Carbohydrate Research 337 (2002) 863–867

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Synthesis and intramolecular transesterifications of pivaloylatedmethyl �-D-galactopyranosides

Vesna Petrovic, Srd-anka Tomic,* Maja MatanovicDepartment of Chemistry, Faculty of Science, Uni�ersity of Zagreb, Strossmayero� trg 14, HR-10000 Zagreb, Croatia

Received 20 September 2001; accepted 16 February 2002

Abstract

Selective pivaloylations of methyl �-D-galactopyranoside have been studied under various reaction conditions. Partiallypivaloylated products were submitted to additional acetylations. The structures were established by 1H and 13C NMRspectroscopies. Both, 2,6- and 3,6-dipivalates underwent intramolecular cyclization in neutral conditions (phosphate bufferedsaline, pH 7.2) to give a stable 2,3-orthoacid with a parallel 6�4 migration of the pivaloyl group. © 2002 Elsevier Science Ltd.All rights reserved.

Keywords: Methyl �-D-galactopyranosides, acylated; Pivalates, intramolecular transesterifications

1. Introduction

We showed in previous work that pivaloylated andacetylated monosaccharides from the D-glucose,1–3 2-acetamido-2-deoxy-D-glucose,4,5 D-mannose,6 and D-xylose7 series of sugars could be regioselectively and/orchemoselectively deacylated in enzymically catalyzedreactions. Enzymes used were esterases from rabbit2,8

and guinea pig9 sera. We now report related studies onmethyl �-D-galactopyranosides. Under hydrolysis con-ditions, acylated galactopyranosides were not shown tobe substrates for esterases in mammalian sera. Instead,acyl migrations were observed in 2,6- and 3,6-dipi-valates proceeding through a stable orthoacid interme-diate, which was the major isolated product. Althoughit was previously reported that acetylated or benzoy-lated sugars undergo acyl migrations under acidic,10

basic,11 and occasionally neutral11,12 conditions, the pi-valoyl group was generally regarded as not prone tosuch migrations. We previously observed migrations ofthe pivaloyl group in reactions catalyzed by an enzymepresent in some mammalian sera.13 Just in a few cases,involving D-mannose6 and D-xylose,7 acyl migrations

were observed in neutral conditions identical to thosewe describe in the case of D-galactose. The mechanismof intramolecular transesterifications has been studiedextensively, and the general conclusion is that it pro-ceeds through orthoacid intermediates.11 This is in ac-cord with our results obtained with pivaloylatedgalactoses in which the intermediate orthoacid wasstable enough to be isolated. In contrast to orthoacids,stable orthoesters are better known and often exploitedas intermediates in stereoselective glycosylations, reac-tions often used for the synthesis of oligosaccha-rides.14,15

2. Results and discussion

Preparation of methyl O-acyl-�-D-galactopyranosides(Table 1).—Various O-pivaloyl derivatives were pre-pared by esterification of methyl �-D-galactopyranoside(1) with different molar equivalents of pivaloyl chloridein pyridine. Acetylation was achieved by using an ex-cess of Ac2O–pyridine mixture.

Thus, the reaction of 1 with four molar equivalentsof pivaloyl chloride for 4 h produced the 2,3,6-tripi-valate 4 as the major product (51%). The 2,6- (2) and3,6-dipivalates (3) were also isolated in yields of 11 and17%, respectively.

* Corresponding author. Fax: +385-1-4819288.E-mail address: srdjanka@rudjer.irb.hr (S. Tomic).

0008-6215/02/$ - see front matter © 2002 Elsevier Science Ltd. All rights reserved.

PII: S0008 -6215 (02 )00055 -1

V. Petro�ic et al. / Carbohydrate Research 337 (2002) 863–867864

With eight molar equivalents of pivaloyl chloride, the2,3,6-tripivalate 4 was formed exclusively in 88% yield.

Acetylation of 2, 3, and 4 produced the expectedacetates 6, 7, and 8 in high yields.

Treatment of 1 with 18 molar equivalents of pivaloylchloride for 7 days at 60 °C produced the correspond-ing tetrapivalate 5 in 55% yield.

