Some aspects of selectivity in the reaction of glycosyl donors

Article abstract of DOI:10.1080/07328300500176528

Some advantages, disadvantages, and anomalies of various donors in glycosidations are discussed. By studying several two-component donor/acceptor-diol reactions, it is shown that regiopreferences are not very sensitive to the type of donor used. However,

Full text of DOI:10.1080/07328300500176528

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Some Aspects of Selectivity in the
Reaction of Glycosyl Donors
Clara Uriel a , Ana M. Gomez a , J. Cristóbal López a & Bert
Fraser‐Reid b
a Instituto de Quimica Organica General (CSIC) , Madrid, Spain
b Natural Products and Glycotechnology Research Institute Inc.
(NPG) , Pittsboro, NC, USA
Published online: 16 Aug 2006.
To cite this article: Clara Uriel , Ana M. Gomez , J. Cristóbal López & Bert Fraser‐Reid (2005) Some
Aspects of Selectivity in the Reaction of Glycosyl Donors , Journal of Carbohydrate Chemistry, 24:4-6,
665-675
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Journal of Carbohydrate Chemistry, 24:665–675, 2005
Copyright # Taylor & Francis, Inc.
ISSN: 0732-8303 print 1532-2327 online
DOI: 10.1080/07328300500176528
Some Aspects of Selectivity
in the Reaction of Glycosyl
Donors
´
´
Clara Uriel, Ana M. Gomez, and J. Cristobal Lopez
Instituto de Quimica Organica General (CSIC), Madrid, Spain
Bert Fraser-Reid
Natural Products and Glycotechnology Research Institute Inc. (NPG), Pittsboro,
NC, USA
Some advantages, disadvantages, and anomalies of various donors in glycosidations are
discussed. By studying several two-component donor/acceptor-diol reactions, it is
shown that regiopreferences are not very sensitive to the type of donor used.
However, in competitive glycosidations within a given type of donor and between
different types of donor, it is shown that regio- and chemoselectivities must be
indexed to donor reactivity.
Keywords Chemo and regioselectivity in glycosylation, Donors in armed/disarmed
competitive glycosylations
The data summarized in Schemes 1a,[1] 3b,[2] and 4a,[3] taken from the large
body of van Boom’s work, are symptomatic of the dilemma facing the
seemingly simple task of coupling a glycosyl donor to an acceptor. Four types
of selectivity—enantio, (dia)stereo, regio, and chemo—are suggested to be
necessary for efficiency in organic synthesis by Trost.[4] In the case of oligosac-
charide assembly, stereoselectivity was the first to be addressed in Isbell’s
historic rationalization[5] of the configurations of products formed in Koenigs
Knorr reactions (see below). This rationalization inspired[6] what was later to
become known as neighboring group participation,[7] whereby the O2
acyl group would foster a trans-coupled product (see Sch. 5). Thus, Isbell,
Received May 12, 2005; accepted May 24, 2005.
Dedicated to the memory of Professor Jacques van Boom.
Address correspondence to Bert Fraser-Reid, Natural Products and Glycotechnology
Research Institute Inc. (NPG), 595f Weathersfield Road, Pittsboro, NC 27312, USA.
E-mail: dglucose@aol.com
665

C. Uriel et al.
666
Scheme 1
60 years ago, established that in the case of sugars, protecting groups do more
than protect, an observation that was seminal in that substitutents at oxygen
are, arguably, the only readily available tools to influence outcomes in synthetic
carbohydrate chemistry.[8]
For the remaining three selectivities, enantioselectivity is usually irrele-
vant since for a given oligosaccharide target, nature specifies D and L
options for the donor and acceptor. In this connection, van Boeckel’s interesting
study[9] showed that, in keeping with Masamune’s principle of “double asym-
metric synthesis”,[10] such D and/or L partners may not be the best “match,”
even where that pairing occurs in nature.
The word “match,” as used in Masamune’s treatise,[10] differs from
Paulsen’s concept.[11] The latter was engendered by years of trial and error,
which prompted Paulsen to conclude, in his historic 1982 review,[12] that
“there are no universal reaction conditions for oligosaccharide synthesis,”
and so the “match” must be determined in each case. Although a crisp prescrip-
tion for eliciting the best “match” was not offered, Paulsen’s studies[13] implied
that appropriate hallmarks are ready coupling and good yields.
Our appreciation of van Boom’s experiments summarized in Scheme 1a
emanated from 1999 observations with the diol 7[14] (Sch. 1b). Glycosidation

