isop), 27.1 (CH3, isop); MS
(ESI+) m/z (rel intensity) 297 [(M+Na)+, 100]; HRMS (ESI-TOF) m/z [M+Na]+ Calcd
for C14H14N2O4Na 297.0851; Found 297.0857.
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(1S,2S,3R)-3-O-formyl-1,2-O-isopropylidene-1,2,3,4-tetrahydrobenzo[4,5]imidazo
[1,2-a]pyridine-1,2,3-triol (23): Following the general procedure, compound 12 (37
mg, 0.13 mmol) afforded, after 2 h, the desired compound 23 (16 mg, 0.06 mmol, 43%)
20
after silica gel chromatography (hexane/EtOAc 70:30) as a white powder: [α]D 105
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(c 0.20, CHCl3); IR: 휈max 3021, 2962, 1733, 1602, 1458, 1173, 1173, 1083 cm-1; H
NMR (500 MHz, CDCl3): δH 8.19 (s, 1H, OCOH), 7.737.71 (m, 1H, Har), 7.547.52
(m, 1H, Har), 7.327.28 (m, 2H, Har), 6.21 (d, J = 4.7 Hz, 1H, H1), 5.48 (ddd, J = 11.3,
5.7, 1.9 Hz, 1H, H3), 4.92 (dd, J = 4.7, 1.9 Hz, 1H, H2), 3.51 (dd, J = 15.5, 11.3 Hz, 1H,
H4), 3.46 (dd, J = 15.5, 5.7 Hz, 1H, H4), 1.52 (s, 3H, isop), 1.16 (s, 3H, isop); 13C NMR
(100.6 MHz, CDCl3): δC 159.7 (CH, OCOH), 147.0 (C, C=N), 143.5 (C, Car), 133.2 (C,
Car), 123.1 (CH, Car), 123.0 (CH, Car), 119.4 (CH, Car), 113.2 (C, isop), 110.4 (CH, Car),
81.2 (CH, C1), 74.1 (CH, C2), 66.8 (CH, C3), 27.2 (CH3, isop), 26.6 (CH3, isop), 25.2
(CH2, C4); MS (ESI+) m/z (rel intensity) 311 [(M+Na)+, 100]; HRMS (ESI-TOF) m/z
[M+Na]+ Calcd for C15H16N2O4Na 311.1008; Found 311.1008.
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(1S,2S,3R,4R)-3-O-formyl-1,2-O-isopropylidene-4-O-methyl-1,2,3,4-tetrahydro
benzo[4,5]imidazo[1,2-a]pyridine-1,2,3,4-tetraol (24): Following the general
procedure, compound 16 (32 mg, 0.10 mmol) afforded, after 1 h, the desired compound
24 (5 mg, 0.02 mmol, 26%) after silica gel chromatography (hexane/EtOAc 70:30) as a
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pale yellow powder: [α]D  (c 0.33, CHCl3); IR: 휈max 3050, 2996, 2928, 1730,
1694, 1539, 1456, 1377, 1226 cm-1; H NMR (500 MHz, CDCl3): δH 8.06 (s, 1H,
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OCOH), 7.837.81 (m, 1H, Har), 7.597.57 (m, 1H, Har), 7.357.33 (m, 2H, Har), 6.22
(d, J = 4.7 Hz, 1H, H1), 6.06 (ddd, J = 4.7, 3.2, 0.9 Hz, 1H, H3), 4.93 (d, J = 3.2 Hz, 1H,
H4), 4.84 (t, J = 4.7 Hz, 1H, H2), 3.73 (s, 3H, OCH3), 1.53 (s, 3H, isop), 1.27 (s, 3H,
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isop); C NMR (125.7 MHz, CDCl3): δC 159.4 (CH, OCOH), 147.4 (C, C=N), 143.3
(C, Car), 133.8 (C, Car), 123.5 (CH, Car), 123.2 (CH, Car), 120.3 (CH, Car), 112.8 (C,
isop), 110.6 (CH, Car), 81.0 (CH, C1), 74.0 (CH, C2), 72.0 (CH, C4), 67.5 (CH, C3), 58.8
(CH3, OCH3), 27.4 (CH3, isop), 26.4 (CH3, isop); MS (ESI+) m/z (rel intensity) 341
[(M+Na)+, 100]; HRMS (ESI-TOF) m/z [M+Na]+ Calcd for C16H18N2O5Na 341.1113;
Found 341.1118.
