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782 Journal of Chemical Education • Vol. 83 No. 5 May 2006 • www.JCE.DivCHED.org

Explosive growth in the field of glycobiology has drawnparticular attention to complex carbohydrate molecules, aclass of natural compounds that plays an important biologi-cal role (1). These findings have resulted in appreciation ofthe difficulties associated with the chemical synthesis ofglycoconjugates and have consequently stimulated the devel-opment of novel methods for stereoselective oligosaccharidesynthesis (2, 3). Students learning organic and biologicalchemistry read about carbohydrates but seldom get a chanceto get the hands-on experience with these fascinating bio-molecules. Despite a number of recent publications intro-ducing simple modifications of carbohydrate molecules, therange of experiments to be performed in the undergraduatelaboratory is quite limited (4–9).

Carbohydrates, as well as other polyfunctional organicmolecules, require extensive use of temporary protectinggroups for their regioselective modification. Selective protect-ing group introduction and removal are major techniquesused in the synthesis of oligosaccharides, antibiotics, steroids,and heterocycles. Protecting groups applied in organic syn-thesis are typically classified as ethers, esters, and acetals (10).

A benzylidene acetal is a commonly used protective groupfor 1,2- and 1,3-diols. In the case of 1,2,3-triols or systemswhere 1,2- or 1,3-diols are possible, the six-membered acetalring is preferentially formed. Two relevant examples of theregioselective benzylidene acetal formation are illustrated inScheme I.

A benzylidene acetal can be introduced under eitheracidic or basic conditions with the use of a range of reac-tants. As detailed below, this protecting group can be eithercompletely removed or regioselectively opened. The versatil-ity of the benzylidene acetal protecting group for the protec-tion of diols has resulted in its broad application in syntheticcarbohydrate chemistry (11), most typically for the protec-tion of 4,6-diol of pyranosides. One relevant example is thesynthesis of methyl 4,6-O-benzylidene-α-D-glucopyranoside(2) from commercially available methyl α-D-glucopyranoside(1, Scheme II). A new chirality center (PhC*H<) is formedin the process of this reaction. Owing to the reversibility ofthe transacetal formation process, the bulky phenyl substitu-ent of 4,6-O-benzylidene acetals generally prefers the ther-modynamically more stable equatorial position. A similarprocedure has been employed for the synthesis of methyl2,3:4,6-di-O-benzylidene-α-D-mannopyranoside, in which asingle stereoisomer for both five (2,3) and six-membered rings(4,6) was formed (12).

Compound 2 is a commonly used building block in syn-thetic carbohydrate chemistry. Early syntheses of 2 employedrather harsh reaction conditions, but better methods havelargely replaced them. Among methods for the introductionof benzylidene acetal, transacetalation procedure with the useof benzylaldehyde dimethyl acetal is by far the mildest ap-proach employed to date (13). DMF is commonly used asthe reaction solvent, but, unfortunately, its use significantlycomplicates this approach because the solvent is difficult toremove prior to workup (DMF bp 153 �C). p-Toluene-sulfonic acid monohydrate is commonly used as the sourceof the protic acid catalyst for this process. Subsequent puri-fication is achieved by means of column chromatography onsilica gel. We have modified the procedure by changing thesolvent, acid catalyst, temperature, and the purification stepin an adaptation of this experiment for use in undergraduateorganic laboratories. The described experiment in suitable forimplementation in the sophomore-level organic laboratoryin its basic setup. In addition, the expanded two-step syn-thesis can be used by upper-level students in the advancedorganic laboratory or a similar course.

Synthesis

The basic experiment for sophomore students is illus-trated in Scheme II. The synthesis of 2 employs thetransacetalation process between 1 and benzylaldehyde dim-

Acetal Protecting Groups in the Organic Laboratory: WSynthesis of Methyl 4,6-O-Benzylidene-ααααα-D-GlucopyranosideAlexei V. Demchenko,* Papapida Pornsuriyasak, and Cristina De MeoDepartment of Chemistry and Biochemistry, University of Missouri–St. Louis, St. Louis, MO 63121;*demchenkoa@umsl.edu

O

OMeHO

OO

HO

Ph

O

OMeHO

HOHO

HO 1. PhCH(OMe)2

2. crystallize

21

Scheme II. The synthesis of methyl 4,6-O-benzylidene-α-D-glucopyranoside (2) from commercially available methyl α-D-glucopyranoside (1).

