Glycosyl fluorides in glycosidations

www.elsevier.nl/locate/carres

Carbohydrate Research 327 (2000) 15–26

Review

Glycosyl fluorides in glycosidationsKazunobu Toshima *

Department of Applied Chemistry, Faculty of Science and Technology, Keio Uni6ersity, 3-14-1 Hiyoshi,Kohoku-ku, Yokohama 223-8522, Japan

Received 20 March 1999

Abstract

This short review deals with the recent progress in chemical O-glycosidation and C-glycosylation methods using gly-cosyl fluorides as glycosyl donors. Pyranosyl and furanosyl fluorides were effectively activated by fluorophilic reagentssuch as SnCl2�AgClO4, SnCl2�TrClO4, SnCl2�AgOTf, TMSOTf, SiF4, BF3·Et2O, TiF4, SnF4, Cp2MCl2�AgClO4 (M=Zror Hf), Cp2ZrCl2�AgBF4, Cp2HfCl2�AgOTf, Bu2Sn(ClO4)2, Me2GaCl, Tf2O, LiClO4, Yb(OTf)3, La(ClO4)3·nH2O,La(ClO4)3·nH2O�Sn(OTf)2, Yb�Amberlyst 15, SO4/ZrO2, Nafion-H®, montmorillonite K-10, and TrB(C6F5)4 to reactwith alcohols to give the corresponding O-glycosides in high yields. Furthermore, several types of C-glycosyl compounds,such as aryl, allyl and alkyl C-glycosyl derivatives, were also obtained by the glycosylation using glycosyl fluorides andthe corresponding nucleophile with or without a Lewis acid. © 2000 Elsevier Science Ltd. All rights reserved.

Keywords: O-Glycosidation; C-Glycosidation; Glycosyl fluoride; O-Glycoside; C-Glycoside

Contents

1. Introduction. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 152. Glycosyl fluorides in O-glycosidations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 163. Glycosyl fluorides in C-glycosylations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26

1. Introduction

Glycosidation is one of the most importantand basic synthetic methods to prepare severaltypes of glycosides and oligosaccharides.Therefore, highly effective chemical glycosida-tion reactions have attracted considerable at-tention in carbohydrate chemistry related tocertain biomolecules and functional materials.From a synthetic standpoint, the efficiency ofthe glycosidation method generally involves ahigh chemical yield, regioselectivity, and

stereoselectivity. This short review concen-trates on recent progress (1981–1998) inchemical O-glycosidation and C-glycosylationmethods using glycosyl fluorides as glycosyldonors, including historically indispensableprotocols [1–3]. This article also particularlyemphasizes the development of new activatingmethods for the glycosyl fluorides.

Glycosyl fluorides have now been widelyand effectively used for O- and C-glycosida-tion reactions. One of the notable advantagesof the glycosyl fluoride as a glycosyl donor isits high thermal and chemical stability as com-pared with the low stability of other glycosylhalides, such as glycosyl chlorides, bromides

* Fax: +81-45-5630446.E-mail address: [email protected] (K. Toshima).

0008-6215/00/$ - see front matter © 2000 Elsevier Science Ltd. All rights reserved.

PII: S 0 0 0 8 -6215 (99 )00325 -0

K. Toshima / Carbohydrate Research 327 (2000) 15–2616

Scheme 1.

and iodides. Therefore, the glycosyl fluoridecan be generally purified by the appropriatedistillation and even by column chromatogra-phy with silica gel. Having such favorable syn-thetic attributes, the use of a glycosyl fluorideas a practical glycosyl donor in the glycosida-tion reaction was first developed byMukaiyama and co-workers in 1981 [4]. Aftertheir significant advance, a number of effectivechemical methods for O-glycosidations and C-glycosylations using a glycosyl fluoride havebeen developed.

2. Glycosyl fluorides in O-glycosidations

Since the use of glycosyl fluoride as a glyco-syl donor with a fluorophilic activator, SnCl2�AgClO4, was introduced by Mukaiyama in1981 (Scheme 1), a number of specific fluoro-philic reagents have been developed for effec-tive O-glycosidation reactions. These activatorsfor O-glycosidations using glycosyl fluorides asglycosyl donors are summarized in Table 1. Af-ter SnCl2 was first employed as an activator ofthe glycosyl fluorides in combination withAgClO4, the other combined use of SnCl2�TrClO4 (Scheme 2) [5] and SnCl2�AgOTf(Scheme 3) [6] were reported by Mukaiyamaand Ogawa, respectively. In these cases, 1,2-cis-a-glycosides were predominantly obtainedin high yields due to the anomeric effect.

