Acyl Transfer as a Problematic Side Reaction in Polymer-Supported Oligosaccharide Synthesis

9030

J . Org. Chem. 1999, 64, 9030-9045

Acyl Tr a n sfer a s a P r oblem a tic Sid e Rea ction in

P olym er -Su p p or ted Oligosa cch a r id e Syn th esis

Tomoo Nukada,^1aAttila Berces,^1band Dennis M. Whitfield*^,1c

The Institute of Physical and Chemical Research (RIKEN), Wako-shi, Saitama 351, J apan, and National

Research Council, Ottawa, Ontario K1A 0R6, Canada

Received April 27, 1999

Under a wide variety of glycosylation conditions acyl transfer to the polymer support competed

with glycoside formation, including the pivaloyl protecting group. As well as acyl transfer, many

glycosylations also led to the formation of polymer-bound â-1,2-linked oligomers of the donor. Using

ethyl 2,6-di-O-pivaloyl-3,4-O-isopropylidene-â-^D-galactothiopyranoside as donor under promotion

of N-iodosuccinimide/silver trifluoromethanesulfonate in the presence of 2-methyl-2-butene, an 82%

yield of glycoside was obtained along with pivaloylated polymer. Subsequent work showed that

increasing the steric bulk about the alcoholic acceptor in conjunction with this 2-O-pivaloyl-protected

glycosyl donor completely suppresses this side reaction, giving a nearly quantitative yield of

glycoside. This contraintuitive approach of decreasing the reactivity of both the donor and the

acceptor to minimize a side reaction is rationalized by assuming that the barrier to acyl transfer

is more sensitive to the protecting groups than that of glycosylation. These developments led to a

polymer-supported synthesis of the branch point trisaccharide of the group B type 1A Streptococcus

capsular polysaccharide.

In tr od u ction

((MPEG)(DOX)OH), has one benzylic ether bond between

the polymer and the linker and one free benzylic hydroxyl

to link to the growing oligosaccharide chain.

As part of our institute’s program to design and develop

vaccines²against group B streptococcal infections the

syntheses of fragments of the serotype specific capsular

polysaccharides are being undertaken.³In particular

polymer-supported synthetic methodologies are under

development. Such methodologies are in principle ame-

nable to the production of the large number of derivatives

and analogues that are critical for vaccine design and

development.⁴Furthermore, the possibilities of develop-

ing automated methods⁵which could allow for the

development of fully synthetic vaccines is alluring.⁶The

present strategy uses the soluble monomethyl ether of

polyethylene glycol (MeO-(CH₂CH₂O)_n-OH, MPEG) as

the polymer support⁷and the dioxyxylene (p-OCH₂-

As a primary target a single repeat unit of the type

1A capsular polysaccharide was chosen, Figure 1. The

syntheses of oligosaccharides corresponding to this struc-

ture were reported from this institute.⁹From a synthetic

point of view this pentasaccharide provides two key

challenges, namely, the 3,4-branching on the â-^D-Gal

residue and the NeuNAc(R2,3)Gal terminal linkage. In

this paper we concentrates on the â-^D-Gal 3,4-branching

problem.

Thus, a ^D-Gal glycosyl donor was needed which could

form a â-linkage and allow for chain extension at O-3

and O-4. As a first design, donors based on the ethyl 2,6-

di-O-acyl-3,4-O-isopropylidene-â-^D-galactothiopyrano-

side structures were tested. The 2-O-acyl group allows

for neighboring group participation to facilitate formation

of â-linkages.¹⁰The 3,4-O-isopropylidene group can be

cleaved by mild acid, and the resulting diol can in

principle be selectively glycosylated at the equatorial O-3

hydroxyl to allow for 3,4-branching.¹¹Unfortunately, our

first attempts at glycosylating the (MPEG)(DOX)OH with

the 2,6-di-O-benzoyl donor 2 produced a complex mixture

of products, Scheme 1. Most of these products can be

traced to a competing acyl transfer from O-2 of the

C₆H₄-CH₂O-, DOX) linker.⁸This combination,

1

(1) (a) RIKEN. (b) Steacie Institute for Molecular Sciences, National

Research Council. (c) Institute for Biological Sciences, National

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Published on Web 11/16/1999

Acyl Transfer in Oligosaccharide Synthesis

J . Org. Chem., Vol. 64, No. 25, 1999 9031

F igu r e 1. Schematic representation of a single repeat unit of group B Streptococcus type 1A capsular polysaccharide. The two

key synthetic challenges are also indicated.

benzoylated acceptor 3, (MPEG)(DOX)OBz, the desired

â-glycoside 4a and the â1,2 oligomers 4b, Table 1. The

OH-2 glycoside 4c is also likely formed as unidentified

isopropylidene resonances were found but not assigned.

Side products 4b and 4c are particularly problematic for

polymer-supported schemes as they create erroneous

sequences.

This side reaction has been reported for the following

F igu r e 2. 2-OH glycoside 4c and orthoester 4d .

leaving groups: halide,¹⁵2,3-diphenyl-2-cyclopropen-1-

yl,¹⁶acetate,¹⁷1,2-O-cyanoethylidene,¹⁸orthoester,¹⁹tri-

Sch em e 1. Com p lex Rea ction Mixtu r e fr om

Rea ction of Don or 2 w ith P olym er -Su p p or ted

chloroacetimidate,²⁰and alkylthio.²¹The acyl group has

Accep tor 1

been unequivocally shown to originate from O-2 by

labeling experiments in two cases.²²In a solution syn-

thesis such products are removed by chromatography,

only affecting the yield of glycoside. In a polymer-

supported synthesis where the acceptor alcohol is at-

tached to the polymer all such products are recovered at

the end of a glycosylation reaction. To efficiently proceed

with polymer-supported methodologies, it is very impor-

tant to establish the mechanism of this reaction so that

conditions can be found to eliminate it.

Following from the results of solution studies the use

of the sterically demanding pivaloyl to minimize ortho-

ester formation has been recommended.²³The relation-

ship between orthoester formation and acyl transfer has

never been clearly elucidated. Several early mechanistic

studies have suggested that orthoesters are intermedi-

(14) (a) Whitfield, D. M. Douglas, S. P.; Tang, T. H.; Csizmadia, I.

G.; Pang, H. Y. S.; Moolten, F. L.; Krepinsky, J . J . Can. J . Chem. 1994,

72, 2225. (b) Liu, M. Z.; Fan, H. N.; Guo, Z. W.; Hui, Y. Z. Carbohydr.

Res. 1996, 290, 233. (c) Lee, R. T.; Lee Y. C. Carbohydr. Res. 1995,

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1990, 31, 4421.

(16) Khorlin, A. Ya.; Nesmeyanov, V. A.; Zurabyan, S. E. Carbohydr.

Res. 1975, 43, 69.

glycosyl donor to the acceptor; cf. 3. The acylated acceptor

3 is a dead end product for polymer-supported synthesis

and is especially devastating if the acyl group is used as

a cleavable protecting group.

(17) (a) Krˇen, V.; Fisˇerova´, A.; Auge´, C.; Sedmera, P.; Havl´ıcˇek, V.;

Sˇ´ıma, P. Bioorg. Med. Chem. Lett. 1996, 4, 869. (b) Nifant’ev, N. E.;

Khatuntseva, E. A.; Shahkov, A. S.; Bock, K. Carbohydr. Lett. 1996,

1, 399. (c) Nilsson, U.; Ray, A. K.; Magnusson, G. Carbohydr. Res. 1994,

252, 117. (d) Pozsgay, V.; Brisson, J .-R.; J ennings, H. J . Can. J . Chem.

1987, 65, 2764.

(18) Tsvetkov, Yu. E.; Kitov, P. I.; Bakinovskii, L. V.; Kochetkov,

N. K. Bioorg. Khim. (Engl. Transl.) 1990 16, 50.

(19) (a) Uvarova, N. I.; Oshitok, G. I.; Elyakov, G. B. Carbohydr.

Res. 1973, 27, 79. (b) Lindhorst, T. K. J . Carbohydr. Chem. 1997, 16,

237. (c) Bochkov, A. F.; Betaneli, V. I.; Kochetkov, N. K. Bioorgan.

Khim. (Engl. Transl.) 1976, 2, 671.

(20) Larson, D. P.; Heathcock, C. H. J . Org. Chem. 1997, 62, 8406.

(b) 14a

(21) Fukase, K.; Kinoshita, I.; Kanoh, T.; Nakai, Y.; Hasuoka, A.;

Kusumoto, S. Tetrahedron 1996, 52, 3897.

(22) (a) Ziegler, T.; Kova´cˇ, P.; Glaudemans, C. P. J . Liebigs Ann.

Chem. 1990, 613. (b) Bochkov, A. F.; Betaneli, V. I.; Kochetkov, N. K.

Izv. Akad. Nauk SSSR, Ser. Khim. (Engl. Transl.) 1974, 6, 1299 (Russ.

p1379).

(23) (a) Kunz, H.; Harreus, A. Liebigs Ann. Chem. 1982, 41. (b) Sato,

S.; Nunomura, T.; Ito, Y.; Ogawa, T. Tetrahedron Lett. 1988, 29, 4097.

(c) Sato, S.; Ito, Y.; Ogawa, T. Tetrahedron Lett. 1988, 29, 5267. (d)

Nunomura, S.; Ogawa, T. Tetrahedron Lett. 1988, 29, 5681.

As exemplified by the quote “Unfortunately, many

carbohydrate chemists do not seem to be aware of these

side-reactions as low yields with O-2 acetylated donors

are still frequently reported”,¹²this side reaction is poorly

understood. Besides the O-acylated acceptor in some

cases the desired â-glycoside is formed but with the O-2

acyl group missing, i.e., a hydroxyl at O-2; cf. 4c Figure

2.¹³As well, the unwanted R-isomers with the hydroxyl

at O-2 can be part of the product mixture. A second type

of side product is 1,2-linked disaccharides.¹⁴With this

knowledge and careful analysis of the ¹H NMR spectrum

of the polymer-bound reaction mixture, the products that

have been identified from the reaction of 1 and 2 are the

(12) Veeneman, G. H. In Carbohydrate Chemistry; Boons, G.-J ., Ed.

Blackie Academic & Professional: London, 1998; p 101.

(13) (a) Lemieux, R. U. Chem. Can. 1964, 16, 14. (b) Whitfield, D.

M.; Douglas, S. P.; Krepinsky J . J . 17th International Carbohydrate

Symposium, Ottawa, Ontario, 1994.

9032 J . Org. Chem., Vol. 64, No. 25, 1999

Ta ble 1. Dia gn ostic ¹H NMR Sp ectr a l Da ta for 3, 4a , 4b, 4d , 13a , 13b, a n d 14^a

H-1 (J _1,2H-2 (J _2,3DOX-CH₂(J ) benzoyl_o(J )

5.35 s 8.07 br d (7.6)

Whitfield et al.

isopropylidene

compd

)

3

4a

4b

4d

5.33 br t (7.7)

5.25 br t^b(7.7)

5.17 br t^c(7.7)

3.89 br d

4.83 d (12.3)

-

4.48 d, -

4.65 brs, 4.49 brs

4.82 d (12.5)

8.10 br d (7.4), 8.02 br d (7.8)

1.65 s, 1.36 s

1.63 s, 1.36 s

shoulders

1.43 s, 1.29 s

1.41 s, 1.29 s

1.64 s, 1.36 s

shoulders

5.76 d (5.0)

5.76 br s

7.99 br d, 7.60 br d

-, 7.63 br d

8.03 br d, 8.00 br d

4d + DBU

13a

13b

4.19 m

5.29 br t (7.8)

5.22^b(7.8)

14

5.12 s

7.93 br d

a

b

See Schemes 1 and 4, Table 2 and Figure 2. Chemical shifts in parts per million and coupling constants in hertz. Disaccharide.

^cTrisaccharide.

Sch em e 2. Ben zoyl Tr a n sfer fr om Rea ction of

Don or 5 a n d 1

SY and TOCSY experiments were used for the complete

structural elucidation of 9d . These experiments estab-

lished connectivities between the â-anomeric proton and

the H-2 of the R-linked residue. Also, the R-anomeric

proton showed connectivity to a benzoyl ortho proton (not

shown), confirming the presence of the benzoate at the

reducing terminus. The results of this glycosylation

experiment suggest that acyl transfer is a property of

donor 2 and not an artifact of the polymer-supported

methodology.

