Joining hydroxyazobenzene and mannose under mitsunobu conditions

Article abstract of DOI:10.1002/ijch.201400211

The Mitsunobu reaction has been revisited to join hydroxyazobenzene and mannose derivatives to supply photoswitchable glycoconjugates. These are suited to modulating and controlling carbohydrate-lectin interactions, as well as to switching bacterial adhesion to surfaces. Employing hydroxyazobenzene in a Mitsunobu protocol, free mannose led to a mixture of azobenzene pyranosides and furanosides. Protected reducing mannose derivatives can give good yields of azobenzene β-D-mannopyranoside, and unprotected alkyl α-D-mannosides can be converted to 6-O-azobenzene-modified glycosides in a single step. Thus, valuable "sweet switches" can be obtained under Mitsunobu conditions, requiring a minimum (or no) protecting group chemistry.

Full text of DOI:10.1002/ijch.201400211

Communication
DOI: 10.1002/ijch.201400211
Joining Hydroxyazobenzene and Mannose under
Mitsunobu Conditions
Julia Hain, Vijayanand Chandrasekaran, and Thisbe K. Lindhorst*[a]
Abstract: The Mitsunobu reaction has been revisited to join
hydroxyazobenzene and mannose derivatives to supply pho-
toswitchable glycoconjugates. These are suited to modulat-
ing and controlling carbohydrate-lectin interactions, as well
as to switching bacterial adhesion to surfaces. Employing
hydroxyazobenzene in a Mitsunobu protocol, free mannose
led to a mixture of azobenzene pyranosides and furano-
sides. Protected reducing mannose derivatives can give
good yields of azobenzene b-d-mannopyranoside, and un-
protected alkyl a-d-mannosides can be converted to 6-O-
azobenzene-modified glycosides in a single step. Thus, val-
uable “sweet switches” can be obtained under Mitsunobu
conditions, requiring a minimum (or no) protecting group
chemistry.
Keywords: azobenzene glycosides · carbohydrates · glycoconjugates · Mitsunobu reaction · photoswitchable glycoconjugates
adhesion of live Escherichia coli bacteria to such glycoar-
rays.[4]
Azobenzene-functionalized carbohydrates have recently
gained interest as photosensitive glycoconjugates. This is
because photoisomerization of the azobenzene N=N
double bond can effect a defined orientational change of
the conjugated carbohydrate ligands, and thus, carbohy-
drate recognition can be modulated or, respectively, con-
trolled. Back in the 1990s, Willner showed that a thio-
phenfulgide-based molecular switch can indeed be used
to influence carbohydrate-lectin interactions.[1] Some time
later, the group of Jayaraman synthesized amide-linked
azobenzene glycoconjugates to photoswitch the interac-
tion between a-d-mannosides and Concanavalin A.[2]
Since 2012, our group has contributed to the synthesis
and investigation of azobenzene-based “sweet switches”
to study photoswitching of cellular adhesion.[3] In fact, we
demonstrated very recently that photoisomerization of
azobenzene a-d-mannoside ligands, ligated to a surface,
allows us to reversibly switch the carbohydrate-specific
As we will extend our studies on the specific photocon-
trol of cell adhesion to glycosylated surfaces, we are si-
multaneously concerned with the chemical synthesis of
the required azobenzene glycosides. In Scheme 1, the typ-
ical synthesis of an azobenzene glycoside, the a-d-manno-
side 7a, is depicted, comprising five steps, starting from
mannose.[5] When more complex azobenzene glycosides
are targeted, even more protecting group chemistry, and
thus, additional reaction steps are required.[6] Thus, we
put further emphasis on making azobenzene glycosides,
without the need for protecting group chemistry. This led
us to the investigation of the Mitsunobu reaction,[7] in
which an alcohol is substituted by an acid/nucleophile
component, mediated by the redox system triarylphos-
phine/dialkyl azodicarboxylate.
A review of the literature shows that the Mitsunobu re-
action has been employed regularly in carbohydrate
chemistry, however, only occasionally in the synthesis of
glycosides. Phenol was especially successfully reacted in
Mitsunobu glycosylation, leading, for example, to phenyl
a-d-mannoside in one step, and with yields greater than
80%.[8] A more complex phenol was employed en route
to hygromycin A.[9] In this case, however, an otherwise
[a] J. Hain, V. Chandrasekaran, T. K. Lindhorst
Otto Diels Institute of Organic Chemistry
Christiana Albertina University of Kiel
Otto-Hahn-Platz 3–4
24118 Kiel (Germany)
e-mail: tklind@oc.uni-kiel.de
Scheme 1. A five-step synthesis furnishes the azobenzene manno-
pyranoside 7a in typically ~60% yield from mannose.[5]
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/ijch.201400211.
Isr. J. Chem. 2015, 55, 383 – 386
ꢀ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
383