(1S,2S,3R,4R)-4-O-formyl-1,2-O-isopropylidene-3-O-methyl-1,2,3,4-tetrahydro
benzo[4,5]imidazo[1,2-a]pyridine-1,2,3,4-tetraol (25): Following the general
procedure, compound 20 (41 mg, 0.12 mmol) afforded, after 1 h, the desired compound
25 (22 mg, 0.07 mmol, 58%) after silica gel chromatography (hexane/EtOAc 1:1) as a
20
pale yellow powder: [α]D  (c 0.54, CHCl3); IR: 휈max 2994, 2938, 1697, 1617,
1459, 1385, 1109 cm-1; 1H NMR (400 MHz, CDCl3): δH 8.42 (s, 1H, OCOH), 7.807.78
(m, 1H, Har), 7.577.54 (m, 1H, Har), 7.327.30 (m, 2H, Har), 6.56 (d, J = 3.2 Hz, 1H,
H1), 6.19 (d, J = 4.8 Hz, 1H, H4), 4.83 (t, J = 4.8 Hz, 1H, H3), 4.39 (dd, J = 4.8, 3.2 Hz,
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1H, H2), 3.55 (s, 3H, OCH3), 1.51 (s, 3H, isop), 1.22 (s, 3H, isop); C NMR (125.7
MHz, CDCl3): δC 160.0 (CH, OCOH), 145.6 (C, C=N), 143.1 (C, Car), 134.0 (C, Car),
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123.4 (CH, Car), 123.0 (CH, Car), 120.1 (CH, Car), 112.7 (C, isop), 110.5 (CH, Car), 81.1
(CH, C1), 77.3 (CH, C2), 74.6 (CH, C3), 66.1 (CH, C4), 60.4 (CH3, OCH3), 27.5 (CH3,
isop), 26.2 (CH3, isop); MS (ESI+) m/z (rel intensity) 341 [(M+Na)+, 100]; HRMS (ESI-
TOF) m/z [M+Na]+ Calcd for C16H18N2O5Na 341.1113; Found 341.1106.
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(1S,2S,3R)-1,2-O-isopropylidene-2,3-dihydro-1H-benzo[d]pyrrolo[1,2-a]imidazolo-
1,2,3-triol (26): Following the general procedure for anomeric benzoate deprotection,
compound 21 (129.5 mg, 0.47 mmol) afforded, after 1 h, the desired compound 26 (93.0
mg, 0.38 mmol, 80%) after silica gel chromatography (hexane/EtOAc 1:1) as a colorless
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oil: H NMR (400 MHz, CDCl3): δH 7.667.64 (m, 1H, Har), 7.527.49 (m, 1H,
Har),7.287.24 (m, 2H, Har), 6.48 (d, J = 4.3 Hz, 1H, H1), 5.40 (s, 1H, H3), 5.26 (d, J =
13
4.3 Hz, 1H, H2), 1.47 (s, 3H, isop), 1.08 (s, 3H, isop); C NMR (100.6 MHz, CDCl3):
δC 158.9 (C, C=N), 147.3 (C, Car), 130.2 (C, Car), 123.6 (CH, Car), 123.3 (CH, Car),
119.7 (CH, Car), 114.5 (C, isop), 110.9 (CH, Car), 90.4 (CH, C1), 86.5 (CH, C2), 69.5
(CH, C3), 27.5 (CH3, isop), 27.0 (CH3, isop); MS (ESI+) m/z (rel intensity) 247
[(M+H)+, 100]; HRMS (ESI-TOF) m/z [M+H]+ Calcd for C13H15N2O3 247.1083; Found
247.1088.