Intramolecular transeserifications.—Under the condi-tions used in previously described enzymic hydrolyses,using phosphate buffered saline (PBS, 0.01 M, pH 7.2),Me2SO, rabbit or guinea pig serum esterase at 37 °C,acylated galactopyranosides were not shown to be sub-strates for esterases in mammalian sera. While tri-sub-stituted and tetra-substituted galactopyranosidesremained unchanged even at prolonged reaction times,2,6- and 3,6-dipivalates underwent intramolecular cy-

clization to give a stable 2,3-orthoacid 10 with a paral-lel 6�4 migration of the pivaloyl group (Scheme 1).Both processes were observed in the presence and in theabsence of enzymes since all enzymic reactions wereperformed with parallel controls containing only thereaction medium (PBS or a mixture of PBS andMe2SO, pH 7.2, 37 °C) without the enzyme. Thus,2,6-dipivalate 2 in PBS at 37 °C gave, after 7 days, theorthoacid 10 as a major product (40%) and some3,6-dipivalate 3 (13%) as a result of complete 2�3pivaloyl migration. Some unreacted 2 was also isolated(18%). The 3,6-dipivalate 3 under the same reactionconditions gave 10 as a major product (34%), some2,6-dipivalate 2 (8%) as a result of complete 3�2pivaloyl migration. Some unreacted 3 (23%) was alsoisolated. Acetylation of 10 produced the diacetate 11(98%) as shown in Scheme 1.

We previously observed migrations of the pivaloylgroup in reactions catalyzed by an enzyme present insome mammalian sera13 but also in just a few cases inneutral conditions identical to those described for D-galactose. Thus, 1�2 migration was observed in 1,6-di-O-pivaloyl-�-D-glucopyranoside,3 2�3 migration inmethyl 2,4-di-O-pivaloyl-�-D-xylopyranoside,7 and 3�2 migration in methyl 3,6-di-O-pivaloyl-�-D-mannopy-ranoside.6 In the case of both methyl 2,6- and3,6-di-O-pivaloyl-�-D-galactopyranose, an unusual 6�4 migration of the pivaloyl group was observed; ingeneral, acyl groups usually migrate towards the pri-mary hydroxyl at C-6 in a hexapyranose derivative.Unusually, the 2�3 and 3�2 migrations of the pival-oyl group were only partially completed since orthoacid10, a presumed intermediate in the mechanism of trans-esterifications, was stable enough to be isolated as amajor product.

3. Experimental

General methods.—Column chromatography wasperformed on Silica Gel (Merck) and TLC on KieselgelG (Merck) with solvent A, EtOAc–benzene (propor-tions are given in the text). The detection was effectedby charring with H2SO4. 1H and 13C NMR spectra wererecorded with a Varian Gemini 300-BB spectrometer(300 MHz, CDCl3, internal Me4Si), and the data aregiven in Tables 2–4,10,16 and in this section. Opticalrotations were measured in CHCl3 at �20 °C using theOptical Activity AA-10 Automatic Polarimeter.

Selecti�e pi�aloylations of methyl �-D-galactopyran-oside (1).— (a) To a solution of 1 (194 mg, 1 mmol) indry pyridine (2.0 mL) was added pivaloyl chloride (0.5mL, 4 mmol). The mixture was stirred at ambienttemperature for 4 h. The reaction was stopped by theaddition of 96% EtOH (2 mL). Water (3×5 mL) wasadded and the mixture of solvents evaporated under

Table 1

R3 R4R1 R2

H H1 H HPivH2 Piv H

Piv HH Piv3H Piv4 Piv PivPiv Piv5 Piv PivAcAc PivPiv6Ac7 PivAc Piv

Piv8 Piv Piv AcAc9 AcAc Ac

Scheme 1.

V. Petro�ic et al. / Carbohydrate Research 337 (2002) 863–867 865

Table 21H NMR data (CDCl3) for ring protons of O-acyl-�-D-galactopyranosides

Chemical shifts � (ppm), J (Hz)Compound

H-1 (J1,2) H-2 (J2,3, J1,2) H-3 (J2,3, J3,4) H-4 H-5 H-6a (J6a,6b, J6a,5) H-6b

5.05 (dd) (10.17,4.86 (d) (3.84) 4.00 (m)2 4.30 (m) 4.00 (m) 4.25 (dd) (11.68, 4.00 (m)3.02) 7.00)4.00 (m) 4.94 (m) 4.44 (m) 4.00 (m)4.94 (m) 4.21 (dd) (11.40,3 4.00 (m)