Communication
protected 1-OH-free carbohydrate had to be used, lead-
ing to an anomeric mixture of arabinosides.
figured mannopyranoside 7b was not seen in any of our
experiments. The scope of this straightforward synthesis
of azobenzene glycosides should be further explored, as it
provides a complementary option in the synthetic arsenal
used for preparation of azobenzene glycoconjugates.
To test whether the yields of the Mitsunobu glycosyla-
tion with hydroxyazobenzene (5) can be improved when
1-OH-free, otherwise protected carbohydrates were em-
ployed as the alcohol components. The readily available
reducing mannose derivative 3 was reacted under Mitsu-
nobu conditions, this time adding diisopropyl azodicar-
boxylate (DIAD) to a mixture of the carbohydrate (3),
the azobenzene 5, and Ph3P. This procedure led to an
anomeric mixture of azobenzene mannopyranosides (6a
and b), with minor amounts of the orthoester 9 formed
besides (Scheme 3). The latter could be readily isolated
The present study aims at testing p-hydroxyazobenzene
(p-hydroxyazophenol, 5) as the nucleophile in a Mitsuno-
bu reaction[10] to explore an alternative means of access
to azobenzene glycoconjugates. As d-mannose is of fore-
most importance in our biological studies, this sugar was
used as the alcohol component. It was planned to vary
the protecting group pattern of the employed mannose al-
cohol; however, in our first attempt, we took the most
direct approach to azobenzene a-d-mannoside, reacting
OH-free mannose and p-hydroxyazobenzene (Scheme 2).
Scheme 2. One-step synthesis of azobenzene mannosides based
on a Mitsunobu protocol.
We were surprised that under Mitsunobu conditions, only
the a-d-configured azobenzene mannopyranoside 7a was
isolated, whereas the b-mannopyranoside was not seen.
On the other hand, both anomeric furanosides, 8a and
8b, were formed. Anomeric configurations of the ob-
tained mannosides were determined according to C1-H1
hetero coupling constants and literature data.[11] Depend-
ing on the reaction conditions, varying pyranoside/furano-
side ratios were isolated. As the Mitsunobu reaction is es-
pecially sensitive to the sequence of reagent addition, sev-
eral modifications were attempted, of which two repre-
sentative prototypes are listed in Table 1. When diethyl
azodicarboxylate (DEAD) was added to a solution of 1,
5, and Ph3P, pyranosidic and furanosidic products were
formed in almost equal amounts. However, when a solu-
tion of p-phenylazophenol and DEAD was added to
a mixture of 1 and Ph3P, only minor amounts of furano-
sides were isolated. At the same time the anomeric ratios
of formed furanosides 8a and 8b varied, whereas a b-con-
Scheme 3. Mitsunobu glycosidation of 5 employing the reducing
mannose derivatives 3 and 10.
after recrystallization, whereas the anomeric mixture of 6
could be purified best after de-O-acetylation, leading to 7
as an anomeric mixture with a :b approx. 1:1. Yields
were around 60%. Notably, the NMR spectra of the ob-
tained azobenzene mannosides (cf. supporting informa-
tion) can be complicated by peaks of the Z-configured
stereoisomers present in the ground photochemical state
of the molecule.
When the same procedure was applied to 2 :3,4 :6-di-
isopropylidene-protected mannose 10, the b-mannopyra-
noside 7b was formed almost exclusively in yields over
80%. Our results with diisopropylidene mannose 10 are
related to literature reports, where 10 was reacted under
similar Mitsunobu conditions with phenol[12] or with 4-
methylumbelliferone,[13] mainly providing the respective
b-d-mannosides in equally pleasant yields. Thus, the re-
ducing mannose derivative 10 can be regarded as an ex-
cellent starting material for the Mitsunobu synthesis of b-
d-mannosides. During the Mitsunobu reaction with 3, on
the other hand, SN1 and SN2 substitution processes are ap-
parently competing, as was discussed previously.[14] We
will employ this reaction to prepare the b-configured gly-
coarray analogues of those photoswitchable surfaces that
Table 1. Typical reaction conditions as applied to the reaction of
Scheme 2. Reactions were performed in THF at room temperature.
Entry
Reaction
mixture
Added
reagents
Reaction
time
Products[c]
7a, 8 (a/b)[d]
1
2
1, 5, PPh3
1, PPh3
DEAD[a]
1 d
4 d
15, 14 (20/80)
33, 5 (36/64)
5, DEAD[b]
[a] Added in THF over 10 min at 08C; [b] Added as a mixture in
THF over 1 h; [c] Isolated yields in %; [d] Anomeric ratios according
1
to H NMR integration.
Isr. J. Chem. 2015, 55, 383 – 386
ꢀ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.ijc.wiley-vch.de
384