Next, the conditions of glycosylation between 1 and 2

were varied to see if factors such as solvent, amounts and

nature of the promoters, or temperature could be ma-

nipulated to minimize acyl transfer. Pertinent results are

presented in Table 2. Entry 1 is the result referred to in

Scheme 1. All of the conditions are typical of NIS/TfOH

glycosylations except that much more triflic acid (1.4

equiv) was used. This necessity of excess promoter is

usual with glycosylations in the presence of PEG deriva-

tives and is ascribed to complexation between the elec-

trophilic promoters and the PEG ether oxygens. Lowering

the amount of triflic acid led to formation of predomi-

nantly the orthoester 4d , Figure 2. Its structure was

assigned on the basis of the R-anomeric proton resonance

at δ ) 5.76 ppm and J ) 5.0 Hz and the upfield-shifted

benzoate resonance at δ ) 7.6 ppm in the ¹H NMR

spectrum. The latter resonance did not disappear after

treatment with 1,8-diazabicyclo[5.4.0]undec-7-ene, DBU,

in methanol, which cleaves benzoyl esters and caused

disappearance of the benzoyl resonance at δ ) 7.99 ppm.

The orthoester was also completely cleaved from the

(MPEG)(DOX) by mild acid treatment. It should be noted

that all NIS/TfOH reactions are quenched with diisopro-

pylethylamine (DIPEA) before workup and it is likely

that the neutral orthoester is formed after this addition.

Changing to the more polar solvent acetonitrile did not

change the result, entry 3. Increasing the temperature

to room temperature led to recovery of starting acceptor

alcohol (MPEG)(DOX)OH, entry 4. Adding the triflic acid

at ice-bath temperature and allowing the reaction to

warm to room temperature also did not improve the

result. Next, the trityl ether (MPEG)(DOX)OTr (10) was

used as acceptor in the hope that this removal of the

proton would minimize acyl transfer. In a previous study

the disruption of an intramolecular hydrogen bond of the

acceptor alcohol was associated with minimization of acyl

transfer.²⁸As well, the triflic acid was replaced with silver

triflate, but the reactivity resembled the parent hydroxyl

with TfOH; cf. entries 6-8 to entries 1-5, and 9. Finally,

the first substantial improvement was made by adding

ates in neighboring-group-assisted glycosylation reactions

which undergo Lewis or Brønsted acid catalysis to the

desired 1,2-trans-glycoside.²⁴In one case kinetic evidence

was presented to support this hypothesis.²⁵Acyl transfer

was postulated as a side reaction of this rearrangement

process.²⁶This work provides experimental results supple-

mented by some theoretical studies which give some ideas

on how to minimize this side reaction.

Resu lts a n d Discu ssion

Model experiments demonstrated that ethyl 2,3,4,6-

tetra-O-benzoyl-â-^D-galactothiopyranoside (5)²⁷glycosy-

lates (MPEG)(DOX)OH in dichloromethane promoted by

N-iodosuccinimide and catalytic trifluoromethanesulfonic

acid (NIS/TfOH) to give 6. However, even this prepara-

tion leads to about 7% benzoyl transfer product 3, Scheme

2. Under very similar conditions the corresponding 3,4-

O-isopropylidene-blocked donor 2 gave the complex mix-

ture (Table 2, entry 1, and Scheme 1). These two results

imply that 2 is more prone than 5 toward acyl transfer.

It was essential to determine if this side reaction was

a property of the polymer or the donor. Thus, acceptor

methyl 4-hydroxymethylbenzoate (7) was reacted with

2 under NIS/TfOH conditions, Scheme 3. This reaction

produced a complex mixture from which the following

products were isolated: the product of benzoate transfer,

methyl 4-benzoyloxymethylbenzoate (8), the expected

â-glycoside 9a , the R-glycoside 9b, the reducing sugar 9c,

a â1,2-linked disaccharide (9d ), and other unidentified

products (see Tables 1 and 2 in the Supporting Informa-

tion for ¹H and ¹³C NMR data). The isolation of donor-

derived â1,2-disaccharide 9d supports the thesis that

such side reactions are a general property of neighboring-

group-assisted glycosylation reactions. 1D selective NOE-

(24) Garegg, P. J .; Konradsson, P.; Kvarnstro¨m, I.; Norberg, T.;

Svensson, S. C. T.; Wigilius, B. Acta Chem. Scand., B 1985, 39, 569.

(25) Wallace, J . E.; Schroeder, L. R. J . Chem. Soc., Perkin Trans 2

1977, 795.

(26) (a) Banoub, J .; Bundle D. R. Can. J . Chem. 1979, 57, 2091. (b)

Magnus, V.; Vikic´-Topic´, D.; Iskric´, S.; Kveder, S. Carbohydr. Res. 1983,

114, 209.

(27) Veeneman, G. H.; van Leeuwen, S. H.; van Boom, J . H.

Tetrahedron Lett. 1990, 31, 1331.

(28) Tang, T. H.; Whitfield, D. M.; Douglas, S. P.; Krepinsky, J . J .;

Csizmadia, I. G. Can. J . Chem. 1994, 72, 1803.

Acyl Transfer in Oligosaccharide Synthesis

J . Org. Chem., Vol. 64, No. 25, 1999 9033

Ta ble 2. Glycosyla tion s of (MP EG)(DOX)OH (1) a n d (MP EG)(DOX)OTr (10) w ith Don or 2

acceptor

donor

entry

(equiv)

promoter (equiv)

temperature, time

solvent, additive

result

1

2

3

4

5

2 (1.5)

1 (1.0)

NIS (5.0), TfOH (1.4)

NIS (5.0), TfOH (0.7)

NIS (5.0), TfOH (1.4)

0-5 °C, 10-15 min

20 °C, 10-15 min

add 0-5 °C to

4Å sieves

20% 4a , 4b, 3, and others

>90% orthoester 4d

>90% 1, and 5% 4d

20% 4a , 4b, 4d , 3, and others

20 °C, 2.5 h

6

7

8

9

10

2 (1.5)

10 (1.0)

10

1 (1.0)

1

NIS (5.0), AgOTf (1.1)

NIS (2.5), AgOTf (1.1)

20 °C, 2.5 h

0-5 °C, 90 h

0-5 to 20 °C 2.5 h

20 to 40 °C, h

4Å sieves

CH₂Cl₂

CH₂Cl₂, 2-Me-but

CH₂Cl₂, 2 Me-but

>90% 1 and 5% of 4d

>90% 1, and traces of others

40% 4a , 4b, 3, and traces of others

55% 4a , 4b, 3, and traces of others

Sch em e 3. Ben zoyl Tr a n sfer a n d

1,2-Disa cch a r id e F or m a tion fr om Solu tion

Rea ction of Don or 2

Sch em e 4. Ch lor oben zoyl Tr a n sfer a n d

1,2-Disa cch a r id e F or m a tion fr om Rea ction of

Don or 11 a n d 1

The pivaloyl group is recommended to prevent orthoester

formation during glycosylation reactions.³¹Increasing the

temperature to room temperature improved the glycoside

to acyl transfer ratio to 5.2:1, corresponding to a 82%

yield. Since the pivaloyl group is a permanent protecting

group, this last result is synthetically useful. The iso-

propylidene protecting group was cleaved by treatment

with 50% aqueous acetic acid at 60 °C for 16 h to give

3,4-diol 17.

To further characterize these compounds, the products

were cleaved from the polymer using the recently

developed conditions of scandium(III) trifluoromethane-

sulfonate, Sc(OTf)₃, and acetic anhydride.³²These condi-

tions cleave the C-O bond between the benzylic carbon

of the DOX and the terminal oxygen of the MPEG. Also,

all hydroxyls are acetylated under these conditions. This

reaction applied to a small portion of the 15 and 16

mixture gave a mixture of products containing predomi-

nantly [4-O-acetoxymethyl]benzyl 3,4-di-O-acetyl-2,6-di-

O-pivaloyl-â-^D-galactopyranoside (18) and 4-O-acetoxym-

ethylpivaloyloxymethylbenzene (19), Scheme 5. Thus, the

isopropylidene protecting group was cleaved under these

conditions. This reaction may have applications to other

synthetic problems. Diol 17 also gave 18 under these

conditions. The polymer is recovered as its O-acetylated

derivative 20.

2-methyl-2-butene as acid scavenger, but still the glyco-

side was only formed in 40% yield, entries 8 and 9.

Increasing the temperature to 40 °C improved the yield

to 55% , entry 10. It should be noted that molecular sieves

are often detrimental to NIS/AgOTf-promoted reactions

(cf. Table 2, entry 6) and 2-methyl-2-butene is a viable

alternative. This optimized condition led to a nearly 3-fold

improvement in yield (55% vs 20%) and may prove of

value for other glycosylations. However, these conditions

are not sufficiently efficient for polymer-supported reac-

tions.

Next, the benzoyl groups were changed to the elec-

tronically deactivated 4-chlorobenzoyl 11 or the sterically

hindered pivaloyl 12. With 11 even with the optimized

conditions a complex mixture (13a b, 14) was formed,

Scheme 4. With donor 12 the first major improvement

was made as the only two products isolated were the

glycoside 15 and the pivaloylated acceptor (MPEG)-

(DOX)OPiv (16), in a ratio of 1.2:1 (start at 0 °C and

increase to rt). To the best of our knowledge this is the

first report of pivaloyl transfer in a glycosylation reaction

although an intramolecular pivaloyl migration has been

reported recently.²⁹Pivalate orthoesters are well known.³⁰

(29) Gildersleeve, J .; Pascal, R. A., J r.; Kahne, D. J . Am. Chem. Soc.

1998, 120, 5961.

(30) Lemieux, R. U.; Morgan, A. R. Can. J . Chem. 1965, 43, 2199.

(31) (a) Kunz, H.; Harreus, A. Liebigs Ann. Chem. 1982, 41. (b) Sato,

S.; Nunomura, T.; Ito, Y.; Ogawa, T. Tetrahedron Lett. 1988, 29, 4097.

(c) Sato, S.; Ito, Y.; Ogawa, T. Tetrahedron Lett. 1988, 29, 5267. (d)

Nunomura, S.; Ogawa, T. Tetrahedron Lett. 1988, 29, 5681.

(32) Mehta, S.; Whitfield, D. M. Tetrahedron Lett. 1998, 39, 5907.

9034 J . Org. Chem., Vol. 64, No. 25, 1999

Whitfield et al.

Sch em e 5. P iva loyl Tr a n sfer bu t No 1,2

Disa cch a r id e F or m a tion fr om Rea ction of Don or

12 a n d 1

predominantly the 3,4-branched trisaccharide 28. Ap-

parently the 1,4-linked 23 is less reactive than 22. The

substitution pattern of 28 was established from a NOESY

spectrum where cross-peaks between GlcNPhth H-1 and

Gal H-3 as well as between Glc H-1 and Gal H-4 were

observed. Another characteristic feature of 28 is an

unusually upfield-shifted acetyl methyl resonance (1.57

ppm, Table 3). A HMBC spectrum was used to assign

this resonance to the O-2 acetyl of the Glc residue (not

shown). The HMBC spectrum also provided additional

proof of the connectivities.

Since the steric bulk of the donor appeared to be

beneficial for minimizing acyl transfer, it was decided to

increase the steric bulk about the acceptor by synthesiz-

ing the modified DOX linkers R,R′-dimethyldioxyxylene,

M2DOX, and R-methyldioxyxylene, MDOX. A limited

study of acceptors in the orthoester glycosylation proce-

dure found that increased steric bulk minimized acyl

transfer, lending credence to this postulate.³⁵

The R,R′-dimethyl linker M2DOX was prepared by a

straightforward series of reactions, Scheme 7. The hy-

droxyl of methyl 4-hydroxymethylbenzoate (29), was

protected as its tert-butyldiphenylsilyl ether 30, and then

30 was reacted with methyl Grignard reagent to produce

the tertiary alcohol 31. This alcohol was protected as its

benzoate 32 followed by cleavage of the silyl ether with

tetrabutylammonium fluoride, TBAF, to give the primary

alcohol 33 and finally reaction to form the trichloro-

acetimidate 34 in an overall yield of 22% from 29.

Compounds 31-34 all stain purple after spraying with

5%_aqH₂SO₄and heating, which facilitates their identi-

fication by TLC. Boron trifluoride etherate promoted

etherification of (MPEG)OH produced 35 which was

converted to alcohol acceptor 36 by basic hydrolysis.

Attempts to glycosylate this acceptor with donor 2 did

not go to completion even with repetitive treatments or

higher temperatures, presumably due to too much steric

hindrance. However, no traces of benzoate 35 were

detected, which supports the hypothesis of the impor-

tance of steric bulk for minimization of acyl transfer.