In 1984, Noyori and co-workers announcedthat the silyl compounds, both SiF4 andtrimethylsilyl trifluoromethanesulfonate (TM-SOTf), were very effective for catalytic glycosi-dation reactions of glucopyranosyl fluoridesand trimethylsilylated alcohols (Scheme 4) [7].In this study, it was found that the stereoselec-tivity of the glycosidation was highly depen-dent on the reaction solvent. Thus, glyco-sidation in MeCN exclusively gave the b-gly-coside, while glycosidation performed in Et2O

predominately afforded the a-glycoside in highyield. Furthermore, this general trend was notaffected by the stereochemistry of the startingglycosyl fluoride.

Nicolaou, Kunz and Vozny independentlyreported that glycosyl fluorides effectively cou-pled with a variety of free alcohols and silylethers using another Lewis acid, BF3·Et2O, inCH2Cl2 to give the corresponding O-glycosidesin moderate to good yields (Scheme 5) [8–11].

On the other hand, metal fluorides such asTiF4 and SnF4 were also used as effective pro-moters of glycosyl fluorides by Thiem and co-workers. The stereoselective glycosidation

Table 1O-Glycosidations of glycosyl fluorides

XActivator Refs.

H [4]SnCl2�AgClO4

HSnCl2�TrClO4 [5]H [6]SnCl2�AgOTfTMSTMSOTf (cat.) [7]

[7]TMSSiF4 (cat.)HBF3·Et2O [8–11]HTiF4 [12]H [12]SnF4

HCp2MCl2�AgClO4 (M=Zr or Hf) [13]HCp2ZrCl2�AgBF4 [14]

[14,15]HCp2HfCl2�AgOTfH [16]Bu2Sn(ClO4)2

HMe2GaCl [17]Tf2O [18]H

H [19]LiClO4

Yb(OTf)3 H [20]La(ClO4)3·nH2O (cat.) TMS [21]

HLa(ClO4)3·nH2O�Sn(OTf)2 [22]Yb�Amberlyst 15 H [24]

H [25]SO4/ZrO2

[25]HNafion-HMontmorillonite K-10 H [25]TrB(C6F5)4 (cat.) H [26]

K. Toshima / Carbohydrate Research 327 (2000) 15–26 17

Scheme 2.

Scheme 3.

Scheme 4.

of 2-deoxyglycosyl fluoride was carried outusing TiF4. In the case of 2-deoxy-a-glycosylfluoride, when hexane was used as the solvent,the b-glycoside was selectively produced withinversion of the anomeric center via an SN2mechanism. On the other hand, when thereaction was performed in Et2O, the a-gly-

coside was obtained as the major product viaa ‘double SN2’ mechanism, which involved theformation of the oxionium cation–ether com-plex (Scheme 6) [12].

Suzuki and co-workers developed new andquite effective glycosidation protocols inwhich the combined activators including the


Scheme 5.

Scheme 6.

Scheme 7.

Scheme 8.


Scheme 9.

Scheme 10.

Scheme 11.

group IVB metallocenes such as Cp2MCl2Ag-ClO4 (M=Zr, Hf) (Scheme 7) [13], Cp2ZrCl2

�AgBF4 (Scheme 8) [14] and Cp2HfCl2�AgOTf [14,15] were used as milder reagents forpromoting the glycosidations of glycosylfluorides. It is interesting to note that thecombinations of Cp2ZrCl2�AgClO4 in benzeneand Cp2HfCl2�2AgClO4 in CH2Cl2 were veryeffective for the highly b-stereoselective glycosi-dations of D-mycinose and D-glucose deriva-tives, respectively. On the other hand, thecombined use of Cp2ZrCl2�AgBF4 was foundto be useful for the stereoselective a-mannopy-ranoside synthesis from totally benzylated a-mannopyranosyl fluoride. Furthermore,Suzuki also reported the novel combined use ofBu2SnCl2�AgClO4 as an effective promoter forthe glycosidation of totally benzylated a-

mannopyranosyl fluoride and several alcohols(Scheme 9) [16].

On the other hand, Me2GaCl and Me2GaOTfwere introduced as new promoters of glycosylfluorides by Kobayashi due to their strongaffinity to fluoride. In this study, it was foundthat the readily available Me2GaCl was moreeffective for the glycosidation reactions of gly-cosyl fluorides, and O-glycosides were obtainedin high yields with moderate stereoselectivities(Scheme 10) [17].