Communication
allowed photocontrol of bacterial adhesion.[4] Through
the biological testing of a surface, analogous to that
tested earlier, we will gain further insight into this novel
biological phenomenon.
Finally, in a third part of this study, regioselective Mit-
sunobu etherification of allyl and propargyl a-d-manno-
sides 11[15] and 12[16] was attempted with hydroxyazoben-
zene (5). This approach is interesting, because the result-
ing azobenzene-modified glycosides have the potential of
a photoswitchable hinge that can be installed onto any
scaffold or surface, respectively, utilizing the allylic or
propargylic aglycon function.
material (1.00 mmol), p-phenylazophenol (5, 1.10 mmol)
and triphenyl phosphine (525 mg, 2.00 mmol) were dis-
solved in dry THF (15.0 mL) under a nitrogen atmos-
phere. The solution was cooled to 08C, and then a solution
of diisopropyl azodicarboxylate (DIAD, 0.42 mL,
2.00 mmol) in dry THF (5.00 mL) was added dropwise
over 30 min, and then the reaction mixture was stirred at
rt until the TLC showed no further consumption of the
carbohydrate starting material.
For NMR data of synthesized azobenzene glycoconju-
gates, see the supporting information.
E-p-(Phenylazo)phenyl a,b-d-mannopyranoside (7a,
7b) and 3,4,6-tri-O-acetyl-1,2-O-(1-E-(p-(phenylazo)phe-
noxyethylidene)-b-d-mannopyranose (9) from 3: Accord-
ing to the general procedure, the acetyl-protected carbo-
hydrate 3 (348 mg, 1.00 mmol) was reacted at rt for 16 h.
Then the mixture was co-evaporated with toluene in
vacuo and the resulting crude product was recrystallized
from CH2Cl2/MeOH (7:20) to give the orthoester 9 in
the form of fine pale yellow crystals (47.6 mg, 0.09 mmol,
9%, mp: 1988C, [a]D21 =+28 (c=0.08, CHCl3). The
mother liquor was evaporated in vacuo, dissolved in dry
methanol (20.0 mL) under a nitrogen atmosphere, and
deprotected with freshly prepared 2 m sodium methoxide
solution (200 mL). After neutralization with Amberlite IR
120 ion exchange resin, the crude product was purified by
column chromatography on silica gel (CH2Cl2/MeOH,
99 :1!9 :1) to give an anomeric mixture (a/b=52 :48) of
the title mannoside as a yellow solid (223 mg, 0.62 mmol,
62%; the product contains minor impurities, cf. NMR
spectrum in the supporting information).
Regioselective Mitsunobu esterifications are rather
common,[17] whereas regioselective etherifications with
phenolic reaction partners are rare. A regioselective Mit-
sunobu 6-O-etherification of methylglucoside with phenol
was reported with 30% yield.[18] Employing mannosides
11 and 12, we were pleased to see that the Mitsunobu
protocol with 5 led to the 6-O-monosubstituted azoben-
zene glycoconjugates 13 and 14 in ~60% yield
(Scheme 4). Encouraged by these results, we will eventu-
ally submit additionally substituted azobenzene alcohols
to the same reaction to achieve carbohydrate-scaffolded