Sch em e 6. F or m a tion a n d Isola tion of Br a n ch ed

Tr isa cch a r id e 28

To increase the reactivity of the acceptor, the R-monom-

ethyl derivative of DOX, MDOX, was prepared, Scheme

8. A modification of a procedure to produce differently

protected trialkylsilyl derivatives of MDOX, cf. 40, was

followed.³⁶Thus, the ketoester, ethyl 4-acetylbenzoate

(37) was reduced nonstereospecifically to the racemic

mixture of alcohols 38. Then 38 was protected as its tert-

butyldiphenylsilyl ether 39 and the ester reduced with

lithium aluminum hydride to the primary alcohol 40. The

alcohol was activated as its trichloroacetimidate 41 in

an overall yield of 68% from 37. Compounds 38-41 also

stain purple after spraying with 5%_aqH₂SO₄and heating,

which facilitates their identification by TLC. Subsequent

etherification of (MPEG)OH produced 42. It should be

noted that for this reaction molecular sieves were abso-

lutely necessary as only unreacted (MPEG)OH was

isolated in their absence. Acetylation of 42 with acetic

anhydride and sodium acetate produced negligible acetyl

incorporation as detected by ¹H NMR, suggesting greater

than 95% incorporation of the linker. The free alcohol

(MPEG)(MDOX)OH (43) was liberated by treatment with

Glycosylation of diol 17 with the known glycosyl donor

3,4,6-tri-O-acetyl-2-deoxy-2-N-phthalimido-â-^D-glucopy-

ranosyl trichloroacetimidate (21)³³led to a mixture of

â-1,3-linked (22) and â-1,4-linked (23) polymer-bound

disaccharides and unreacted 17 in a ratio of 3.0:1.4:1.0,

Scheme 6. Their identities were confirmed by cleavage

mediated by Sc(OTf)₃to yield disaccharides 24 and 25.

The synthesis was completed by reaction with known

donor 2,3,4,6-tetra-O-acetyl-R-^D-glucopyranosyl-O-trichlo-

roacetimidate (26)³⁴to yield a mixture of products.

Cleavage from the polymer mediated by Sc(OTf)₃led to

(35) Garegg, P. J .; Kvarnstro¨m, I. Acta Chem. Scand. B 1977, 31,

509.

(36) Pilcher, A. S.; Hill, D. K.; Shimshock, S. J .; Waltermire, R. E.;

DeShong, P. J . Org. Chem. 1992, 57, 2492.

(33) Leung, O.-T.; Douglas, S. P.; Pang, H. Y. S.; Whitfield, D. M.;

Krepinsky, J . J . New J . Chem. 1994, 18, 349.

(34) Schmidt, R. R.; Stumpp, M. Liebigs Ann. Chem. 1983, 1249.

Acyl Transfer in Oligosaccharide Synthesis

J . Org. Chem., Vol. 64, No. 25, 1999 9035

Ta ble 3. ¹H NMR Sp ectr a l Da ta for 24, 25, a n d 28^a

comp

H-1 (J _1,2

)

H-2 (J _2,3

)

H-3 (J _3,4

)

H-4 (J _4,5

)

H-5 (J _5,6

)

H-6 (J _5,6′

)

H-6′ (-J _6,6′

)

Bn-CH₂(-J _H,H)

24

GlcN

Ga l

5.31 d

(8.0)

4.37 d

(7.8)

4.17 dd

(10.8)

4.96 dd

(9.7)

5.81 dd

(9.0)

3.95 dd

(3.7)

5.17 br t

(10.1)

5.38 br d

(<0.5)

3.75 br dt 4.35 dd

(2.4)

3.82 br t

4.17 dd

(2.7)

4.13 m

5.05 s (2)

(12.1)

4.13 m

4.74 d, 4.46 d

(11.9)

ArH

Ac

Piv

Phth 7.79 m (2), 7.70 m (2); DOX 7.24 d (2), 7.18 d (2) J ) 8.0

2.13 s (3), 2.12 s (3), 2.07 s (3), 2.01 s (3), 1.82 s (3)

1.20 s (9), 1.01 s (9)

25

GlcN

Ga l

5.30 d

(8.4)

4.33 d

(7.7)

4.40 dd

(10.7)

4.81 dd

(10.6)

5.92 dd

(9.3)

4.83 dd

(2.5)

5.15 br t

(10.0)

4.00 br d

(<0.5)

3.82 br dt 4.21 m

4.21 m

5.06 s (2)

3.64 br dd 4.33 dd

(4.0)

4.26 dd

(7.7)

4.72 d, 4.54 d

(12.0)

(12.4)

ArH

Ac

Piv

Phth 7.79 br m (2), 7.69 m (2); DOX 7.26 d (2), 7.20 d (2) J ) 7.9

2.11 s (3), 2.08 s (3), 2.02 s (3), 1.96 s (3), 1.87 s (3)

1.20 s (9), 0.85 s (9)

28

GlcN

Ga l

5.33 d

(8.3)

4.31 d

(7.8)

5.07 d

(8.1)

4.24 m

(10.7)

4.95 dd

(10.1)

4.86 dd

(9.5)

5.89 dd

(9.1)

4.09 dd

(3.4)

5.61 br t

(9.7)

5.19 br t

(9.9)

4.11 br d

(<0.5)

5.11 br t

(9.7)

3.76 br dt

(4.4)

3.66 br dd

(4.4)

4.38 dd

(4.2)

4.17 m

(6.9)

4.19 dd

(12.3)

4.17 m

5.04 s (2)

4.64 d, 4.43 d

(11.7)

Glc

3.92 br dt

4.26 m

ArH

Ac

Piv

Phth 8.10 m (1), 7.79 br m (1), 7.71 m (2); DOX 7.23 d (2), 7.18 d (2) J ) 8.0

2.12 s (3), 2.11 s (3), 2.07 s (6), 2.02 s (3), 2.00 s (3), 1.85 s (3), 1.57 s (3)

1.24 s (3), 1.18 s (6), 1.16 s (9)

a

See Scheme 6. Chemical shifts in parts per million and coupling constants in hertz.

Ta ble 4. ¹³C NMR Sp ectr a l Da ta for Oligosa cch a r id es 24, 25, a n d 28^a

C-2 C-3 C-4 C-5

54.8 70.8 71.9

compd

C-1

C-6

61.2

Bn-CH₂

24GlcN

Ga l

99.8

97.4

70.0

74.3

65.9

68.9

69.9 69.2

71.2

62.2

CdO

ArC

Piv

Piv 178.0, 176.3; Ac 170.9, 170.8, 170.1, 170.0, 169.4; Phth 168.1 br

DOX 136.6, 135.5, 128.2, 128.1; Phth 134.2, 131.5, 123.6

C 38.8, 38.7;

CH₃27.1, 26.8

Ac

21.0, 20.8, 20.64, 20.61, 20.4

25GlcN

Ga l

98.2

98.9

54.4

67.7

70.0

72.5

69.1

74.6

71.5

72.2

61.9

63.7

66.0

68.8

CdO

ArC

Piv

Piv 178.0, 175.3; Ac 170.9, 170.8, 170.5(2), 170.1, 169.4

DOX 137.1, 135.5, 128.3, 128.2; Phth 134.0 br, 131.6 br, 126.8 br

C 38.8, 38.7; CH₃27.2, 26.8

Ac

21.0, 20.7, 20.6, 20.5, 20.4

28GlcN

Glc

Gal

98.8

99.5

96.6

71.5

55.0

71.0

71.4

69.07

77.5

69.8

69.07

72.06

73.0

72.5

72.14

61.7

61.8

63.5

66.0

68.8

CdO

ArC

Piv

Piv 178.2, 176.4; Ac 170.8, 170.8, 170.5, 170.3, 169.9, 169.7, 169.5(3); Phth 167.6 br

DOX 137.0, 135.3, 128.2, 128.1; Phth 134.2 br, 131.5, 124.9, 123.6

C 38.9, 38.7;

CH₃27.1, 26.8

Ac

21.0, 20.77 (2), 20.75, 20.7, 20.6, 20.5, 19.3

a

See Scheme 6. Chemical shifts in parts per million.

1

TBAF. The H and ¹³C NMR spectral data for all linkers

from the diastereomers at the R-methyl benzyl carbon of

the MDOX and not from endo/exo isomers at the orthoe-

ster carbon since the analogous orthoester with DOX has

only one set of resonances in its ¹H NMR spectrum.

Orthoesters 47a b could be isomerized to the glycoside

44a b by treatment with TfOH at -20 to -10 °C, but

benzoyl transfer (see product 45) and some hydrolysis

also occurred.

To proceed with the synthesis, the isopropylidene group

was cleaved to yield 3,4-diol 48a b, Scheme 10. The

structure of 48a b was confirmed after cleavage from the

polymer with Sc(OTf)₃/Ac₂O to yield 3,4-di-O-acetate

49a b. Traces of other unidentified products and the

expected product from benzoyl transfer, 4-acetoxymethyl

(1-(R,S)-benzoyloxyethyl)benzene (50), as well as traces

of 4-acetoxymethyl (1-(R,S)-acetoxyethyl)benzene (51)

were the only other products isolated. The diacetate 51

could result from unreacted 43. Isopropylidene 48a b also

led to diacetate 29a b; cf. 15 to 18. The ¹³C NMR spectra

of 50 and 51 were unequivocally assigned by use of a

long-range ¹³C-¹H correlation, HMBC, experiment. Thus,

the aromatic resonances of the MDOX at 142 ( 1 and

are compiled in Tables 3 and 4 of the Supporting

Information, respectively.

In this case alcohol 43 could be glycosylated success-

fully under carefully controlled conditions by treatment

with donor 2 at -10 to -20 °C using NIS and two

additions of TfOH to yield 44a b and 15% benzoyl transfer

product 45, Scheme 9. The a b nomenclature will be used

for 44a b and all subsequent products in which the

diastereomers can be distinguished by NMR, Tables 5

and 6. Benzoate 45 had been produced by an alternate

1

less efficient synthesis of 22 (not shown), so its H NMR

spectrum was known (see the Supporting Information).

The combination of NIS/AgOTf and 2-methyl-2-butene at

temperatures between 0 and 40 °C in dichloromethane

did not work with 43 as acceptor due to formation of a

product tentatively identified as the styrene 46 from its

¹H NMR spectrum (Table 8, Supporting Information).

Reaction temperatures lower than -20 °C or only one

addition of TfOH led to the isolation of predominantly

the isomeric orthoesters 47a b. The source of the chirality

in 47a b is not known for certain but is presumed to arise

9036 J . Org. Chem., Vol. 64, No. 25, 1999

Whitfield et al.

Sch em e 7. P r ep a r a tion of

r,r′-Dim eth yld ioxyxylen e, M2DOX, Lin k er

Sch em e 9. Op tim iza tion of Glycosyla tion of

(MP EG)(MDOX)OH (43), by Don or 1

Sch em e 10. F or m a tion a n d Isola tion of

3,4-Dia ceta te 49a b a n d Ela bor a tion in to 1,3-Lin k ed

Disa cch a r id es 54a b

Sch em e 8. P r ep a r a tion of r-Meth yld ioxyxylen e,

MDOX, Lin k er

126.5 ( 1 ppm are assigned as ipso and ortho to the

benzylic CH, whereas those at 135.5 ( 1 and 128.5 ( 1

ppm are ipso and ortho to the benzylic CH₂. These

assignments are assumed for all MDOX-containing com-

pounds, Table 6. The isolation and characterization of

50 unequivocally establishes that benzoyl transfer is to

the secondary hydroxyl of 43.

with the Hyperchem program package suggests that the

R-isomer should have the phenyl group projected toward

C-6. A weak cross-peak between a MDOX aromatic

proton and a H-6 proton in a NOESY spectrum allows

for a tentative assignment of the absolute stereochem-

1

istry. This isomer has a H chemical shift for H-1 at 4.42,

Most of the ¹H NMR resonances of 49a b were doubled;

see Figure 3. It proved possible to isolate a small quantity

of one isomer by preparative TLC and hence to assign

the spectra into two sets of resonances. Molecular model-

ing of these isomers using the version of MM³⁷supplied

whereas the other isomer has a shift at 4.76 ppm, Table

5. Such a large chemical shift difference is not observed

for other chiral secondary aglygons attached to glucose.³⁸

(38) (a) Trujillo, M.; Morales, E. Q.; Va´zquez, J . T. J . Org. Chem.

1994, 59, 6637. (b) Morales, E. Q.; Padro´n, J . I.; Trujillo, M.; Va´zquez,

J . T. J . Org. Chem. 1995, 60, 2537. (c) Padro´n, J . I.; Va´zquez, J . T.

Chirality 1997, 9, 626.

(37) Burkert, U.; Allinger, N. L. Molecular Mechanics; American

Chemical Society: Washington, DC, 1982.