(2S,3R,4R)-3-O-methyl-1,2,3,4-tetrahydrobenzo[4,5]imidazo[1,2-a]pyridine-1,2,3,4-
tetraol (27): To a stirred solution of 25 (20 mg, 0.06 mmol) in DCM (0.6 mL, 10
mL/mmol) at rt was added trifluoroacetic acid (TFA) (0.6 mL, 10 mL/mmol), and the
resulting reaction mixture was stirred for 24 h, then concentrated under vacuum, and the
crude was purified by column chromatography on silica gel (EtOAc) to afford the
compound 27 (6 mg, 0.02 mmol, 40%) as a colorless oil: MS (ESI+) m/z (rel intensity)
273 [(M+Na)+, 100]; HRMS (ESI-TOF) m/z [M+Na]+ Calcd for C12H14N2O4Na
273.0851; Found 273.0864.
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1H and C NMR spectra for all new compounds. This material is available free of
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ACKNOWLEDGMENTS
Financial support from Gobierno de Canarias (Research Program ProID20100088) and
the MINECO/FEDER (SAF-2013-48399-R) is gratefully acknowledged. EA-J thanks
the University of Nantes for a fellowship. AGS thanks the I3P-CSIC Program for a
fellowship and MINECO for a JdC-incorporación contract.
REFERENCES AND FOOTNOTES
(1) (a) Ajani, O. O.; Aderohunmu, D. V.; Ikpo, C. O.; Adedapo, A. E.; Olanrewaju, I. O.
Functionalized Benzimidazole Scaffolds: Privileged Heterocycle for Drug Design in
Therapeutic Medicine. Arch. Pharm. Chem. Life Sci. 2016, 349, 475–506. (b) Carvalho,
L. R. C.; Fernandes, E.; Marques, M. M. B. Developments Towards Regioselective
Synthesis of 1,2-Disubstituted Benzimidazoles. Chem. Eur. J. 2011, 17, 12544–12555.
(2) Bonnett, R. Chemistry of Vitamin B12 Group. Chem. Rev. 1963, 63 (6), 573–605.
(3) Bansal, Y.; Silakari, O. The Therapeutic Journey of Benzimidazoles: A Review.
Bioorg. Med. Chem. 2012, 20, 6208–6236.
(4) For syntheses of iminosugar-bearing tetrahydrobenzimidazo[1,2-a]pyridines, see: (a)
Yadav, L. D. S.; Awasthi, C. Efficient One-pot Synthetic Protocols for Iminosugar-
bearing Imidazo[1,2-a]pyridines from Carbohydrates. Carbohydr. Res. 2010, 345, 318–
323. (b) Sallam, M. A. E.; Waagen, V.; Anthonsen, T. Synthesis of N-Bridgehead
Heterocycles from Saccharide Benzimidazoles. Carbohydr. Res. 2008, 343, 388–391.
For hybrid aromatic heterocycles/carbohydrates, see: (c) Sánchez-Fernández, E. M.;
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Gonçalves-Pereira, R.; Rísquez-Cuadro, R; Plata, G. B.; Padrón, J. M.; García
Fernández, J. M.; Ortiz Mellet, C. Influence of the Configurational Pattern of sp2-
Iminosugar Pseudo N-, S-, O- and C-Glycosides on their Glycoside Inhibitory and
Antitumor Properties. Carbohydr. Res. 2016, 429, 113-122. (d) Zhong, M.; Meng, X.-
B.; Li, Z.-J. Synthesis of C-Aminobutyric Acid Analogs Based on Carbohydrate
Scaffolds. Carbohydr. Res. 2010, 345, 1099−1106. (e) Worch, M.; Wittmann, V.
Unexpected Formation of Complex Bridged Tetrazoles via Intramolecular 1,3-Dipolar
Cycloaddition of 1,2 O-Cyanoalkylidene Derivatives of 3-Azido-3-deoxy-D-allose.
Carbohydr. Res. 2008, 343, 2118−2129. (f) Anegundi, R. I.; Puranikb, V. G.; Hotha, S.
Synthesis of 1,2,3-Triazole and 1,2,3,4-Tetrazole-fused Glycosides and Nucleosides by
an Intramolecular 1,3-Dipolar Cycloaddition Reaction. Org. Biomol. Chem. 2008, 6,
779−786. (g) Couri, M. R.; Luduvico, I.; Santos, L.; Alves, R.; Prado, M. A.; Freitas
Gila, R. Microwave-assisted Efficient Preparation of Novel Carbohydrate Tetrazole
Derivatives. Carbohydr. Res. 2007, 342, 1096−1100. (h) Li, J.; Zheng, M.; Tang, W.;
He, P.-L.; Zhu, W.; Li, T.; Zuo, J.-P.; Liua, H.; Jianga, H. Syntheses of Triazole-
modified Zanamivir Analogues via Click Chemistry and anti-AIV Activities. Bioorg.