Reaction of Glycosyl Donors
667
with the 2-O-benzoylated donor 5 went only at O6, while the 2-O-benzyl
counterpart 8 went mainly at O2.[15] The n-pentenyl orthoester (NPOE) 6
followed the same exclusive course as 5. With the hindsight of our results,
the 84% vs. 10% yields of 3 in Scheme 1a implied that the 2-O-benzoyl
donors 2, as in the case of 5, is a good “match” for the O6-OH of inositol accep-
tors, but not O2-OH.
Good regioselectivity was observed with several diols,[16] a most impressive
case being the altroside 11.[17] As seen in Scheme 2a, the thioglycoside and
trichloroacetimidate donors 10a and b gave exclusive O3-OH regioselectivities,
as did the NPOE 9.[18] By contrast, the 2-O-benzyl donors 12a, b, and c all
Scheme 2: (a) Regioseletivities in tow-component reactions of diol 11. (b) Products from
three-component reactions of diol 11 with pairs of donors.

C. Uriel et al.
668
displayed the same major and minor preferences for the O3-OH and O2-OH
respectively.
Further, to the two-component reactions in Schemes 1b and 2a we have
reported three-component reactions in which two different n-pentenyl donors
are presented simultaneously to diol acceptors.[19] The results with a 1 : 1 : 1
mixture of diol 11 and n-pentenyl donors 9 and 12a (Scheme 2b, entry i) typi-
cally gave only 13 (of four possible trisaccharides) and 14a (of four possible
disaccharides).[17]
We extended the three-component double glycosidation of diol 11 to the
thioglycoside donors 10a and 12b (Scheme 2b, entry ii). Again the only trisac-
charide was compound 13, albeit formed in lesser amount. Compound 14b was
now added to the mix of products. With the trichloroacetimidates 10b and 12c
(Sch. 2b, entry iii), 13 was again the only trisaccharide, although in almost
negligible amounts. The yield of disaccharide 14a was also lower. On the
other hand, disaccharide 14b was increased, and 15 was obtained for the
first time in a three-component reaction.
The fact that all three types of donors exhibited the same trends in the two-
component (Sch. 2a) but not in the three-component (Sch. 2b) reactions was
curious, and caused us to reflect on the 1988 armed/disarmed experiments
(Sch. 3a), which showed that an O2-protecting group could be used to control
chemoselective acceptor/donor choices in NPG couplings.[20] This phenomenon
was soon extended to thioglycosides by van Boom (Sch. 3b).[2]
Scheme 3

Reaction of Glycosyl Donors
669
However, to our knowledge, the armed/disarmed protocol had not been
reported for trichloroacetimidates. Accordingly, the experiments in
Scheme 3c were undertaken. It is seen that 12c and 23 do not exhibit the
armed/disarmed behavior (even at –788C) as do NPG and thioglycoside
analogs. Thus, formation of the 1,6-anhydrosugar 26, which was not seen in
Schemes 3a and b, suggests that the condition for chemocontrol is not met.
With regard to NPGs and thioglycosides, van Boom’s experiment in
Scheme 4a represented a case of chemoselectivity, as well as the first
example of a reaction between orthogonal donors.[3] The selectivity was
Scheme 4