Acyl Transfer in Oligosaccharide Synthesis

J . Org. Chem., Vol. 64, No. 25, 1999 9037

Ta ble 5. ¹H NMR Sp ectr a l Da ta for 49a b, 54a b, 60a b, 64a b, a n d 66^a

H-6′

Bn-CH₂

(-J _H,H

Bn-CH

(J _H,H

comp

H-1 (J _1,2

)

H-2 (J _2,3

5.54 dd

)

H-3 (J _3,4

5.08 dd

)

H-4 (J _4,5

5.48 br d

(<0.5)

)

H-5 (J _5,6

3.96 br t

)

H-6 (J _5,6′

4.58 dd

)

(-J _6,6′

)

49a (R) 4.42 d

4.32 dd

(11.2)

5.00 s

4.87 q

(6.5)

(8.1)

(10.7)

(3.4)

(6.7)

(6.8)

ArH

acetyl

CH₃

Bz_o8.02 d (2) J ) 7.8, 7.93 d (2) J ) 7.2, Bz_p7.58 m (2), Bz_m7.45 m (4); MDOX 7.04 br s (4)

2.20 s (3), 2.10 s (3), 1.87 s (3)

1.41 d

49b(S) 4.76 d

5.54 dd

(10.7)

5.24 dd

(3.4)

5.52 br d

(<0.5)

4.03 br t

(6.5)

4.43 dd

(6.9)

4.25 dd

(11.2)

5.00 s

4.96 s

4.81 q

(6.5)

(7.9)

ArH

acetyl

CH₃

Bz_o8.02 d (2) J ) 7.8, 7.97 d (2) J ) 7.1, Bz_p7.58 m (2), Bz_m7.45 m (4); MDOX 7.25 d (2), 7.19 d (2) J ) 7.9

2.17 s (3), 2.06 s (3), 1.90 s (3)

1.32 d

54a Ga l 4.2 m

5.12 m

3.82 m

(3.4)

5.60 dd

(9.1)

5.51 br d

(<0.5)

5.29 m

3.82 m

(7.5)

3.73 m

4.43 dd

(5.3)

4.2 m

4.36 dd

(11.5)

4.2 m

4.73 q

(6.5)

GlcN

5.29

(8.3)

4.2 m

(10.4)

ArH

acetyl

CH₃

Bz_o8.00 br d (2) J ) 7.9, 7.55 m; Phth 7.73 brm (2), 7.55 brm; Bz_p7.55 m Bz_m7.45 m, 7.32 br t(2); MDOX 7.10 d (2), 7.07 d (2) J ) 8.3

2.19 s (3), 2.09 s (3), 2.03 s (3), 1.96 s (3), 1.74 s (3)

1.25 d (3)

54bGa l 4.56 d

5.12 m

(10.1)

4.2 m

(10.7)

3.97 dd

(3.4)

5.64 dd

(9.1)

5.55 br d

(<0.5)

5.29 m

3.91 br t

3.82 m

4.2 m

4.92 s

4.65 q

(6.5)

(8.0)

GlcN

5.45 d

(8.3)

ArH

acetyl

CH₃

Bz_o8.07 br d (2) J ) 7.9, 7.66 br d (2) J ) 7.9; Phth 7.82 brm (2H), 7.55 brm, Bz_p7.55 m Bz_m7.45 m; MDOX 6.90 d (2), 6.82 d (2) J ) 7.9

2.15 s (3), 2.05 s (3), 2.03 s (3), 1.98 s (3), 1.76 s (3)

1.12 d (3)

60a Ga l 4.08 d

4.88 m

(9.9)

4.10 m

(10.6)

4.88 m

(9.9)

3.78 dd

(3.7)

5.75 m

(9.8)

3.91 dd

(3.7)

5.75 m

(9.8)

5.26 br d

(<0.5)

5.11 m

(10.1)

5.29 br d

(<0.5)

5.11 m

(10.3)

3.60 br t

5.9

3.70 dt

(2.6)

3.96 dd

7.3

4.31 dd

3.89 dd

11.4

4.12 dd

(12.1)

4.06 m

4.97 s

5.01 s

4.76 q

(6.6)

(7.7)

GlcN

5.26 d

(8.1)

60bGa l 4.45 d

3.69 br t

4.06 m

4.29 dd

4.72 q

(6.6)

(7.7)

GlcN

5.18 d

(8.1)

4.10 m

(10.6)

3.65 dt

(2.2)

4.08 m

(12.1)

ArH

acetyl

CH₃

Phth 7.74 m (4), 7.65 m (4); MDOX 7.21 d (2), 7.17 d (2) J ) 8.1; 7.18 d (2), 7.13 d (2) J )8.1

2.081, 2.077, 2.05, 2.039, 2.037, 2.00, 1.95, 1.94, 1.76, 1.70 10s (3)

1.30 d (3), 1.27 d (3)

pivaloyl 1.19, 1.16, 1.09, 1.06, 0.94 (36)

64a Ga l 4.06 d

4.89 dd

(10.4)

4.2 m

(10.7)

4.94 dd

(9.9)

4.01 dd

(3.4)

5.91 dd

(9.3)

5.649 dd

(9.4)

4.07 br d

(<0.5)

5.20 dd

(9.8)

5.14 dd

(9.8)

3.51 dd

3.75 m

4.2 m

5.09 s

4.84 q

(6.4)

(7.7)

GlcN

Glc

5.28 d

(8.3)

5.07 d

(8.0)

4.2 m

3.94 ddd

(4.3)

4.39 dd

4.2 m

(12.5)

ArH

acetyl

CH₃

MDOX 7.24 d (2), 7.23 d (2) J )8.3

2.14 s (3), 2.12 s (3), 2.10 s (6), 2.05 s (6), 1.88 s (3), 1.25 s (3)

1.36 d (3)

64bGa l 4.47 d

4.96 dd

(10.4)

4.2 m

(10.7)

4.92 dd

(9.9)

3.93 dd

(3.4)

5.95 dd

(9.3)

5.650 dd

(9.4)

4.10 br d

(<0.5)

5.23 dd

(9.5)

5.12 dd

(9.8)

3.61 dd

3.75 m

4.2 m

5.05 s

4.81 q

(6.4)

(7.6)

GlcN

Glc

5.35 d

(8.2)

5.09 d

(8.0)

4.2 m

3.94 ddd

(4.5)

4.41 dd

4.2 m

(12.5)

ArH

acetyl

CH₃

Phth 8.13 m (2), 7.82 m (2), 7.74 m (4); MDOX 7.27 d (2), 7.19 d (2) J ) 8.3

2.16 s (3), 2.08 s (3), 2.06 s (6), 2.03 s (6), 1.87 s (3), 1.29 s (3)

1.34 d (3)

pivaloyl 1.2 m

66rGa l 5.10 m

4.79 dd

(10.7)

4.2 m

4.43 m

5.82 dd

5.49 dd

4.1 m

4.39 m

4.1 m

4.38 dd

4.1 m

-

GlcN

Glc

5.42 d

(8.3)

4.97 d

(8.3)

5.13 dd

5.04 dd

4.1 m

3.77 m

3.84 m

3.62 m

3.73 m

3.84 m

4.78 dd

66âGa l 4.33 d

4.69 dd

(10.2)

4.2 m

(8.5)

GlcN

Glc

5.37 d

(8.3)

4.97 d

(8.3)

5.81 dd

(9.9)

5.49 dd

(9.2)

5.15 dd

4.78 dd

5.04 dd

(9.5)

ArH

Phth 8.04 m (2), 7.72 m (2), 7.66 m (4)

acetyl

2.08, 2.05, 2.01, 1.98, 1.93, 1.80, 1.18, 1.11

pivaloyl 1.1-1.2

a

See Schemes 10 and 11. Chemical shifts in parts per million and coupling constants in Hertz.

These R,S assignments are consistent with the assign-

ments for model alkyl â-^D-Gal compounds.³⁹For both

isomers strong cross-peaks are observed between H-1 and

the benzylic methine but only weak or absent cross-peaks

to the MDOX aromatics, suggesting that the CHO-1---

C*HPh(MDOX) bond has the benzylic methine gauche

and the phenyl anti to the C-1 methine.

Treatment of 48a b with trichloroacetimidate 21 pro-

duced predominantly one product (52a b) along with its

1,4 regioisomer 53a b, Scheme 10. The 3-O-selective

(39) Padro´n, J . I.; Morales, E. Q.; Va´zquez, J . T. J . Org. Chem. 1998,

63, 8247.

9038 J . Org. Chem., Vol. 64, No. 25, 1999

Whitfield et al.

1

F igu r e 3. Partial H NMR spectrum of diastereomeric glycosides 49a b.

glycosylation was confirmed by cleaving 52a b + 53a b

from the polymer using Sc(OTf)₃/Ac₂O, which in this case

produced the diastereomeric peracetylated MDOXyl dis-

accharide glycosides 54a b as the major products along

with the regioisomers 55a b and small amounts of uni-

dentified products. Most of the resonances in the ¹H NMR

spectrum of 54a b were doubled due to the diastereomers

at the R-methylbenzyl carbon of the MDOX linker, Table

5. The two diastereomers were not equally abundant (55:

45), and so it was possible to assign sets of resonances.

However, the absolute stereochemistry is not known. The

tentative R,S assignments follow from those for 49a b.

The downfield shifts of the Gal H-4 (δ ) 5.55 and 5.51

ppm) resonances confirmed the O-3 regiochemistry of the

glycosylation and the J ) 8.3 Hz coupling of the H-1

resonances of the GlcNPhth resonances confirmed the â

stereochemistry of the linkage. Since all the easily

measured coupling constants are identical the source of

this pronounced difference in the two diastereomers of

54a b is not known. Subtle conformational changes about

the glycosidic linkages may be the origin of the induced

shifts.

Although preparation of disaccharides 54a b was pos-

sible by this route, the reaction was still not sufficiently

efficient for polymer-supported synthesis. Therefore,

alcohol 43 was reacted with donor 12, and in this case

only diastereomeric glycosides 56ab were isolated, Scheme

11. Thus, the contraintuitive approach of decreasing the

reactivity of both the donor and the acceptor by increas-

ing the steric bulk leads to conditions that favor glyco-

sylation over acyl transfer. Applying the sequence of

hydrolysis of the isopropylidenes to afford diols 57a b and

then subsequent glycosylation with 21 led to the 1,3

disaccharides 58a b and 1,4 disaccharides 59a b. The 1,3

to 1,4 selectivity was 2.2 to 1. The structures were

confirmed by cleavage from the MPEG using Sc(OTf)₃/

Ac₂O to yield 60a b and 61a b. Finally, disaccharides 58a b

+ 59a b were glycosylated with known donor 26 to yield

62a b and 4,3 regioisomers 63a b, respectively. Subse-

quent cleavage using Sc(OTf)₃/Ac₂O yielded 64a b and

65a b. The overall yield for 64a b was 12% for three

glycosylations and two functional group transformations.

This yield is based on 100% derivitization of the MPEG-

5000. Comparable solution chemistry is unlikely to

proceed in yields higher than 80% for glycosylations or

90% for functional group transformations, i.e., 80 × 80

× 80 × 90 × 90 ) 41%. Given that the whole sequence

can be done in six working days, once the building blocks

are available, this represents a significant development.

Again, oligosaccharides 60a b and 64a b exhibit re-

markable diastereotopic chemical shifts between the two

isomers. Figure 4 shows the HSQC partial spectrum of

the ring protons and carbons of disaccharides 60a b.

Many resonances show diastereomeric shifts in both the

carbon and proton dimensions including the H-6 protons

of the terminal GlcNPhth residue which are 10 bonds

removed from the MDOX chiral center, Table 5. The R,S

assignments follow from previous assumptions. The Gal

H-1 resonances of the R-isomers at 4.05 (60a ) and 4.06

(64a ) ppm are unusually upfield and characteristic of

these compounds. The connectivities in 64a b were con-

firmed by observation of ROESY cross-peaks between

GlcNPhth H-1 and Gal H-3 as well as between Glc H-1

and Gal H-4 (not shown). A characteristic of trisaccha-

rides 64a b is one unusually high-field acetyl methyl

resonance 1.25 ppm (64a ), 1.29 ppm (64b). A similar

resonance was observed in the analogous trisaccharide

derived from DOX 28 and was assigned to the O-2 acetyl

of the Glc residue, Table 3. These resonances remained

at high field after hydrogenation of 64a b, which removed

the MDOX linker, leading to the reducing trisaccharide

66R/â, Table 5.