Med. Chem. Lett. 2006, 16, 5009−5013. (i) van der Peet, P.; Gannon, C. T.; Walker, I.;
Dinev, Z.; Angelin, M.; Tam, S.; Ralton, J. E.; McConville, M. J.; Williams, S. J. Use of
Click Chemistry to Define the Substrate Specificity of Leishmania -1,2-
Mannosyltransferases. ChemBioChem 2006, 7, 1384−1391. (j) Koumbis, A. E.; Gallos,
J. K. 1,3-Dipolar Cycloadditions in the Synthesis of Carbohydrate Mimics. Part 3:
Azides, Diazo Compounds and Other Dipoles. Curr. Org. Chem. 2003, 7, 771−797.
(5) For review, see: Suárez, E.; Rodríguez, M. S. -Fragmentation of Alkoxyl Radicals:
Synthetic Applications. In Radicals in Organic Synthesis; Renaud, P., Sibi, M. P.; Eds.,
Wiley-VCH: Weinheim, 2001, Vol. 2, pp 440–454. For selected papers on -
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Fragmentation of the research group, see: (a) Hernández-Guerra, D.; Rodríguez, M. S.;
Suárez, E. Fragmentation of Carbohydrate Anomeric Alkoxyl Radicals: Synthesis of
Chiral Polyhydroxylated beta-iodo and Alkenylorganophosphorus (V) Compounds. Eur.
J. Org. Chem. 2014, 23, 50335038; b) Francisco, C. G.; González, C. C.; Kennedy, A.
R.; Paz, N. R.; Suárez, E. Fragmentation of Carbohydrate Anomeric Alkoxyl Radicals:
New Synthesis of Chiral 1-Fluoro-1-halo-1-iodoalditols. Chem. Eur. J. 2008, 14, 6704–
6712; (c) Francisco, C. G.; González, C. C.; Kennedy, A. R.; Paz, N. R.; Suárez, E.
Synthesis and Stability of Mixed Nonfluorinated 1,1,1-trihalo-alkanes. Tetrahedron Lett.
2006, 47, 35–38; (d) González, C. C.; Kennedy, A. R.; León, E. I.; Riesco-Fagundo, C.;
Suárez, E. Fragmentation of Carbohydrate Anomeric Alkoxy Radicals: New Synthesis of
Chiral 1-Halo-1-iodo Alditols. Chem. Eur. J. 2003, 9, 5800–5809; (e) Francisco, C. G.;
González, C. C.; Kennedy, A. R.; Paz, N. R.; Suárez, E. Fragmentation of Carbohydrate
Anomeric Alkoxyl Radicals: A New Synthesis of Chiral 1,1-Difluoro-1-iodo Alditols.
Org. Lett. 2003, 5, 4171–4173.
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(6) Francisco, C. G.; Martin, C. G.; Suárez, E. Synthesis of -Iodoalkyl Esters and -
Iodoalkyl Carbonates from Carbohydrates. Formation of Convenient Chiral Synthetic
Intermediates. J. Org. Chem. 1998, 63, 8092–8093.
(7) (a) Santana, A. G.; Paz, N. R.; Francisco, C. G.; Suárez, E.; González, C. C. Synthesis
of Branched Iminosugars Through a Hypervalent Iodine(III)-Mediated Radical-Polar
Crossover Reaction. J. Org. Chem. 2013, 78, 7527–7543. (b) Paz, N. R.; Santana, A. G.;
Francisco, C. G.; Suárez, E.; González C. C. Synthesis of Tetrazole-Fused Glycosides by
a Tandem Fragmentation-Cyclization Reaction. Org. Lett. 2012, 14, 3388–3391; (c)
Francisco, C. G.; Freire R.; González, C. C.; Suárez, E. Fragmentation of Carbohydrate
Anomeric Alkoxy Radicals. Synthesis of Azasugars. Tetrahedron:Asymmetry 1997, 8,
1971–1974.
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