Wessel and co-workers announced that Tf2Owas shown to be a highly reactive activator forthe glycosidation of glycosyl fluorides (Scheme11) [18]. In this report, it was interestinglysuggested that the sequence of relative reactiv-ity for catalysts examined for the glycosylfluorides was TMSOTfBSnCl2�AgOTfBTiF4BTf2O.


On the other hand, Waldmann and Bohmreported the use of 1 M solutions of LiClO4 inCH2Cl2, which is a milder Lewis acid, for theglycosidation of fucosyl fluoride under neutralconditions (Scheme 12) [19]. In this glycosida-tion protocol, CsF was employed as an effec-tive acid scavenger. Although LiClO4 could beused for the activation of other glycosyldonors such as a glycosyl trichloroimidate,glycosyl fluorides were found to be most effec-tively activated.

Recently, Shibasaki and co-workers devel-oped the rare earth metal salts, such asLa(ClO4)3·nH2O and Yb(OTf)3, for the gly-cosidations with glycosyl fluorides [20–23].The use of either Yb(OTf)3 or YbCl3 in the pre-sence of CaCO3 and 4 A, molecular sieves in

Et2O was found to be effective for a-selectiveglycosidation of the glucosyl fluoride. On theother hand, for b-selective glycosidation, theutilization of Yb(OTf)3 in MeCN containingK2CO3 and 4 A, molecular sieves gave a goodresult (Scheme 13) [20]. Furthermore, it wasfound that glycosidations of glycosyl fluorideswith trimethylsilylated alcohols were pro-moted more effectively using a catalyticamount of La(ClO4)3·nH2O [21], and the com-bined use of La(ClO4)3·nH2O and Sn(OTf)2

was very useful for b-stereoselective mannosy-lation, which was one of the most difficultstereoselective glycosidations (Scheme 14) [22].

Along this line, Wang and co-workers re-ported the glycosidation of glucosyl fluoride inmethanol promoted by the lanthanide(III) cat-

Scheme 12.

Scheme 13.


Scheme 14.

Scheme 15.

Scheme 16.

alyst supported on an ion-exchange resin,Yb�Amberlyst 15 (Scheme 15) [24]. Interesting-ly, it was found that methyl glucosides werestereospecifically produced with complete in-version of the anomeric center of the glucosylfluoride.

Very recently, Toshima and co-workersdemonstrated that environmentally friendlyheterogeneous catalysts such as montmorillniteK-10, Nafion-H® and SO4/ZrO2 were very ef-fective for practical glycosidations of glycosylfluorides (Scheme 16) [25]. Among them, SO4/ZrO2 was shown to be superior for the stereo-controlled glycosidation with a-manno-pyranosyl fluoride. Thus, the glycosidations of

perbenzylated a-mannopyranosyl fluoride andalcohols using SO4/ZrO2 in MeCN at 40 °Cgave exclusively the corresponding a-glycosidesin high yields. On the other hand, the corre-sponding b-glycosides were obtained selectivelyby the glycosidations employing SO4/ZrO2 inthe presence of 5 A, molecular sieves in Et2O at25 °C.

Furthermore, Mukaiyama and Takeuchiquite recently reported the stereoselective b-gly-cosidation of glucopyranosyl fluoride and freealcohols using a catalytic amount of TrB(C6F5)4

in tBuCN�benzotrifluoride (BTF) (Scheme 17)[26]. This is the first example of activation of theanomeric C�F bond using a trityl cation.


3. Glycosyl fluorides in C-glycosylations

An efficient chemical C-glycosylation withhigh regio- and stereoselectivity is of particu-lar interest as well as O-glycosidations. Severaltypes of C-glycosyl compounds such as alkyl,allyl and aryl C-glycosyl derivatives are nowwell recognized to be useful chiral buildingblocks for the syntheses of optically activebiomolecules and functional materials. Fur-thermore, C-linked glycosyl compounds, sta-ble analogs of naturally occurring O-glycosides and glycosylamines, have becomethe subject of considerable interest in medici-nal chemistry. After Nicolaou and co-workersfirst announced the use of glycosyl fluorides inC-glycosylations in 1984 [27], several types ofC-glycosylations using glycosyl fluorides havebeen reported.

The reaction of glycosyl fluoride andtrimethylsilyl cyanide with a Lewis acid wasdeveloped by two groups. Nicolaou and co-workers first reported that the C-glycosylationof totally benzylated glucopyranosyl fluorideand trimethylsilyl cyanide using BF3·Et2O as a

Lewis acid gave the corresponding cyanogly-coside in high yield (Scheme 18) [27]. Alongthis line, Ishido and co-workers confirmedthat the anomeric selectivity was dependent onthe amount of Lewis acid, and increasing theamount of BF3·Et2O resulted in the highlystereoselective formation of the a anomer [28].Ishido also reported the C-glycosylation of thetotally benzylated ribofuranosyl fluoride withtrimethylsilyl cyanide using a catalytic amountof BF3·Et2O (Scheme 18) [28].