The central question in this work is the cause of the

acyl transfer to the glycosyl acceptor. In a previous

theoretical study we determined the most important

intermediates of the neighboring-group-assisted glyco-

sylation reaction between 2,6-di-O-acetyl-3,4-O-isopro-

pylidene-1-deoxy-^D-galactopyranosyl cation and metha-

Acyl Transfer in Oligosaccharide Synthesis

J . Org. Chem., Vol. 64, No. 25, 1999 9039

Ta ble 6. ¹³C NMR Sp ectr a l Da ta for Oligosa cch a r id es 49a b, 54a b, 60a b, 64a b, a n d 66^a

compd

C-1

C-2

C-3

C-4

C-5

C-6

Bn-CH₂

66.0

Bn-CH

77.5

49a

CH₃

49b

CH₃

CdO

ArC

98.5

24.0

99.8

21.0

69.8

70.94

67.33

70.85

61.70

69.5

70.94

67.29

70.90

61.65

65.9

75.9

Ac 170.9, 170.3, 170.2, 170.1; Bz 165.94, 165.96, 165.1, 165.0

MDOX_ip142.5, 142.0, 135.6, 135.1; MDOX_o128.4, 128.2, 126.8, 126.4; Bz_p133.39, 133.35; Bz_o130.0, 129.7; Bz_ip129.5, 129.4;

Bz_m128.53, 128.48

Ac

21.02, 20.98, 20.76, 20.72, 20.6, 20.5

50

66.0

72.6

72.0

CdO

ArC

CH₃

Ac

Ac 170.9, 170.3

DOX_ipCH141.9; DOX_ipCH2135.5; DOX_oCH2128.5; DOX_oCH126.3

22.2

21.3, 21.0

51

66.0

CdO

ArC

CH₃

Ac

Ac 170.9 Bz 165.8

DOX_ipCH141.8; DOX_ipCH2135.5; DOX_oCH2128.3; DOX_oCH126.3; Bz_p133.0; Bz_o129.6; Bz_m128.5

22.4

21.0

54a Ga l 98.1

GlcN 98.4

54bGa l 99.5

68.8

54.5

68.71

54.5

77.55

70.3

77.41

70.3

69.4

71.5

69.3

71.5

71.1

71.8

71.4

71.8

62.7

61.4

62.8

61.4

65.0

65.9

77.0

75.3

GlcN

CdO

ArC

98.4

Ac 170.8 (2), 170.1 (2), 170.0 (4), 169.4 (2); Bz 166.1 (2), 164.5, 164.4; Phth 168.5 br

MDOX_ip142.6, 142.0, 135.4, 134.9, MDOX_o128.9, 128.1, 126.8, 126.2; Bz_p132.9, 133.2, 133.3; Bz_o129.8, 129.7, 129.5; Bz_ip129.34,

129.28, 128.8; Bz_m128.5, 128.4, 128.2; Phth_o133.9; Phth_ip130.9; Phth_o123.2 br

CH₃

Ac

22.2, 23.9

21.0, 20.8, 20.7, 20.6, 20.3;

60a Ga l 97.8

GlcN 97.3

60bGa l 98.7

70.95

54.8

71.0

54.8

74.31

69.9

74.48

69.9

69.4

69.0

68.9

69.2

71.1

71.9

71.1

71.9

61.9

61.2

62.6

61.0

66.1

65.9

75.0

75.1

GlcN

CdO

ArC

CH₃

Ac

97.3

Piv 176.2 (4); Ac 170.8 (2), 170.0 (6), 169.4 (2); Phth -

MDOX_ip143.1, 141.4, 135.7, 134.9; MDOX_o128.2, 128.1, 127.1, 126.1; Phth_o134.1; Phth_ip131.5; Phth_o123.6 br

23.4, 21.8

21.0, 20.8, 20.6, 20.4

64a Ga l 97.6

71.61

55.1

71.57

71.8

55.1

71.52

74.7

69.8

73.1

74.9

69.8

73.00

73.02

69.1

69.08

72.4

69.1

69.08

72.1

72.13

71.3

71.8

72.13

71.3

64.0

61.8

61.7

63.3

61.8

61.7

66.0

66.2

74.2

74.1

GlcN

Glc

96.6

98.9

64bGa l 98.4

GlcN

Glc

96.6

98.8

CdO

ArC

Piv

Piv 178.2, 176.3; Ac 170.8, 170.5, 170.3, 169.9, 169.7, 169.6, 169.5, 167.5; Phth -

MDOX_ip143.5, 141.7, 135.5, 134.5; MDOX_o128.2, 128.0, 127.3, 126.0; Phth_o134.2; Phth_ip131.6; Phth_o124.9 br, 123.6 br

31.2, 29.7, 27.2, 26.9

CH₃

Ac

23.2, 21.7

21.0, 20.8, 20.7, 20.6, 20.5, 20.4, 19.37, 19.33

66rGa l 90.3

70.8

55.2

71.2

73.8

55.0

71.6

71.1

70.0

73.0

74.3

70.0

73.0

73.5

69.3

69.1

72.5

69.3

69.1

68.4

72.2

71.3

72.3

72.2

71.3

63.5

61.8

61.9

63.5

61.8

61.9

GlcN

Glc

96.7

100.0

66âGa l 96.1

GlcN

Glc

96.8

100.0

CdO

ArC

Piv

Piv 178.1, 177.4; Ac 170.8, 170.5, 1702, 169.6, 169.2; Phth 167.5

Phth 134.3, 125.0

29.7, 27.1, 26.8

Ac

20.8, 20.7, 20.6, 20.4, 19.3, 19.2

a

See Schemes 10 and 11. Chemical shifts in ppm.

nol.⁴⁰It is valuable to reconsider the theoretical model

in light of the current experimental findings and update

our understanding of the reaction mechanisms. The most

important intermediates and the corresponding energy

diagram are shown in Schemes 12 and 13, respectively.

An S_N1 mechanism leads to the reactive intermediate

B, which can isomerize by an intramolecular ring closure

into intermediate C. This intramolecular reaction is

expected to be very fast with a modest barrier of less than

4 kcal mol^-1due to conformational change. The modest

conformational barrier can be attributed to the isopro-

pylidene protecting group, which restricts the accessible

conformations to the pseudorotational itinerary. Alter-

natively, B can react with the nucleophile in a highly

favorable and fast reaction with a minimal barrier to

form H, the intermediate leading to the wanted â-glyco-

side. The formation of intermediate G, leading to the

R-glycoside, is significantly less favorable. The formations

of C and H from B are competitive, and the intramo-

lecular formation of C is expected to be favored for

entropic reasons.⁴¹

Intermediates such as C can be isolated and probably

play a central role in the reaction mechanism.⁴²The

reaction of C with the nucleophile methanol leads to ion-

dipole complexes D and F , which are in equilibrium in

favor of D, which is the overall most stable intermediate.

It is proposed that D can lead to either orthoester

formation or acyl transfer. In most mechanistic schemes,

(40) Nukada, T.; Be´rces, A.; Zgierski, M. Z.; Whitfield, D. M. J . Am.

Chem. Soc. 1998, 120, 13291.

(41) Paulsen, H. Adv. Carbohydr. Chem. Biochem. 1971, 26, 127.

(42) Crich, D.; Dai, Z.; Gastaldi, S. J . Org. Chem. 1999, 64, 5224.

9040 J . Org. Chem., Vol. 64, No. 25, 1999

Whitfield et al.

F igu r e 4. Partial ¹³C-¹H correlation spectrum (gHSQC) of diastereomeric disaccharides 60a b.

Sch em e 11. F or m a tion a n d Isola tion of Br a n ch ed

Tr isa cch a r id es 64a b a n d 66

to the ion:dipole character of these complexes, the dotted

lines in Scheme 12 are not chemical bonds in the usual

sense and simply indicate the closest points of approach.

The details of the mechanism of orthoester formation

and acyl transfer from D is a subject of current theoreti-

cal studies. So far, we have established that the transfer

of the proton from the nucleophile to O-2 is the most

crucial step which leads to the breaking of the C-1---O-1

and O-2---C(dO) bonds. The proton transfer and the

orthoester formation (from D) both involve significant

barriers which are sensitive to electronic and steric

factors from the O-2 protecting group and the oxygen

nucleophile. The C-1---O-1 bond strength is highly sensi-

tive to the nature of the protecting group at O-2, and

proton transfer requires specific geometric orientation.

Consequently, the bulky pivaloyl protecting group and

the sterically hindered MDOX linker are expected to

provide unfavorable conditions for acyl transfer. We also

expect that glycosylation is less sensitive to steric hin-

drance of the nucleophile and the nature of the O-2

protecting group. The successful use of the O-2 pivaloyl

group and the MDOX linker is consistent with this

hypothesis.

High reaction temperatures and high promoter con-

centrations were found to favor â-glycoside formation at

the expense of orthoesters and acyl transfer. The â-gly-

coside is the lowest energy product in the calculation not

counting decomposition pathways of the acyl transfer.

Consequently, higher temperature can be favorable for

â-glycosylation, provided that the barrier to acyl transfer

is sufficiently high. Higher temperature can also affect

the speciation of the promoter (for example, ion pairing),

which complicates simple comparisons. According to our

mechanism the â-glycoside is formed directly from B

which is favorably affected by higher promoter concen-

tration.

orthoester intermediates such as D are drawn with a

short C---O bond to the incoming nucleophile in accord

with the original proposal.⁴³However, our calculated

intermediate D has a long C---O separation (2.79 Å). Due

The explanation regarding the thermodynamic factor

(43) Garegg, P. J .; Kvarnstro¨m, I. Acta Chem. Scand. B 1976, 30,

655.

to obtain â-glycoside assumes that intermediate D or the

Acyl Transfer in Oligosaccharide Synthesis

J . Org. Chem., Vol. 64, No. 25, 1999 9041

Sch em e 12. P ostu la ted Mech a n ism for th e F or m a tion of Acyl Tr a n sfer P r od u cts a n d Glycosid es in

Neigh bor in g-Gr ou p -Assisted Glycosyla tion s of 3,4-O-Isop r op ylid en e Don or s^a

a

The dotted lines in D, F , and G are not bonds and only indicate the nearest points of contact. Energies were calculated using DFT

and are in kilocaories per mole relative to solvated isolated cation B + solvated methanol. The scheme is modified from ref 40 to include

E.

Sch em e 13. Rela tive En er gies (k ca l m ol^-1) for th e

P r op osed In ter m ed ia tes in Sch em e 12^a

way to H follows directly from the oxonium intermediate

B formed in the first step of the S_N1 reaction. The

transformation D to H, however, does not necessarily

involve the high-energy intermediate B, but another ion-

dipole complex. Most likely, the transformation of D to

H involves at least three steps: (i) the breaking of the

R-glycosidic bond, (ii) the conformational change of the

pyranose ring, (iii) the formation of the â-glycoside bond.

The relatively simple model of Scheme 12 neglects

specific solvation and ion-pairing. More complex mech-

anisms can be considered involving nucleophilic attack

on ion pairs, leading to trimolecular intermediates. Also,

our results are only directly applicable to this fused ring

system. Present efforts are focused on assessing the

importance of pyranose ring conformational interconver-

sions by extending this study to more flexible intermedi-

ates.

a

The dotted lines indicate the barrier to acyl transfer.

corresponding orthoester can be converted to H. The

textbook explanation of such a mechanism involves an

S_N2 mechanism, cf. F , which we have previously shown

to be a high-energy pathway in accord with earlier

suggestions by Dewar.⁴⁴The most straightforward path-

Con clu sion s

Thus, for the first time the branched trisaccharide

corresponding to part of a single repeat unit of the group

B Streptococcus type 1A capsular polysaccharide has been

(44) Dewar, M. J . S.; Dougherty R. C. The PMO Theory of Organic

Chemistry; Plenum Publishing Corp.: New York, 1975; p 263.

9042 J . Org. Chem., Vol. 64, No. 25, 1999

Whitfield et al.

or a mixture of glycerol and thioglycerol were used as the FAB

matrix. Typically 10-15 full-range, low-resolution MS scans

were averaged to yield a low-resolution mass spectrum. For

high-resolution MS, the electric sector was scanned over the

range of interest. Typically polyethylene glycol or polypropy-

lene glycol was used as an internal mass standard, and

between 75 and 150 scans were averaged.

synthesized by polymer-supported methods. This syn-

thesis required only three chromatographic separations,

one for the final product and two for the preparation of

building blocks 12 and 26. Several important observa-

tions and developments were made in the process. From

a practical synthetic perspective the NIS/AgOTf/2-meth-

yl-2-butene promoter system for thioglycosides is a valu-

able addition. The versatility of the ScOTf₃/Ac₂O MPEG-

DOX cleavage system is a useful improvement to the

(MPEG)(DOX) polymer linker combination and should

find widespread application. Similarly the observation

that isopropylidenes can be directly converted to di-O-

acetates by this reagent system may have other applica-

tions.

