Carbohydrate scaffolds for combinatorial syntheses that allow selective deprotection of all four positions independent of the sequence

Article abstract of DOI:10.1002/anie.200352919

Irrespective of the sequence, the carbohydrate scaffold 1 as a molecular platform for combinatorial diversification can be selectively deprotected and equipped with side chains, for example amino acids or peptides, in every position (see picture). This allows the flexible arrangement of functional groups and structural elements. PG = protective group.

Full text of DOI:10.1002/anie.200352919

Communications
reactions,[3] selective transformations on multifunctional scaf-
folds are increasingly attractive for combinatorial applica-
tions. In this sense cyclopeptides with various side chains,[4]
bile acid derivatives,[5] aryl-[6] and squaric acid building
blocks[7] have been combinatorially applied as scaffolds for
the introduction of side chains.
Carbohydrates, more specifically monosaccharides, hold
an extraordinary density of functional groups and chiral
information per molecular unit. This feature has previously
been used in synthesizing peptido-[8] and aminoglycoside
mimics.[9] In the form of immobilized scaffolds on solid
support, monosaccharides have proved suitable for combina-
torial diversification as aminouronic acid derivatives[10] at
three positions, in the case of selectively deprotectable
glucose and galactose scaffolds at four,[11] and even at all
five positions.[13] Nevertheless these systems require a certain
sequence of deprotection and introduction of the desired side
chains. So far it has not been feasible to selectively remove an
individual protective group from any arbitrary position, whilst
leaving the protective groups on the other positions
untouched. Herein we report the first successful application
of such a strategy for 2,6-diaminoglucose scaffolds. The
strategy is based on the protective group concept 1 shown
in Scheme 1.
Solid-Phase Synthesis
Carbohydrate Scaffolds for Combinatorial
Syntheses That Allow Selective Deprotection of
All Four Positions Independent of the Sequence**
Scheme 1. Protective group concept for a selectively deprotectable
carbohydrate scaffold. Aloc=allyloxycarbonyl.
Preserving the thioglycosidic anchoring group[11,12,14] and
all other protected functionalities, the tBuMe2Si (TBS) group
is removable with fluoride and at the same time stable
towards all deprotection conditions for the other function-
alities. Corresponding demands have to be applied to the
reduction of the 6-azido group and the oxidative cleavage of
the p-methoxy benzyl (Pmb) group in position 4. For the
protection of the 2-amino group the phthaloyl (Pht) as well as
the allyloxycarbonyl (Aloc) group have been utilized. The
thioglycosidic linker of the scaffold, which can be activated
for glycosylations with bromine as has been shown previ-
ously,[11,12] was attached to tentagel[14] resin through the acid-
labile Rink benzhydryl amide linker.[15] The Rink linker
facilitates cleavage of the products from the solid phase
without affecting any other functional group incorporated in
the scaffold. In this way, removal from the solid support can
be achieved after every synthetic step and the products can be
analyzed.
Udo Hünger, Janina Ohnsmann, and Horst Kunz*
Combinatorial chemistry[1] has accelerated the syntheses of
novel active compounds in both pharmaceutical and agro-
chemical industries. To carry out solution-phase parallel
combinatorial syntheses, sophisticated robots have been
developed, but the applied procedures are limited by the
total number of synthetic operations achievable. Thus,
combinatorial syntheses on solid support remain particularly
important for the generation of libraries of active compounds
with highly diverse structures. Besides linear sequential
syntheses of peptides and peptidomimetic compounds[1] as
the origin of combinatorial chemistry,[2] and multicomponent
[*] Dr. U. Hünger, Dipl.-Chem. J. Ohnsmann, Prof. Dr. H. Kunz
Institut für Organische Chemie der Universität Mainz
Duesbergweg 10–14, 55128 Mainz (Germany)
Fax: (+49)6131-392-4786
To synthesize the completely protected diaminoglucose
scaffold, glucosamine hydrochloride was N-acylated with
either phthalic anhydride or allyl chloroformate and directly
per-O-acetylated to form the products 2a or 2b, respectively
(Scheme 2). BF3·Et2O-Induced stereoselective glycosylation
E-mail: hokunz@uni-mainz.de
[**] This work was supported by the Deutschen Forschungsgemein-
schaft and the Fonds der Chemischen Industrie. U.H. is grateful to
the Fonds der Chemischen Industrie for a KekulØ Fellowship.