Scheme I. Two examples of the regioselective benzylidene acetalformation.

O

HO

RHO

OH

O

RHO

HOHO

OO

Ph

O

RHO

O

OPh

OH

+

+

*

*

major product

major product

(1)

(2)

OH OH

HOO O

HO

Ph

O OHO

Ph

* *

In the Laboratory

www.JCE.DivCHED.org • Vol. 83 No. 5 May 2006 • Journal of Chemical Education 783

ethyl acetal (α,α-dimethoxytoluene, DMT). We havechanged the protic acid catalyst to 10-camphorsulfonic acid(CSA) and the solvent to acetonitrile (MeCN). Thus, 1 isfirst suspended in MeCN, followed by the addition of DMTand CSA (5 mol %). The reaction is complete in 16 h atroom temperature or in 15–20 minutes in refluxing MeCN,without a notable difference in yield.

It should be noted that owing to the equilibrium of thereaction, a small amount of unreacted 1 (< 5 %) is alwayspresent in the reaction mixture and can be detected even ifthe reaction is refluxed for 2 h or longer. Upon completion(targeted 95% conversion of the starting material, monitoredby TLC), the reaction mixture is cooled and neutralized withtriethylamine. The reaction mixture is then diluted with ethylacetate and subjected to aqueous workup. Small amounts ofunreacted 1 are removed at this stage because of its high solu-bility in the aqueous media. The organic phase is then driedand solvents are evaporated. The residue is redissolved indichloromethane and precipitated as white crystals by slowaddition of hexanes to afford 2 after filtration; no furtherpurification is typically required. The isolated yields fromclasses of 14–18 students per section have ranged from 50 to75%. There are no major problems that have been encoun-tered by students using this procedure. The students verifythe structure of 2 by TLC and melting point determinations(166–167 �C) (14). 1H NMR technique could be also usedif covered by the scope of the course.

The students in the advanced laboratory synthesize 2using the basic experiment described above, and then acety-late its free 2,3-hydroxyls to obtain methyl 2,3-di-O-acetyl-4,6-O-benzylidene-α-D-glucopyranoside (Scheme III, 2a, P= Ac). Thus, 2 is first dissolved in pyridine, followed by theaddition of acetic anhydride. The reaction is complete in 16h at room temperature or could be left until the next classperiod without a notable difference in yield.

Upon completion (targeted quantitative conversion ofthe starting material, monitored by TLC), the reaction isquenched with MeOH and then evaporated to dryness. Theresidue is then dissolved in ethyl acetate and subjected toaqueous workup. The organic phase is then dried and sol-vent is evaporated. The residue is redissolved in dichlo-romethane and precipitated as white crystals by slow additionof hexanes to afford 2a after filtration; no further purifica-tion is typically required. The isolated yields from classes ofadvanced students have ranged from 50 to 70%. There areno major problems that have been encountered by studentsusing this procedure. The students verify the structure of 2aby TLC, melting point determinations (108 �C, and 1H-NMR spectroscopy).

Characterization

Typically a melting point determination, TLC analysis,or a single scan 1H NMR spectrum can serve as unambigu-ous proof of the synthesized compound’s identity and pu-rity. Nevertheless, it is entirely up to the instructor to exposethe students to other common analytical techniques such asmass spectrometry, FTIR spectroscopy, 13C NMR, and po-larimetry and compare the acquired data with values in theliterature (14). In addition, 2 gives well resolved COSY andHMQC spectra that can be successfully used in the class-

room setting for 2D NMR spectroscopy studies. Examplesof these are included in the Supplemental Material.W

Hazards

Dichloromethane (methylene chloride, CH2Cl2) shouldbe handled with particular care as it is anticipated to be acarcinogen. Overexposure may lead to fatigue, weakness,sleepiness, nausea, lightheadedness, and irritation of eyes andskin. Although no other particular hazards are involved withthis experiment, typical precautions for the undergraduateorganic laboratory must be applied. The reagents should behandled in a well vented fumehood, and it is recommendedthat the students wear protective goggles, shoes, clothing, andgloves at all times.