The observation that pivaloyl transfers from O-2 of the

conformationally restricted 3,4-O-isopropylidene pro-

tected donors to the acceptor is unprecedented. This

transfer suggests that in some way this structural feature

promotes this side reaction. Such fused rings have been

shown to “torsionally” deactivate other donor systems,

and such deactivation is likely operative in these ex-

amples.⁴⁵

This work provides a general methodology for prepar-

ing modified DOX linkers and is applicable to other linker

systems which can be used with both soluble and

insoluble PEG derivatives. The application of the Sc-

(OTf)₃/Ac₂O cleavage conditions to the diastereomeric

R-methyl-substituted MDOX glycosides led to the obser-

vation of large ¹H NMR chemical shift differences. These

diastereotopic shifts are larger than those reported for a

large number of O-glycosides epimeric at the aglyconic

carbon. By finding conditions to eliminate acyl transfer,

a method to achieve polymer-supported synthesis of a

branched trisaccharide corresponding to a portion of the

type 1A capsular polysaccharide of group B Streptococcus

was created.

(MPEG)(DOX)OH (1) was synthesized according to ref 46.⁴⁶

Eth yl 2,6-Di-O-ben zoyl-3,4-O-isop r op ylid en e-â-D-ga la c-

toth iop yr a n osid e (2). Ethyl â-^D-galactothiopyranoside⁴⁷was

converted to its 3,4-O-isopropylidene according to ref 48.⁴⁸To

a round-bottomed flask containing this 2,6-diol (1.74 g, 6.6

mmol) was added pyridine (20 mL), and the mixture was cooled

in an ice bath under an atmosphere of argon. To this mixture

were added benzoic anhydride (4.6 g) and 4-dimethylaminopy-

ridine (145 mg), and the mixture was left to stir and warm to

rt overnight. The solvents were removed by distillation at high

vacuum followed by codistillation with toluene. The residue

was taken up in warm 100% ethanol (230 mL) and filtered by

gravity through a warm filter paper, and warm water (175

mL) was slowly added. After slow cooling to rt, white needles

were separated, collected by filtration, and washed with cold

water. The crystals were dissolved in dichloromethane, any

residual water was removed in a separatory funnel, and the

dichloromethane solution was dried with MgSO₄, filtered, and

evaporated to yield 2 (2.69 g, 86%): mp 148-149 °C; [R]_D46.5

(c, 0.51, CHCl₃); MS FAB +ve 490 (M + H₂O⁺), 473 (MH⁺),

411 (M - SEt⁺); HRMS C₂₅H₂₉O₇S 473.1661 calcd 473.1634.

Anal. Calcd for C₂₅H₂₉O₇S (473.1661): C, 63.40, H, 6.00.

Found: C, 63.12, H, 6.25.

(MP EG)(DOX)yl 2,6-Di-O-ben zoyl-3,4-O-isopr opyliden e-

â-^D-ga la ctop yr a n osid e (4a ) a n d (MP EG)(DOX)-O-ben -

zoa te (3) [Ta ble 2, En tr y 1]. Donor 2 (71 mg, 0.15 mmol),

(MPEG)(DOX)OH (512 mg, 0.1 mmol), and activated 4 Å

molecular sieves (about 300 mg) were dried together overnight

at high vacuum. Dichloromethane (3 ml) was added and the

mixture cooled in an ice-salt bath. After the mixture was

stirred for 0.5 h, N-iodosuccinimide (NIS; 112 mg, 0.5 mmol)

was added followed by trifluoromethanesulfonic acid (TfOH;

200 µL of a dilute solution in CH₂Cl_2,, 0.14 mmol). After being

stirred for 15 min, the reaction was quenched with DIPEA (50

µL) and the polymer precipitated with tert-butyl methyl ether

(TBME; 40 mL). The solid was recovered by filtration and after

being rinsed with TBME was recrystallized from absolute

ethanol (25 mL). The resulting solid was collected by filtration

and after being washed with cold ethanol and diethyl ether

was taken up in CH₂Cl₂, filtered, and evaporated to yield the

mixture (510 mg) described in Scheme 1 and Tables 1 and 2.

[Ta ble 2, En tr y 10]. Donor 2 (55 mg, 0.12 mmol) and

(MPEG)(DOX)OH (397 mg, 0.078 mmol) were dried together

overnight at high vacuum. Dichloromethane (2.5 ml) was

added and the mixture heated in an oil bath to 40 °C.

2-Methyl-2-butene (410 µL of a 1:10 solution in CH₂Cl₂, 0.4

mmol) and NIS (86 mg, 0.5 mmol) were added followed by

silver trifluoromethanesulfonate (AgOTf, 22 mg, 0.86 mmol).

After being stirred for 40 min, the reaction was cooled in an

ice bath and quenched with ammonium bicarbonate (100 mg)

and the solid precipitated with TBME (40 mL). The solid was

recovered by filtration and after being rinsed with TBME was

recrystallized from absolute ethanol (25 mL) containing 0.5%

w/v imidazole. The resulting solid was collected by filtration

and after washed with cold ethanol and diethyl ether was

taken up in CH₂Cl₂, filtered, and evaporated to yield the

mixture (380 mg) containing 55% 4a .

Exp er im en ta l Section

Ma ter ia ls a n d Gen er a l Meth od s. Silica gel (230-400

mesh) was used for flash chromatography. All starting materi-

als were dried overnight in vacuo (10^-3mmHg) over KOH or

P₂O₅prior to use, and the solvents were distilled from

appropriate drying agents. Solutions were concentrated at 1

mmHg of pressure in a rotary evaporator. Optical rotations

were measured (λ ) 589 nm) at room temperature in a 10 cm

1 mL cell. The ¹H and ¹³C NMR spectra were recorded in

deuteriochloroform solution at 500.1 or 600.2 MHz and 125.8

or 150.9 MHz, respectively. ¹H NMR spectra in CDCl₃were

referenced to residual CHCl₃at 7.24 ppm, and ¹³C NMR

spectra to the central peak of CDCl₃at 77.0 ppm. Assignments

were made by standard ¹H-¹H COSY and ¹³C-¹H COSY

1

experiments. H chemical shifts are reported to two decimal

places and ¹³C shifts to one. In the case of closely separated

resonances an additional figure is added to show that they are

separately identifiable. For polymer-bound samples, the MPEG

methylenes were saturated and quantitation was made by

comparing integrals to the terminal methyl of the MPEG.

Assignments were made by comparison to the spectra of

building blocks and cleaved compounds. Advantage was also

taken of gradient-enhanced 1D selective TOCSY and NOESY

experiments. Typically 256 transients were used for TOCSY

spectra with mixing times from 20 to 150 ms and 4k transients

for NOESY spectra with mixing times of 200-500 ms.

(MP EG)(DOX)yl 2,3,4,6-tetr a -O-ben zoyl-â-^D-ga la ctop y-

r a n osid e 6 a n d (MP EG)(DOX)-O-ben zoa te (3). Donor 5²⁷

(545 mg, 0.85 mmol), (MPEG)(DOX)OH (2.9 g, 0.57 mmol), and

4 Å molecular sieves (about 1 g) were dried together overnight

The mass spectral analysis was done on a forward mass

spectrometer. Fast atom bombardment (FAB) MS was per-

formed using xenon atom at 6 kV as the source. Thioglycerol

(46) Krepinsky, J . J .; Douglas, S. P.; Whitfield, D. M. Methods

Enzymol. 1994, 242, 281.

(47) Aberg, P.-M.; Blomberg, L.; Lo¨nn, H.; Norberg, T. J . Carbohydr.

Chem. 1994, 13, 141.

(45) Fraser-Reid, B.; Wu, Z.; Andrews, C. W.; Skowronski, E. J . Am.

Chem. Soc. 1991, 113, 1434.

(48) Catelani, G.; Colonna, F.; Marra, A. Carbohydr. Res. 1988, 182,

297.

Acyl Transfer in Oligosaccharide Synthesis

J . Org. Chem., Vol. 64, No. 25, 1999 9043

at high vacuum. Dichloromethane (12 mL) was added and the

mixture cooled in an ice-salt bath to 0 °C. After the mixture

was stirred for 1 h, NIS (638 mg, 2.8 mmol) was added followed

by TfOH (750 µL of a dilute solution in CH₂Cl_2,, 0.28 mmol).

After being stirred for 15 min, the reaction was quenched with

DIPEA (700 µL) and the polymer precipitated with tert-butyl

methyl ether (TBME; 150 mL). The solid was recovered by

filtration and after being rinsed with TBME was recrystallized

from absolute ethanol (175 mL). The resulting solid was

collected by filtration and after being washed with cold ethanol

and diethyl ether was taken up in CH₂Cl₂, filtered, and

evaporated to yield the mixture (2.76 g) of 6 and 3 (7%).

[4-Ca r bom eth oxy]ben zyl 2,6-Di-O-ben zoyl-3,4-O-iso-

p r op ylid en e-â-^D-ga la ctop yr a n osid e (9a ), [4-Ca r bom eth -

oxy]benzyl 2,6-Di-O-benzoyl-3,4-O-isopropylidene-r-^D-galacto-

p yr a n osid e (9b), [4-Ca r bom eth oxy]ben zyl-O-ben zoa te

(8), a n d 2-O-[2,6-Di-O-ben zoyl-3,4-O-isop r op ylid en e-â-^D-

galactopyranosyl]1,6-di-O-benzoyl-3,4-O-isopropylidene-r-^D-

ga la ctop yr a n ose (9d ). Donor 2 (278 mg, 0.59 mmol), [4-car-

bomethoxy]benzyl alcohol (165 mg, 0.39 mmol), and 3 Å

molecular sieves (500 mg) were dried together overnight at

high vacuum. Then CH₂Cl₂(10 mL) was added and the

mixture cooled in an ice-salt bath to 0 °C. After the mixture

was stirred for 0.5 h, NIS (219 mg, 0.98 mmol) was added

followed by trifluoromethanesulfonic acid (TfOH, 320 µL of a

dilute solution in CH₂Cl_2,, 0.20 mmol). After being stirred for

30 min, the reaction was quenched with DIPEA (100 µL) and

evaporated to dryness. The residue was purified by flash

chromatography on silica gel eluting with 80:15:5 hexanes-

ethyl acetate-CH₂Cl₂to yield in increasing polarity a viscous

oil (8) (19 mg, 19%), a waxy solid (9b) (4 mg, 2%) ([R]_D215.5

(c, 0.02, CHCl₃); MS FAB +ve 575 (M - H⁺), 545 (M - OCH₃⁺),

411 (M - C₉H₉O₃⁺); HRMS C₃₂H₃₂O₁₀Na 599.1859, calcd

599.1893), and two fractions which were mixtures. The first

was rechromatographed eluting with 88:12 toluene-ethyl

acetate to yield an amorphous solid (9a ) (71 mg, 31%) (mp

123-124 °C; [R]_D-10.7 (c, 2.2, CHCl₃); MS FAB +ve C₃₂H₃₂O₁₀

575 (M - H⁺), 545 (M - OCH₃⁺), 411 (M - C₉H₉O₃⁺). Anal.

Calcd for C₃₂H₃₂O₁₀: C, 66.66; H, 5.59. Found: C, 66.56; H,

5.56) and a viscous oil (9c) (78 mg, 47%) (MS FAB +ve 451

(MNa⁺), 411 (M - OH⁺); HRMS C₂₃H₂₄O₈Na 451.1361, calcd

451.1369). The second fraction was further purified by pre-

parative TLC eluting two times with 9:1 toluene-ethyl acetate

to yield a viscous oil (9d ) (8 mg, 3%): [R]_D10.0 (c, 0.03, CHCl₃);

MS FAB +ve C₄₆H₄₆O₁₅861 (M + Na⁺), 717 (M - OBz⁺), 411

(C₂₃H₂₃O₇⁺); HRMS C₄₆H₄₆O₁₅Na 861.2763, calcd 861.2734.