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Scheme 2. a) 2a: 1. Phthalic anhydride, NaHCO3 aq., dioxane, 208C, 6 h; 2. Ac2O, pyridine, 208C, 12 h, 72%; 2b: 1. AlocCl, NaHCO3 aq., dioxane,
208C, 10 h; 2. Ac2O, pyridine, 208C, 12 h, 73%; b) methyl mercaptoalkylcarboxylate, BF3·OEt2, CH2Cl2, 208C, 12 h, 46%–82%; c) NaOMe, MeOH,
208C, 4 h, quant.; d) anisaldehyde dimethyl acetal, TsOH, DMF, 508C, 20 mbar, 6 h, 68–70%; e) TBSOTf, pyridine, CH2Cl2, 0–208C, 4 h, 78–97%;
f) NaBH3CN, TMSCl, acetonitrile, 0–208C, 12 h, 56–85%; g) 1. MsCl, pyridine, CH2Cl2, 0 À208C, 6 h; 2. NaN3, DMF, 758C, 50–84%. Ts=p-tolyl-
sulfonyl; Ms= methanesulfonyl.
of methyl mercaptopropionate (to form 3a or 3b) and methyl
4-mercaptobutyrate (to form 3c) introduced the respective
thioglycosidic anchor. After the O-acetyl groups were
removed upon treatment with catalytic amounts of sodium
methoxide in methanol, the 4,6-p-methoxybenzylidene acetal
was installed to furnish carbohydrates 3a–c. tert-Butyldime-
thylsilyltriflate (TBSOTf) was used to silylate the 3-OH group
in the presence of pyridine. Subsequently, 6-OH deprotection
was achieved regioselectively by reductive ring opening of the
benzylidene acetal with sodium cyanoborohydride and tri-
methylsilyl chloride.[16] The resulting free hydroxy group was
converted to the corresponding methanesulfonate. The latter
was then displaced using sodium azide in DMF to give access
to the scaffolds 4a–c.
in the presence of Hünigs base (Scheme 3c). The coupling
reactions were monitored by both FTIR and high-resolution
magic angle spinning (HRMAS) NMR spectroscopy.[19]
Following the coupling reaction unreacted benzhydryl func-
tionalities were end-capped with methanesulfonyl chloride in
the presence of pyridine.
Selective deprotections of the polymer-bound scaffolds
6a–c were conducted to prove the concept of the orthogonal
stability for the chosen set of protective groups. By using
tetrabutylammonium fluoride (TBAF) trihydrate in THF or
its HF adduct, the TBS groups were, according to HRMAS-
NMR spectroscopy, quantitatively removed[20] yielding the 3-
OH-deprotected scaffolds 7a–c (Scheme 4, route a).
To attach the scaffold to the Rink amide tentagel resin it
was necessary to convert the methyl ester of the anchor to the
carboxylic acid without affecting any base-labile protective
groups incorporated in scaffolds 4a–c. In case of 4a the ester
was cleaved with lithium chloride and thiophenol[17] in DMF
at 758C under SN2conditions (Scheme 3a). The base-stable
Aloc group of 4b and 4c remains after saponification with
lithium hydroxide in a mixture of water/THF (Scheme 3b).
The resulting carboxylic acids 5a–c were coupled to the
Rink benzylhydrylamine resin in high yields by using O-(1H-
benzotriazolyl)-N,N,N’,N’-tetramethyluronium hexafluoro-
phosphate (HTBU)[18] and 1-hydroxy-benzotriazole (HOBt)
Scheme 4. a) TBAF, THF; b) DDQ, CH2Cl2/H2O; c) 1. Bu3P, DMF; 2.
Et3N, DMF/H2O; d) 6b and 6c: [Pd2(dba)3], p-toluenesulfinic acid,
DMF. dba=trans,trans-dibenzylideneacetone.
Treatment of 6a–c with 2,3-dichloro-5,6-dicyano-1,4-qui-
none (DDQ)[21] in CH2Cl2 and 10 vol% water led to the
selective removal of the methoxybenzyl ether portective
groups and thus to the resin-bound compounds 8a–c
(Scheme 4, route b). The 6-azido groups were reduced
under Staudinger conditions with tri-n-butylphosphane, fol-
lowed by hydrolysis of the resulting phosphine imines with
triethylamine in DMF in the presence of water to give 9a–c
(Scheme 4, route c). Only the cleavage of the phthaloyl group
Scheme 3. a) PhSH, LiCl, DMF, 758C, 12 h, 52%; b) LiOH, H2O/THF,
208C, 5 h, 86%; c) HBTU, HOBt, DMF, NEtiPr2, Rink amide tentagel,
90%.