The reaction of glycosyl fluoride and allylsi-lane with a Lewis acid was also announcedfrom the same groups. Thus, the C-glycosyla-tions of glycosyl fluoride and allyltrimethylsi-lane with a Lewis acid, BF3·Et2O, wasreported by Nicolaou (Scheme 19) [27] and asimilar result was demonstrated by Ishido(Scheme 19) [28,29]. In most cases, the allylC-glycosyl compounds was obtained withhigh a-stereoselectivity.

Nicolaou [27] and Ishido [30] also indepen-dently reported effective C-glycosylations ofglycosyl fluorides and several types of silylenol ethers. In these reactions, BF3·Et2O was

Scheme 17.

Scheme 18.


Scheme 19.

Scheme 20.

Scheme 21.

also found to be an effective catalyst. Ishidoand co-workers clearly indicated that the a-glycoside was obtained exclusively with highstereoselectivity and the chemical yield andthe stereoselectivity was highly independent ofthe amount of catalyst (Scheme 20) [30].

Furthermore, two different types of arylC-glycosylations with glycosyl fluorides andelectron-rich aromatic compounds usingCp2ZrCl2�AgClO4 were reported by Suzukiand Matsumoto. One is the Friedel–Craftstype reaction (Scheme 21) [31], and another is


the C-glycosylation of naphthol derivatives viathe O–C migration pathway (Scheme 22) [32].In both cases, the thermodynamically stablearyl b-glycosides were generally obtained inhigh yields and excellent stereoselectivity.

Alternatively, the reaction of ribofuranosylfluoride with some indoles using BF3·Et2O wasreported by Yokoyama and co-workers(Scheme 23) [33]. The stereoselectivity wasdependent on the reaction temperatures andsolvents; the b-glycoside was selectively ob-tained under such conditions as −15 to−40 °C in EtNO2, while the a-glycoside waspreferred at −78 °C in EtCN.

On the other hand, the reactions of glycosylfluoride and organoaluminum reagents weredeveloped. For example, cyanations of glyco-syl fluorides using aluminated cyanides suchas Me2AlCN (Scheme 24) [27] and Et2AlCN[34] were reported. In the case of the C-glyco-sylation of an a-mannopyranosyl fluoride withEt2AlCN, a mixture of isocyanoglycoside andcyanoglycoside was produced in a 7:3 ratio[34]. Posner and Haines reported that severalfuranosyl and pyranosyl fluorides weresmoothly reacted with alkyl, alkenyl, andalkynyl organoaluminum reagents under mildconditions to afford the corresponding C-gly-

Scheme 22.

Scheme 23.

Scheme 24.

Scheme 25.


Scheme 26.

Scheme 27.

Scheme 28.

cosyl compounds in high to excellent yields(Scheme 25) [35]. Furthermore, the C-glycosy-lations of glycosyl fluorides and aluminatedheterocycles were also demonstrated byMcKenzie and co-workers. It was interestingto note that when glycopyranosyl fluorideswere used as the glycosyl donors, the C-glyco-sylations proceeded with retention of configu-ration at the anomeric center. On the otherhand, reactions of the same aluminated hete-rocycles with ribofuranosyl fluorides selec-

tively afforded the b-ribofuranosyl hetero-cycles (Scheme 26) [36].

Glycosyl fluorides were also found to beexcellent substrates for the radical mediatedC-glycosylations. Thus, Nicolaou and co-workers reported that the C-glycosylation oftotally benzylated glucopyranosyl fluoride andacrylonitrile using Bu3SnH and AIBNsmoothly proceeded to afford the corresponding C-glycosyl derivatives in moderate yieldwith high a-selectivity (Scheme 27) [27].


Very recently, Yokoyama and co-workersdemonstrated C-glycosylations of glycosylfluorides with several Grignard reagents with-out any activators. Thus, the glycosylations ofaryl magnesium bromides with a totally ben-zylated furanosyl fluoride gave the corre-sponding aryl b-C-glycosyl derivatives inmoderate yields. Furthermore, the totally ben-zylated glucopyranosyl fluoride was found toreact with N-methylpyrrol-2-yl magnesiumbromide and allyl magnesium bromide to af-ford the corresponding C-glycosyl derivativesin moderate to high yields (Scheme 28) [37].

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Glycosyl fluorides in glycosidations