photosensitive molecular joints. Additionally, it is worth
mentioning that the column chromatographic purification
of the azobenzene-functionalized products is chromo-
phore-supported.[19]
In conclusion, our study shows that p-phenylazophenol
can be employed readily in Mitsunobu reactions to regio-
selectively obtain various photoswitchable glycoconju-
gates from free sugars (5”OH), and protected reducing
sugars (1”OH, anomeric), as well as from glycosides (4”
OH, 6-selective). Some interesting mechanistic aspects of
our findings, as well as the photochromic properties of
the synthesized azobenzene derivatives, will be investigat-
ed next and the scope of this reaction will be tested with
substituted phenylazophenol derivatives. Yields and
anomeric selectivity will be further optimized, employing,
for example, ultrasound and trialkyl phosphines, such as
Bu3P.
E-p-(Phenylazo)phenyl a,b-d-mannopyranoside (7a,
7b) from 10: According to the general procedure, the iso-
propylidene-protected
carbohydrate
10
(260 mg,
1.00 mmol) was reacted at rt for 3 d. Then water
(5.00 mL) was added, followed by dropwise addition of
trifluoroacetic acid (consecutively 2”1.70 mL) to effect
deprotection. After the isopropylidene groups were com-
pletely removed, the mixture was co-evaporated with tol-
uene in vacuo and the resulting crude product was puri-
fied by column chromatography on silica gel (CH2Cl2/
MeOH, 99 :1!9 :1) to give an anomeric mixture (a/b=
5 :95) of the title mannoside as a dark orange solid
(298 mg, 0.83 mmol, 83%).
Allyl 6-O-E-[p-(phenylazo)phenyl] a-d-mannopyrano-
side (13): According to the general procedure, allyl a-d-
mannopyranoside (11, 220 mg, 1.00 mmol) was reacted at
rt for 5 d and subsequently evaporated in vacuo. The
crude product was purified by column chromatography
on silica gel (CH2Cl2/MeOH. 9 :1) to give the title man-
noside as a yellow syrup (253 mg, 0.63 mmol, 63%); Rf =
0.44 (CH2Cl2/MeOH, 9 :1); [a]2D1 = +36 (c=0.52, MeOH).
Propargyl 6-O-E-[p-(phenylazo)phenyl] a-d-mannopyr-
anoside (14): According to the general procedure, prop-
argyl a-d-mannopyranoside (12, 218 mg, 1.00 mmol) was
reacted at rt for 3 d and subsequently evaporated in
Scheme 4. Regioselective Mitsunobu reaction of 5 with manno-
sides 11 and 12.
Experimental
Mitsunobu reaction: General procedure employed for the
conversion of 3, and of 10–12: The carbohydrate starting
Isr. J. Chem. 2015, 55, 383 – 386
ꢀ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.ijc.wiley-vch.de
385

Communication
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Received: December 31, 2014
Accepted: February 9, 2015
Published online: March 27, 2015
Isr. J. Chem. 2015, 55, 383 – 386
ꢀ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.ijc.wiley-vch.de
386

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