O-Tr ityl-(MP EG)(DOX) (10). (MPEG)(DOX)OH (1) (2.4 g,

0.47 mmol) was dissolved in CH₂Cl₂(15 mL), chlorotriphenyl-

methane (653 mg, 2.4 mmol) followed by 4-dimethylaminopy-

ridine (705 mg, 0.70 mmol) was added, and the mixture was

left to stir overnight. The polymer was precipitated by adding

TBME (225 mL) and collected by filtration. The solid was

recrystallized twice from absolute ethanol (125 mL) to yield

10 (2.3 g, 92 %): ¹H NMR 7.51 br d (6H) (J ) 8 Hz); 7.28 m

(13H); 4.56 s (2H); 4.10 s (2H); 3.38 s (3).

la te (16). Donor 12 (940 mg, 1.0 mmol) and 1 (2.0 g, 0.39

mmol) were dried together overnight at high vacuum. Dichlo-

romethane (9.5 ml) was added and the mixture stirred at rt

under an argon atmosphere. 2-Methyl-2-butene (2.0 mL of a

1:10 solution in CH₂Cl₂, 2.0 mmol) and NIS (439 mg, 2.0 mmol)

were added followed by AgOTf (110 mg, 0.42 mmol). After

being stirred for 3¹/₂h the reaction was cooled in an ice bath

and quenched with ammonium bicarbonate and the solid

precipitated with TBME (125 mL). The solid was recovered

by filtration and after being rinsed with TBME was recrystal-

lized from absolute ethanol (125 mL) containing 0.5% w/v

imidazole. The resulting solid was collected by filtration and

after being washed with cold ethanol and diethyl ether was

taken up in CH₂Cl₂, filtered, and evaporated to yield the

mixture of 15 and 16 (4.2:1, 2.07 g).

(MP EG)(DOX)yl 2,6-Di-O-p iva loyl-â-^D-ga la ctop yr a n o-

sid e (17). The mixture of 15 and 16 (1.78 g, 0.33 mmol) was

dissolved in 50% AcOH_aq(40 mL) and heated at 60 °C for 16

h. The liquids were removed by evaporation followed by

coevaporation with toluene. The residue was recrystallized

from ethanol (100 mL) and collected by filtration. After being

washed with cold ethanol and diethyl ether it was dissolved

in CH₂Cl₂, filtered, and evaporated to yield 15 and 16 (1.75 g,

96%).

[4-O-Acet oxym et h yl]b en zyl 3,4-Di-O-a cet yl-2,6-d i-O-

p iva loyl-â-^D-ga la ctop yr a n osid e (18) a n d 4-O-Acetoxy-

m eth ylp iva loyloxym eth ylben zen e (19). The mixture of 15

and 16 (208 mg; 0.04 mmol) was dissolved in CH₂Cl₂(1.0 mL)

and Ac₂O (1.0 mL) under an atmosphere of argon at rt. To

this solution was added Sc(OTf)₃(10 mg, 0.02 mmol) and the

stirring continued for 16 h. After the mixture was cooled with

an ice bath the polymer was precipitated with TBME (40 mL)

and collected by filtration. The filtrate was evaporated. The

solid was recrystallized from ethanol (20 mL) and collected

by filtration. The filtrate was combined with the residue from

TBME and evaporated. The residue was purified by MPLC

on silica eluting with 70:30 hexanes-ethyl acetate to yield

viscous oil (19) (1 mg): C₁₅H₂₀O₄MS FAB +ve 265 (MH⁺), 205

(M - OAc⁺), 163 (M - OPiv⁺); HRMS C₁₅H₂₀O₄Na 287.1250,

calcd 287.1259; and viscous oil 18 (12 mg, 76%) [R]_D-75.0 (c,

0.06, CHCl₃); MS FAB +ve 595 (MH⁺), 415 (M - DOX⁺);

HRMS C₃₀H₄₂O₁₂Na 617.2579, calcd 617.2574.

(MP EG)(DOX)yl

3-O-[3,4,6-tr i-O-a cetyl-2-d eoxy-2-

p h t h a lim id o-â-^D-glu cop yr a n osyl]-2,6-d i-O-p iva loyl-â-D-

ga la ctop yr a n osid e (22) a n d (MP EG)(DOX)yl 4-O-[3,4,6-

Tr i-O-a cetyl-2-d eoxy-2-p h th a lim id o-â-^D-glu cop yr a n osyl]-

2,6-d i-O-p iva loyl-â-^D-ga la ctop yr a n osid e (23). Diol 17 (1.75

g, 0.3 mmol) and 3,4,6-tri-O-acetyl-2-deoxy-2-phthalimido-â-

D-glucopyranosyl-O-trichloroacetimidate (21) (220 mg, 0.38

mmol) were dried together overnight at high vacuum. Under

an atmosphere of argon with cooling in an ice bath CH₂Cl₂

(12 mL) was added followed by BF₃‚OEt₂(39 µL, 0.32 mmol),

and the mixture left to stir for 4 h. The reaction was quenched

with DIPEA (50 µL) and the polymer precipitated with TBME

(85 mL). After filtration and rinsing with TBME the polymer

was recrystallized from ethanol (125 mL). It was collected by

filtration and after being rinsed with ethanol and diethyl ether

was taken up in CH₂Cl₂, filtered, and evaporated to yield 22,

23, and residual 17 (3.0:1.4:1.0, 1.78 g).

Eth yl 2,6-Di-O-[4-ch lor oben zoyl]-3,4-O-isop r op ylid en e-

â-^D-ga la ctoth iop yr a n osid e (11). Donor 11 was prepared as

for 2 except that 4-chlorobenzoyl chloride was used as acylating

reagent. The product was purified by chromatography on silica

eluting with hexanes-ethyl acetate (4:1) to yield an amorphous

solid (11; 47%): [R]_D48.0 (c, 0.72, CHCl₃); MS FAB +ve 541

(M³⁵Cl₂+ H⁺), 479 (M³⁵Cl₂- SEt⁺); HRMS C₂₅H₂₆O₇³⁵Cl₂Na

[4-O-Acetoxym eth yl]ben zyl 3-O-[3,4,6-Tr i-O-a cetyl-2-

d e oxy-2-p h t h a lim id o-â-^D-glu cop yr a n osyl]-4,O-a ce t yl-

d i-2,6-O-p iva loyl-â-^D-ga la ctop yr a n osid e (24) a n d [4-O-

Acetoxym eth yl]ben zyl 4-O-[3,4,6-Tr i-O-a cetyl-2-d eoxy-2-

p h t h a lim id o-â-^D-glu cop yr a n osyl]-3-O-a ce t yl-2,6-d i-O-

p iva loyl-â-^D-ga la ctop yr a n osid e (25). The mixture of 22, 23,

and 17 (781 mg) was treated with Sc(OTf)₃as described above

for 15, which after MPLC purification eluting with toluene-

ethyl acetate (80:20 to 60:40) yielded 19 (2 mg) and viscous

oil 24 (34 mg, 26%) ([R]_D3.7 (c, 0.46, CHCl₃); MS FAB +ve

992 (M + Na⁺), 790 (M - DOX⁺), 418 (C₂₀H₂₀NO₉⁺);⁴⁹HRMS

563.0674, calcd 563.0674. Anal. Calcd for

C₂₅H₂₆O₇Cl₂S

(541.4462): C, 55.46, H, 4.84. Found: C, 55.88, H, 4.85.

E t h yl 2,6-Di-O-p iva loyl-3,4-O-isop r op ylid en e-â-^D-ga -

la ctoth iop yr a n osid e (12). Donor 12 was prepared as for 2

except that pivaloyl chloride was used as acylating reagent.

The product was purified by chromatography on silica eluting

with hexanes-ethyl acetate (9:1) to yield a white solid (12;

41%): mp 85-86 °C; [R]_D29.2 (c, 0.53, CHCl₃); MS FAB +ve

C

₂₁H₃₆O₇S 439 (M + Li⁺), 371 (M - SEt⁺). Anal. Calcd for

₂₁H₃₆O₇S (432.5752): C, 59.11; H, 8.50. Found: C, 58.90; H,

C

₄₈H₅₉NO₂₀Na 992.3546, calcd 992.3528) and viscous oil 25 (29

8.57.

(MP EG)(DOX)yl 2,6-Di-O-pivaloyl-3,4-O-isopr opyliden e-

â-D-ga la ctop yr a n osid e (15) a n d (MP EG)(DOX)-O-p iva loy-

(49) Whitfield, D. M.; Pang, H.; Carver, J . P.; Krepinsky, J . J . Can.

J . Chem. 1990, 68, 942.

9044 J . Org. Chem., Vol. 64, No. 25, 1999

Whitfield et al.

mg; 23%) ([R]_D-13.4 (c, 0.19, CHCl₃); MS FAB +ve 992 (M +

the polymer was precipitated with TBME (200 mL) and

recovered by filtration. The residue was recrystallized twice

from ethanol to yield 43 (4.73 g, 92%). A small portion was

acetylated by being dissolved in acetic anhydride (0.7 mL) and

stirred with KOAc (5 mg) for 16 h. The acetylated polymer

was recovered by precipitation with TBME followed by recov-

ery by filtration and recrystallization from EtOH. ¹H NMR

analysis of the product indicated >95% derivatization.

1-Meth yl(DOX)(P EG)yl-2,6-d i-O-ben zoyl-3,4-O-isop r o-

p ylid en e-â-^D-ga la ctop yr a n osid e (44a b). Thioglycoside 2

(224 mg, 0.47 mmol), 43 (975 mg, 0.19 mmol) and activated 3

Å molecular sieves (500 mg) were placed in a 100 mL round-

bottomed flask and dried in vacuo in a desiccator containing

P₂O₅. The desiccator was opened to argon and the flask

maintained under an argon atmosphere. To the flask was

added CH₂Cl₂(5 mL), and it was cooled in an ethylene glycol-

water cooling bath with the temperature maintained between

-20 and -15 °C by a cold finger. To this stirred mixture was

added NIS (107 mg, 0.48 mmol) followed by TfOH (17 µL, 0.19

mmol). After 1.5 h another aliquot of TfOH (17 µL, 0.19 mmol)

was added and the stirring continued for another 1 h. The

reaction was quenched with DIPEA (100 µL) and the polymer

precipitated with TBME (100 mL). After recovery of the solids

by filtration through a coarse glass frit the residue was

dissolved in warm EtOH and filtered through the same glass

frit. The filtrate was placed in a -20 °C freezer and after

precipitation was recovered by filtration with a medium glass

frit and rinsed with EtOH and Et₂O. The residue was dissolved

in CH₂Cl₂and filtered through the same glass frit. The filtrate

was evaporated to yield 44a b (859 mg, 81%).

Na⁺), 790 (M - DOX⁺), 418 (C₂₀H₂₀NO₉⁺); HRMS C₄₈H₅₉NO₂₀

-

Na 992.3597, calcd 992.3528).

(MP EG)(DOX)yl

3-O-[3,4,6-Tr i-O-a cetyl-2-d eoxy-2-

p h t h a lim id o-â-^D-glu cop yr a n osyl]-4-O-[2,3,4,6-t et r a -O-

a cetyl-â-^D-glu cop yr a n osyl]-2,6-d i-O-p iva loyl-â-D-ga la cto-

p yr a n osid e (27) a n d [4-O-Acetoxym eth yl]ben zyl 3-O-

[3,4,6-Tr i-O-a ce t yl-2-d e oxy-2-p h t h a lim id o-â-^D-glu co-

p yr a n osyl]-4-O-[2,3,4,6-t et r a -O-a cet yl-â-^D-glu cop yr a n o-

syl]-2,6-d i-O-p iva loyl-â-^D-ga la ct op yr a n osid e (28). The

mixture of 22 and 23 (974 mg, 0.16 mmol) was dried at high

vacuum overnight with 2,3,4,6-tetra-O-acetyl-â-^D-glucopyra-

nosyl-O-trichloroacetimidate 26 (160 mg, 0.33 mmol). After the

mixture was cooled in an ice bath under an atmosphere of

argon, CH₂Cl₂(5 mL) was followed by TESOTf (25 µL; 0.11

mmol) and the mixture left to warm to rt and stir for 1¹/₄h.

The reaction was quenched with DIPEA (50 µL) and the

polymer precipitated with TBME (80 mL). After filtration and

rinsing with TBME the polymer was recrystallized from

ethanol (100 mL). It was collected by filtration and after being

rinsed with ethanol and diethyl ether was taken up in CH₂-

Cl₂, filtered and evaporated to yield a mixture containing 27

(0.99 g). This was dissolved in CH₂Cl₂(4 mL) and Ac₂O (4 mL).

To this solution was added Sc(OTf)₃(40 mg; 0.08 mmol). After

the mixture was stirred for 16 h at rt, the polymer was

precipitated with TBME (90 mL) and collected by filtration.

The polymer was recrystallized from ethanol (75 mL), and the

combined filtrates were evaporated to dryness. The residue

was purified by MPLC on silica eluting with 60:40 ethyl

acetate-hexanes to yield a viscous oil (28) (23 mg; 11%): [R]_D

-24.6 (c, 0.07, CHCl₃); MS FAB +ve C₆₀H₇₅NO₂₈1078 (M -

DOX⁺); MS MALDI +ve 1280.3 (M + Na⁺).