6.73)4 5.15 (dd) (10.71,4.99 (d) (3.84) 5.29 (dd) (10.57, 4.05 (m)4.43 (m) 4.05 (m) 4.26 (dd) (11.54,

7.14)3.17)3.57)5.09 (dd) (10.00,5.01 (d) (3.63) 5.45 (m)5 5.45 (m) 4.13 (m) 4.25 (m) 4.01 (m)3.53)5.26 (m) 5.32 (m) 5.50 (m)6 4.08 (m)4.97 (d) (3.57) 4.22 (m) 4.08 (m)5.07 (m) 5.45 (m) 5.45 (m) 4.10 (m)7 4.20 (m)4.99 (d) (3.57) 4.10 (m)5.11 (dd) (10.72, 4.06 (m)5.40 (dd) (10.72,5.01 (d) (3.57) 5.50 (m)8 4.06 (m) 4.21 (m)3.84) 3.30)

9 5.15 (dd) (10.99,5.00 (d) (3.57) 5.35 (dd) (10.85, 4.11 (m)5.43 (m) 4.11 (m) 4.20 (m)3.44)3.57)

Table 31H NMR data (CDCl3) for functional groups of O-acyl-�-D-galactopyranosides

Chemical shifts (�, ppm)Compound

OAc (s)OPiv (s) OMe (s)

3 4 6 22 3 4 6

1.222 1.26 3.463 1.25 1.23 3.38

1.21 1.184 1.22 3.301.19 1.27 1.131.205 3.41

1.196 1.12 2.07 2.13 3.431.19 1.18 1.97 2.15 3.4071.18 1.141.208 2.13 3.39

2.079 2.09 2.15 1.99 3.41

reduced pressure. The remaining traces of water wereremoved by co-distillation with toluene. Column chro-matography of the residue (solvent A, 1:1) gave, firstly,methyl 2,3,6-tri-O-pivaloyl-�-D-galactopyranoside (4;228 mg, 51%): mp 86–87 °C (EtOAc– light petroleum);[� ]D +131° (c 1.00); Rf �0.66 (solvent A, 1:1). Anal.Calcd for C22H38O9: C, 59.17; H, 8.58. Found: C, 58.97;H, 8.72. Eluted next was methyl 2,6-di-O-pivaloyl-�-D-galactopyranoside (2; 38 mg, 11%): mp 77–79°C (EtOAc– light petroleum); [� ]D +121° (c 0.86);Rf �0.42 (solvent A, 1:1). ESMS m/z [M+Na]+

385.2. Anal. Calcd for C17H30O8: C, 56.34; H, 8.34.Found: C, 56.38; H, 8.57. Eluted last was methyl3,6-di-O-pivaloyl-�-D-galactopyranoside (3; 62 mg,17%): mp 76–78 °C (EtOAc– light petroleum); [� ]D+129° (c 1.00); Rf �0.36 (solvent A, 1:1). Anal. Calcdfor C17H30O8: C, 56.34; H, 8.34. Found: C, 56.03;H, 8.57.

Conventional acetylation of 4 (52 mg, 0.11 mmol)with Ac2O–pyridine, followed by column chromatogra-phy (solvent A, 5:1) gave the 4-acetate 8 (51 mg, 90%)as oil: [� ]D +109° (c 1.00); Rf �0.56 (solvent A, 5:1).ESMS m/z [M+Na]+ 488.6. Anal. Calcd forC24H40O10: C, 59.00; H, 8.25. Found: C, 58.88; H, 8.35.Likewise, 2 (20 mg, 0.05 mmol) gave the 3,4-diacetate 6(28 mg, 95%) as an oil: [� ]D +92° (c 0.99); Rf �0.43(solvent A, 5:1). ESMS m/z [M+Na]+ 446.5. Anal.Calcd for C21H34O10: C, 56.49; H, 7.68. Found: C,56.32; H, 7.81. Compound 3 (40 mg, 0.1 mmol) underthe same reaction conditions gave the 2,4-diacetate 7 (53mg, 98%) as an oil: [� ]D +96° (c 1.34); Rf �0.47(solvent A, 5:1). Anal. Calcd for C21H34O10: C, 56.49; H,7.68. Found: C, 56.20; H, 7.85.

(b) Pivaloylation of 1 (50 mg, 0.26 mmol) withpivaloyl chloride (0.26 mL, 2.1 mmol) for 4 h at ambienttemperature gave the 2,3,6-tripivalate 4 (101 mg, 88%).