C. Uriel et al.
670
conceivably based on the preference of iodonium ion (Iþ) to react with sulfur
rather than an alkene. However, recent experiments in our laboratory
indicate that the O2-protecting group profoundly affects orthogonal prefe-
rence.[17] Thus, in Schemes 4b and c the armed donors 12a and 12b completely
shut out the disarmed competitor 10a to give disaccharide 31 as the main
product.[17] On the other hand, Schemes 4d and 4e illustrate the preference
of iodonium ion (Iþ) to react with an alkene rather than sulfur when NPOEs
33 and 9 react instead of either disarmed or armed thioglycosides 10a and
12b, to yield disaccharides 34 and 32, respectively.
The issue of reactivity requires that the role of the O2-protecting group be
examined more closely. Isbell’s landmark insight[5] was aimed at rationalizing
the stereochemical outcome of the Koenigs-Knorr reaction (Sch. 5), and the
concept of neighboring group participation met this need. However, subsequent
examination of the phenomenon revealed a kinetic aspect, which came to be
known as “anchimeric assistance”.[7] It therefore seemed anomalous that the
trans-2-O-acyl donors, for example, 17 and 21 (Sch. 3), be “disarmed” with
respect to the 2-O-alkyl counterparts 16 and 20, respectively.
We have probed the matter theoretically using the tetrahydropyranyl chlor-
ides 35 and 36 to represent the armed and disarmed donors and 38 and 39 as
the carbocations derived from them, respectively.[21] The transition energy is
seen to be higher for the disarmed species by approximately 5.6 kcal (Scheme
6). The dioxolenium ion 41 was found, but calculations also revealed a trioxole-
nium counterpart 40 with charge being distributed to each of the three oxygens.
Scheme 5

Reaction of Glycosyl Donors
671
Scheme 6
These same ions would also be generated from the bicyclic precursor 37 at much
less energetic cost, as is clear from the results in Scheme 4d and 4e.
The formation of cross-coupled products 18 and 22 (Sch. 3) is consistent
with 16 and 20 being the donors; otherwise, self-coupled products would
have been formed. By corollary, 17 and 21 could not have benefitted from anchi-
meric assistance leading to di/trioxolenium ions, or they would not have
behaved as acceptors.
The contrasting results between pentenyl glycosides/thioglycosides (9/
10a) on the one hand and trichloroacetimidate 10b on the other (Sch. 2)
could imply that in the case of the latter, reaction proceeds directly through
the di/trioxolenium ion (40/41), whereas the former follows the more energetic
pathway, going first to 39 before plunging to 40/41.
SUMMARY
The results above draw attention to advantages and disadvantages, as well as
the anomalies, of donor reactivity in glycosidation. Chemoselective coupling

C. Uriel et al.
672
within a given type (e.g., armed/disarmed in Sch. 3a and 3b) and between
different types (i.e., orthogonal, Sch. 4a, 4d) can be advantageous. On the
other hand, the failure to observe armed/disarmed coupling for trichloroaceti-
midates, even at 2788C (Sch. 3c) can be considered a disadvantage. This is
evident in the results in Scheme 2. All three types of armed and disarmed
donors showed the same regiopreferences in the two-component reactions
(Sch. 2a). However, when these donors were made to compete in three-
component reactions, the success of regioselective, double differential glycosi-
dations to give 13 was best with NPGs, the least reactive of the three sets of
donors examined in Scheme 2.
The results in Schemes 2 and 3 indicate that regio- and chemoselectivities
must be indexed to donor reactivities. Thus, the transition energy gaps in
Scheme 6 could be increased or decreased depending on the models chosen.
This should be amenable to theoretical analysis, and appropriate studies are
currently underway.
EXPERIMENTAL
General Methods
1
Optical rotations were determined at the sodium D line. H NMR spectra
were recorded at 200, 300, or 500 MHz; chemical shifts (d) are relative to
residual nondeuterated solvent as internal reference. TLC was conducted in pre-
coated Kieselgel 60 F254. Detection was first by UV (254 nm), then charring with a
solution of ammonium molybdate (VI) tetrahydrate (12.5 g) and cerium (IV)
sulfate tetrahydrate (5.0 g) in 10% aqueous sulfuric acid (500 mL). Column
chromatography was carried out on Kieselgel (230–400 mesh) and, unless other-
wise noted, mixtures of hexane-EtOAc were used as eluant. All reactions were
conducted under atmosphere of argon. Anhydrous MgSO4 or Na2SO4 were
used to dry the organic solutions during work-ups and the removal of the
solvents was done under vacuum with a rotavapor. Unless otherwise noted,
materials were obtained from commercially available sources and used without
further purification. Solvents were dried and purified using standard methods.
General glycosidation procedure for thioglycoside and n-pentenyl
donors. To a solution containing the glycosyl acceptor (42.3 mg, 0.15 mmol)
and the glycosyl donor (0.15 mmol) in anhydrous CH2Cl2 (3 mL) were added
˚
molecular sieves 5A (1 mg/mg donor) and N-iodosuccinimide (1 eq.) at
2308C. After stirring the mixture for 5 min BF3OEt2 (0.045 mmol) was
added. The reaction was monitored by TLC and, after disappearance of the
acceptor, solutions of saturated NaHCO3 and Na2S2O3 were added. The
reaction mixture was extracted with methylene chloride and dried over