1-Met h yl(DOX)(P E G)yl-2,6-d i-O-b en zoyl-â-^D-ga la ct o-

p yr a n osid e (48a b). Polymer-bound diol 48a b was prepared

as for 17 to yield 48a b (813 mg, 97%).

r,r′-Dim et h yl(DOX)(P E G)yl-O-b en zoa t e (35). The

monomethyl ether of poly(ethylene oxide) 5000, (MPEG)OH

(2.0 g, 0.4 mmol) was dissolved in CH₂Cl₂(10 mL), and 34 (0.32

g, 0.8 mmol) dissolved in CH₂Cl₂(2 mL) was added along with

molecular sieves 3 Å (1 g) under an atmosphere of argon. After

the mixture was in an ice bath, BF₃‚OEt₂(55 µL, 0.14 mmol)

was added dropwise to the mixture. The reaction mixture was

allowed to slowly warm to rt over 4 h and then recooled in an

ice bath. The reaction was quenched with diisopropylethy-

lamine (DIPEA) (6 drops) and the polymer precipitated with

tert-butyl methyl ether (TBME) (200 mL). The solid was

recovered by filtration and after being rinsed with TBME was

recrystallized from EtOH (100 mL). The product was recovered

by filtration and after being rinsed with EtOH and Et₂O was

taken up in CH₂Cl₂, filtered, and evaporated to yield 35 (2.1

g, 99%). A small portion was acetylated by being dissolving in

acetic anhydride (0.7 mL) and stirred with KOAc (5 mg) for

16 h. The acetylated polymer was recovered by precipitation

with TBME followed by recovery by filtration and recrystal-

lization from EtOH. ¹H NMR analysis of the product suggested

>95% derivitization.

r,r′-Dim eth yl(DOX)(P EG)ol (36). Ester 35 (1.96 g, 0.37

mmol) was dissolved in 0.5 M NaOH_aq(20 mL) and the mixture

left to stir for 16 h. After neutralization with 1 M HCl_aqthe

water was evaporated and residual water removed by coevapo-

ration with toluene several times. The residue was taken up

in CH₂Cl₂, filtered, and evaporated and the residue recrystal-

lized from EtOH (150 mL). The product was recovered by

filtration and after being rinsed with EtOH and Et₂O it was

taken up in CH₂Cl₂, filtered and evaporated. This last process

was repeated again to yield 36 (1.65 g, 87%).

[4-O-Acet oxym et h yl-(1-(R,S)-et h yloxy)]p h en yl-3,4-d i-

O-a cetyl-2,6-d i-O-ben zoyl-â-^D-ga la ctop yr a n osid e (49a b).

The mixture containing 44a b was treated as for 15-18. The

residue was purified by MPLC eluting with 2:1 hexanes-ethyl

acetate to yield in increasing order of polarity 4-acetoxymethyl-

(1-(R,S)-benzoyloxyethyl)benzene (50) (2 mg, 20%), 4-acetoxym-

ethyl-(1-(R,S)-acetoxyethyl)benzene (51) (<1 mg) and (49a b)

(8.8 mg, 31%): [R]_D-15.9 (c 0.24, CHCl₃); MS FAB +ve 649

(M + H⁺), 455 (M - MDOX⁺); HRMS C₃₅H₃₆O₁₂Na (M + Na⁺)

671.2103, calcd 671.2104. A small portion was repurified by

preparative TLC eluting with 3:1 hexanes-ethyl acetate and

isolating the least polar band yielded predominantly 49a (<1

mg): [R]_D-59.4 (c 0.02, CHCl₃).

1-Meth yl(DOX)(P EG)yl-3-O-(3,4,6-tr i-O-a cetyl-2-d eoxy-

2-p h th a lim id o-â-^D-glu cop yr a n osyl)-2,6-d i-O-ben zoyl-â-D-

ga la ctop yr a n osid e (52a b) a n d 1,4 r egioisom er (52a b).

Disaccharide 52a b was prepared as for 22 + 23 to yield a

mixture of 52a b and 53a b (1.32 g, 92%).

[4-O-Acetoxym eth yl-(1-(R,S)-eth yloxy)]ph en yl-3-O-(3,4,6-

tr i-O-a cetyl-2-d eoxy-2-p h th a lim id o-â-^D-glu cop yr a n osyl)-

4-O-acetyl-2,6-di-O-ben zoyl-â-^D-galactopyr an oside (54ab).

Polymer-bound mixtures 52a b and 53a b (350 mg, 0.06 mmol)

were treated with Sc(OTf)₃as for 44a b above. The residue (55

mg) was purified by chromatography eluting with ethyl

acetate-hexanes (50:50) to yield four fractions in increasing

order of polarity (50 (5 mg), 51 (2 mg), a mixture of monosac-

charides and disaccharides (9 mg), and 54a b (5 mg, 8%)): [R]_D

38.8 (c 0.32, CHCl₃); MS ES +ve C₅₃H₅₃NO₂₀1046 (M + Na⁺),

1024 (M + H⁺), 830 (M - MDOX⁺).

1-Meth yl(DOX)(P EG)yl-2,6-d i-O-p iva loyl-3,4-O-isop r o-

p ylid en e-â-^D-ga la ctop yr a n osid e (56a b). Glycoside 56a b

was prepared as for 44a b except thioglycoside 12 was used as

donor to yield 56a b (83 %).

1-Met h yl(DOX)(P E G)yl-2,6-d i-O-p iva loyl-â-^D-ga la ct o-

p yr a n osid e (57a b). Diol 57a b was prepared as for 48a b to

yield 57a b (95 %).

1-Meth yl(DOX)(P EG)yl-3-O-(3,4,6-tr i-O-a cetyl-2-d eoxy-

2-p h th a lim id o-â-^D-glu cop yr a n osyl)-2,6-d i-O-p iva loyl-â-D-

ga la ctop yr a n osid e (58a b) a n d 1,4 r egioisom er (59a b).

Diol 57a b was glycosylated as for 33a b except the temperature

was maintained at -15 to -20 °C to yield a 2.2:1.0 mixture of

58a b and 59a b and only traces of unreacted 57a b.

r-Meth yl(DOX)(P EG)-O-ter t-bu tyld ip h en ylsilyl Eth er

(42). Diether 42 was prepared as for 35 from (MPEG)OH (7.5

g, 1.5 mmol) and trichloroacetimidate 41 (2.42 g, 4.5 mmol)

except that after the first treatment ¹H NMR analysis indi-

cated that the reaction was only 70-75% complete. Therefore,

the polymer was retreated with 41 (1.61 g, 3.0 mmol) to yield

42 (7.0 g, 91%).

r-Meth yl(DOX)(P EG)ol (43). Silyl ether 42 (5.1 g, 1.0

mmol) was dissolved in dry THF (100 mL) under an argon

atmosphere to which was added 4 Å molecular sieves (1 g) and

1 M TBAF in THF (2 mL, 2 mmol). The mixture was heated

at 40 °C for 3 h. After the mixture was cooled in an ice bath,

Acyl Transfer in Oligosaccharide Synthesis

J . Org. Chem., Vol. 64, No. 25, 1999 9045

[4-O-Acetoxym eth yl-(1-(R,S-eth yloxy)]ph en yl-3-O-(3,4,6-

tr i-O-a cetyl-2-d eoxy-2-p h th a lim id o-â-^D-glu cop yr a n osyl)-

4-O-acetyl-2,6-di-O-pivaloyl-â-^D-galactopyr an oside (60ab)

a n d [4-O-Acetoxym eth yl-(1-(R,S)-eth yloxy)]-p h en yl-4-O-

(3,4,6-tr i-O-a cetyl-2-d eoxy-2-p h th a lim id o-â-^D-glu cop yr a -

n osyl)-3-O-a cet yl-2,6-d i-O-p iva loyl-â-^D-ga la ct op yr a n o-

sid e (61a b). Polymer-bound mixture 58a bs and 59a b (285 mg,

0.048 mmol) were treated with Sc(OTf)₃as for 52a b above.

The residue was purified by chromatography eluting with ethyl

acetate-hexanes (50:50) to yield two fractions in increasing

order of polarity (60a b (3.2 mg, 7%) ([R]_D-103.7 (c 0.03,

CHCl₃); MS-FAB +ve 1022 (M + K⁺), 1006 (M + Na⁺), 790

(M - MDOX⁺), 418 (C₂₀H₂₀NO₉⁺);⁴⁹HRMS C₄₉H₆₁NO₂₀Na (M

3-O-(3,4,6-Tr i-O-a cet yl-2-d eoxy-2-p h t h a lim id o-^D-glu -

cop yr a n osyl)-4-O-[2,3,4,6-t et r a -O-a cet yl-â-^D-glu cop yr a -

n osyl]-2,6-di-O-pivaloyl-â-^D-galactopyr an oside 66r/â.Trisac-

charide 64a b (13 mg, 0.01 mmol) was dissolved in acetic acid

(2 mL) and ethyl acetate (2 mL) and placed in a screw cap

test tube. To this tube was added palladium black (15 mg),

and the tube was placed in a cotton-filled Parr bottle and

hydrogenated on a Parr apparatus at 45 psi of H₂for 16 h.

The solids were removed by filtration through celite and rinsed

well with methanol. The filtrates were evaporated, and the

residue was purified on a preparative TLC plate eluting two

times with 60:40 toluene-ethyl acetate to yield 66R/â (5 mg,

46%): [R]_D-4.6 (c, 0.17, CHCl₃); MS FAB +ve C₅₀H₆₅NO₂₆

1134 (M + K⁺), 1118 (M + Na⁺), 1078 (M - OH⁺).

+ Na⁺) 1006.3662, calcd 1006.3685. Anal. Calcd for C₄₉H₆₁

-

NO₂₀(983): C, 59.44, H, 6.13, N, 1.44. Found: C, 60.07, H,

6.51; N, 1.38) and a mixture of 59a b and 60a b (8.9 mg, 19%)).

The latter was further purified by preparative TLC eluting

with 70:30 toluene-ethyl acetate to yield 60a b (1 mg) as a

75:25 mixture of diastereomers: [R]_D2.9 (c 0.12, CHCl₃); MS-

Ack n ow led gm en t. We thank E. Eichler for mea-

suring optical rotations, K. Chan for measuring FAB

MS spectra, D. Krajcarski for measuring the MALDI

MS spectra, and J . R. Brisson for advice with the NMR

spectra. This is NRC paper 40402.

FAB +ve 1006 (M + Na⁺), 790 (M - MDOX⁺); HRMS C₄₉H₆₁

-

NO₂₀Na (M + Na⁺) 1006.3618, calcd 1006.3685.

[4-O-Acetoxym eth yl-(1-(R,S)-eth yloxy)]ph en yl-3-O-(3,4,6-

tr i-O-a cetyl-2-d eoxy-2-p h th a lim id o-â-^D-glu cop yr a n osyl)-

4-O-[2,3,4,6-tetr a -O-a cetyl-â-^D-glu cop yr a n osyl]-2,6-d i-O-

p iva loyl-â-^D-ga la ctop yr a n osid e (64a b). Trisaccharide 64a b

was prepared as for 28 above. The residue was purified by

MPLC on silica eluting with a gradient of 70:30 to 50:50

toluene-ethyl acetate to yield a viscous oil (64a b) (32 mg; 15%)

[R]_D-20.4 (c, 0.48, CHCl₃); MS FAB +ve 1078 (M - MDOX⁺);

HRMS C₆₁H₇₇NO₂₈Na (M + Na⁺, 1294.4624), calcd 1294.4520.

Anal. Calcd for C₆₁H₇₇NO₂₈(1271): C, 57.28, H, 6.01, N, 1.11.

Found: C, 57.53; H, 6.38; N, 0.83) and a more polar fraction

(probably 65a b) (10 mg) which also gave a MS FAB +ve peak

at 1294 but was not possible to purify to homogeneity.

Su p p or tin g In for m a tion Ava ila ble: Synthetic proce-

dures for linker precursors 30-34 and 38-41 as well as their

¹H and ¹³C NMR spectral data, ¹H and ¹³C NMR spectral data

are provided for compounds 2, 6-9, 11, 12, 15, 17-19, 22-

24a b, 44a b-47a b, 50-53a b, 56a b-59a b, and 61a b, and

copies of the spectra of 8, 9b-d , 15, 16, 18, 19, 24, 25, 28, 32,

33, 34, 38, 39, 42, 43, 45, 49a b, 50, 51, 54a b, 61a b, and 66.

This material is available free of charge via the Internet at

http://pubs.acs.org.

J O990712B

Article Doi

Acyl Transfer as a Problematic Side Reaction in Polymer-Supported Oligosaccharide Synthesis

DOI: 10.1021/jo990712b

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Article abstract of DOI:10.1021/jo990712b

Full text of DOI:10.1021/jo990712b

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