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from 6a with hydrazine hydrate, which can be
achieved readily in solution for an analogous
compound, gave unsatisfactory results on the
resin. In DMF at 308C the Rink linker was
obviously cleaved and the structure of the polymer
was affected in a way, which the intended reaction
could not proceed. In contrast, the Aloc groups in
6b and 6c were cleaved selectively by palla-
dium(0)-catalyzed allyl transfer reactions in p-
toluenesulfinic acid[22] to form carbohydrates 10b
and 10c without any problems (Scheme 4, route
d). The 2-N-Aloc-protected frameworks 6b and 6c
satisfy all requirements for a multifunctional chiral
scaffold selectively deprotectable in any position
independently of the sequence. In the case of the
phthaloyl-protected analogue 6a, diversity at the
2-amino group can be applied after cleavage from
the solid phase.
Position-selective introduction of side chains
as demonstrated herein for the esterification and
amidation with Fmoc-protected amino acids, was
conducted without affecting any of the protecting
groups. To esterify position 3 in 7a or position 4 in
8a the Fmoc-amino acids were activated with
N,N’-diisopropylcarbodiimide (DIC) and 4-dime-
thylaminopyridine (DMAP; Scheme 5).[23] All
reactions gave quantitative conversion of the
starting materials at ambient temperature, as has
been shown by UV-control of the Fmoc-cleav-
age[24] from analytical resin samples. Neither the
phthaloyl group nor the Pmb (11) or the TBS
groups (12) showed instability in the presence of
DMAP.
Scheme 5. a) PG-AA-OH, DIC, DMAP, DMF, 208C; b) PG-AA-OH, DIC, HOBt, Bu3P,
THF, 208C; c) TFA/CH2Cl2 1:1, mercaptomethyl polystyrene, 30 min, 208C.
PG=protective group. Yields given refer to products purified by HPLC (purity
>95%); purities of crude products determined by HPLC-MS directly after detach-
ment from the resin are also quoted.
To prevent side reactions at the base-labile phthaloyl
group during modification of the 6-position, model compound
6a was converted directly to the amide in one step without
isolating intermediate 9a by combining the Staudinger
reaction[25] with the subsequent amide coupling (Scheme 5).
To this end, 6a was treated with a solution of N- and side-
chain-protected amino acids, DIC, HOBt,[26] and tri-n-butyl-
phosphane in THF at ambient temperature. According to the
UV spectroscopic determination of N-(9-fluorenylmethyl)pi-
peridine after Fmoc cleavage from analytical samples of 13a
and 13b, the reactions proceed quantitatively.
Finally the products 13a and 13b, like 11 and 12, were
detached from the Rink linker with trifluoroacetic acid (50%
in dichloromethane). Simultaneously all acid-labile protective
groups, such as the Pmc group[27] of 13a and the Boc group of
13b or the trityl group of 12 as well as the Pmb and TBS
groups were cleaved. mercaptomethyl polystyrene was used
as scavenger nucleophile. The yields given for products 14a
and 14b as well as those from 11 and 12 are based on 6a as the
starting material and involve at least three steps.
To introduce substituents in position 2, Aloc-protected
scaffold 6b was converted to 2-amino compound 10b, which
was subjected to peptide condensation with different amino
acids. To monitor the reaction, analytical samples of 15a and
15b were treated with piperidine and the resulting Fmoc-
adduct was examined by UV spectroscopy (Scheme 6).
Scheme 6. a) Fmoc-Lys(Fmoc)-OH, DIC, DMAP, DMF, 208C; b) Fmoc-
Arg(Pmc)-OH, DIC, DMAP, DMF, 208C; c) TFA/CH2Cl2 1:1, mercapto-
methyl polystyrene, 30 min, 208C. Pmc: 2,2,5,7,8-pentamethylchro-
mane-6-sulfonyl. All yields (based on 6b) refer to products purified by
HPLC (purity >95%); purities of crude products determined by
HPLC-MS are also quoted.
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The described results show that 2,6-diaminoglucose 6 can
be selectively deprotected and functionalized in every posi-
tion of the polyfunctional chiral scaffold. The potential of this
methodology lies in the ability to generate high diversity
especially if the diversifications would be performed not only
with Fmoc-protected amino acids, as demonstrated herein,
but with an array of different compounds. This applies
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[12]crown-4 did the cleavage of the methyl ester begin (yield:
47%)
Received: September 19, 2003 [Z52919]
Keywords: carbohydrates · combinatorial chemistry ·
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