Reaction of Glycosyl Donors
673
anhydrous sodium sulfate. Solvents were removed and the residue was purified
by flash chromatography.
Preparation of trichloroacetimidates 23 and 12c. The “disarmed”
trichloroacetimidate 23 [1H NMR (300 MHz) d: 8.53 (s, 1H) 7.90–7.79
(m, 5H), 7.75–7.15 (m, 10H), 6.73 (d, 1H), 6.22 (t, 1H, J ¼ 9.8 Hz), 5.54
(t, 1H, J ¼ 9.8 Hz), 5.48 (dd, 1H, J ¼ 10.2, 3.5 Hz), 4.16 (m, 1H), 3.71 (m, 2H)]
was prepared from the known[22] phenyl 2,3,4-tri-O-benzoyl-6-O–(t-butyldi-
phenylsilyl)-1-thio-b-D-glucopyranoside (A) by oxidative hydrolysis with NBS
in aqueous acetone, and standard[23] reaction with DBU and CNCCl3,
followed by desilylation with HF/pyridine complex. The “armed” counterpart
12c was prepared likewise from thioglycoside 12b.
Glycosidation procedure for trichloroacetimidate donors. To a
solution of glycosyl trichloroacetimidates 12c and 23 (0.09 mmol each) in anhy-
˚
drous CH2Cl2 (3 mL) were added molecular sieves 5A (1 mg/mg donor) and
BF3OEt2 (0.3 eq.) at 2788C for 23: The reaction was quenched after 10 min
with saturated aqueous sodium bicarbonate, extracted with dichloromethane,
dried over Na2SO4, and concentrated. The mixture was purified by flash chrom-
atography (hexane: EtOAc, 9 : 1 to 7 : 3) to give known monosaccharides 24
(16 mg, 33%) and 26 (28 mg, 66%) and disaccharide 25 (18 mg, 18%).
1
Data for compound 34. H NMR (300 MHz) d: 3.02 (dd, 1H, J ¼ 9.98,
8.85 Hz, H-4), 3.14 (dd, 1H, J ¼ 9.61, 3.58 Hz, H-2), 3.40 (s, 3H, OMe), 3.43
(s, 3H, OMe), 3.27–3.54 (m, 2H), 3.56 (s, 3H, OMe), 3.63 (dd, 1H, J ¼ 11.30,
1.70 Hz), 3.69–4.02 (m, 7H), 4.43–4.53 (m, 4H), 4.63–4.74 (m, 5H), 4.81 (d,
1H, J ¼ 10.9 Hz), 4.99 (d, 1H, J ¼ 1.88 Hz, H-10), 5.58 (m, 1H, H-20), 7.09–
7.31 (m, 18H, Ar) 7.98–8.01(m, 2H, Ar). 13C NMR (75 MHz) d: 55.4, 59.4,
60.9, 61.2 (4 ꢀ OMe), 66.4, 69.4, 71.8, 73.8, 75.6 (5 ꢀ CH2), 69.2, 70.1, 72.1,
74.6 (C-2, C-3, C-4, C-5), 78.2, 79.9, 82.2, 84.0 (C-20, C-30, C-40, C-50) 97.6,
98.5 (C-1, C-10), 127.8, 127.9, 128.0, 128.3, 128.5, 128.6, 128.7, 128.8, 130.3,
130.4, 133.5, 138.9 (Ar) 166.1 (C ¼ O).

C. Uriel et al.
674
ACKNOWLEDGMENTS
B.F.R. is grateful to the National Science Foundation (CHE-0236946) and the
Mizutani Foundation of Japan for partial support of this work. J.C.L. and
´
A.M.G. are grateful to the Direccion General de Ensen˜anza Superior (Grant:
´
PPQ2003-00396) and the Ministerio de Ciencia y Tecnologıa for financial
support. C.U. thanks the CSIC for financial support.
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