Discussion

We have developed and tested a rapid and efficient ap-proach to the synthesis of benzylidene acetal 2 for organicstudents to illustrate an important reaction in carbohydratechemistry. Typically, 0.5 g scale was used in the sophomoreorganic laboratory. It is noteworthy that consistent results (theisolated yield is in the range of 50–75%) have been achievedby differently qualified undergraduate students. In the de-velopment of the experiment, both dichloromethane and themore benign ethyl acetate were used as the organic phase sol-vent. The dichloromethane procedure is slightly more labo-rious, but it results in the higher yields (up to 90% vs.50–75% for ethyl acetate), with slightly higher hazards.

The described experiment is successful on both macro(up to 15–20 g) and micro (10–20 mg) scales (data notshown). Although we do not expect our undergraduate stu-

O

OMePO

OO

PO

Ph

b

da

2a

c

O

OMePO

POBnO

OH

D

O

OMePO

POHO

OBn

C

O

OMePO

POBzO

Br

B

O

OMePO

POHO

OH

A

Scheme III. The building block (2a) can be employed for the syn-thesis of a range of selectively protected carbohydrate derivatives,where P = Ac, Bn = benzyl, and Bz = benzoyl.

In the Laboratory

784 Journal of Chemical Education • Vol. 83 No. 5 May 2006 • www.JCE.DivCHED.org

dents to do so, it is possible to routinely obtain yields of 90–95% if dry reaction conditions (argon), anhydrous solvents,dichloromethane extraction, and chromatographic purifica-tion are used.

While the basic experiment can serve as a first step forthe common carbohydrate derivatization, the remaining 2,3-trans-diol could be subsequently protected as ethers or estersif necessary. In our advanced organic laboratory, students scaleup the basic experiment to 1.0–1.5 g, and then perform acety-lation of the remaining hydroxyls (acetic anhydride–pyridine)in order to confirm the regioselectivity of the benzylideneacetal. This is easily achieved by comparison of the chemicalshifts of H-2,3 as in compound 2a (P = Ac) the indicatedprotons are shifted downfield (∆δ 1.2–1.5 ppm) in compari-son to that of 2. Subsequently, the building block (2a, SchemeIII) can be employed for the synthesis of a range of selec-tively protected carbohydrate derivatives (12). Thus, cleav-age of the benzylidene acetal under mildly acidic (20% TFAin CH2Cl2) or neutral conditions (H2, Pd�C) affords the diolA, whereas oxidative (pathway b) or reductive (pathways cor d) cleavage affords a variety of monohydroxyl derivativesB, C, or D, respectively. Excellent regioselectivity of the re-ductive opening can be achieved by varying the reducingagents or solvents (15).

Conclusion

In conclusion, we developed an efficient laboratory pro-cedure for the regio- and stereoselective introduction of a ben-zylidene acetal. This procedure is applied to the synthesis ofmethyl 4,6-O-benzylidene-α-D-glucopyranoside (2). Purifi-cation of the target compound is achieved by simple precipi-tation from organic solvents. The described procedure wassuccessfully applied in multistep syntheses. In addition, othercommon substrates, such as O-methyl, O-trimethylsilyl, S-ethyl, or S-phenyl glycosides of the D-glucose, D-galactose,and D-mannose series have been successfully converted intothe corresponding cyclic O-benzylidene acetals in our labo-ratory (16–18).

Acknowledgments

This work was supported by the University of Missouriat St. Louis. The authors thank Jonathan Silva for the ex-perimental assistance and Harold Harris and ChristopherSpilling for valuable suggestions.

WSupplemental Material

Notes for the student, suggestions for the instructor, ex-perimental procedure, suggested application in a multistepsynthesis, and sample characterization data are available inthis issue of JCE Online.

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