V. Petro�ic et al. / Carbohydrate Research 337 (2002) 863–867866

Table 413C NMR data (CDCl3) for O-acyl derivatives of methyl �-D-galactopyranoside

Chemical shifts (�, ppm)Compound

C-2 C-3 C-4 C-5 C-6 OCH3C-1 (CH3)3CCO (CH3)3CCO (CH3)3CCO CH3CO CH3CO

67.84 68.18 67.272 72.6099.47 62.88 55.35 �180 38.67 27.0638.94

97.283 68.08 69.15 67.44 71.31 62.83 55.35 178.65 38.82 26.92178.98 27.06

67.89 67.98 67.33 69.59 62.93 55.3296.92 177.144 38.63 26.86177.79 38.82 27.02178.28

97.125 67.49 67.67 66.54 68.22 61.64 55.47 177.77 38.61 26.85177.03 38.96 26.97176.84 27.08

66.23 67.28 67.28 68.01 61.69 55.326 177.8697.22 38.62 26.97 20.52 170.11176.84 26.70 20.65 169.70

67.35 67.86 66.09 68.08 61.55 55.497 177.8097.14 38.61 26.74 20.43 170.0026.96 20.53 169.73

96.978 67.28 67.89 66.11 68.20 61.69 55.47 176.90 38.62 26.84 20.52 169.73177.82 26.96

97.029 67.38 67.96 65.99 67.96 61.67 55.37 20.55 169.8520.60 170.1220.74 170.29

(c) Pivaloylation of 1 (50 mg, 0.26 mmol) with pival-oyl chloride (0.6 mL, 4.8 mmol) for 7 days at 60 °Cgave the 2,3,4,6-tetrapivalate 5 (76 mg, 55%): mp 73–74 °C (EtOAc– light petroleum); [� ]D +79° (c 1.02); Rf

�0.69 (solvent A, 2:1). Anal. Calcd for C27H46O10: C,61.11; H, 8.74. Found: C, 61.08; H, 9.00.

(d) Conventional acetylation of the starting com-pound 1 (194 mg, 1 mmol) with Ac2O–pyridine gavethe 2,3,4,6-tetraacetate 9 (348 mg, 93%): mp 86–87 °C(1:1 Et2O– light petroleum), lit.17 87 °C; [� ]D +134° (c1.00), lit.17 +133° (c 2.88); Rf �0.41 (solvent A, 5:1).Anal. Calcd for C15H22O10: C, 49.72; H, 6.12. Found:C, 49.58; H, 6.24.

Methyl 4-O-pi�aloyl-[2,3-O-(1-hydroxy)-2,2-dime-thylpropylidene]-�-D-galactopyranoside (10).— (a) Thesolution of methyl 2,6-di-O-pivaloyl-�-D-galactopyran-oside (2; 40 mg, 0.1 mmol) in PBS (pH 7.2) was stirredat ambient temperature for 5 days. The solvent wasevaporated under reduced pressure. Column chro-matography of the residue (solvent A, 1:1) gave first thestarting compound 2 (7 mg, 18%) followed by 3 (5 mg,13%). Eluted last was 10 as an oil (16 mg, 40%): [� ]D+96° (c 1.08); Rf �0.2 (solvent A, 1:1). 1H NMR(CDCl3): � 5.33 (d, 1 H, J3,4 3.99 Hz, H-4), 4.87 (d, 1H, J1,2 3.85 Hz, H-1), 4.11 (m, 2 H, H-2, H-3), 4.07 (m,1 H, H-5), 3.99 (dd, 1 H, J6a,6b 9.89, J5,6a 3.50 Hz,H-6a), 3.79 (dd, 1 H, J6a,6b 10.03, J5,6b 3.6 Hz, H-6b),3.45 (s, 3 H, MeO), 1.20 (s, 9 H, Me3CO-2,3), 1.25 (s,

9 H, PivO-4); 13C NMR (CDCl3): � 178.23 (C�OH),177.87 (CO), 99.25 (C-1), 69.59 (C-5, C-6), 69.51 (C-4),67.06 (C-3), 62.12 (C-2), 55.47 (OMe), 39.12, 38.61 (2Me3C), 27.04, 27.01 (2 Me3C). ESMS m/z [M+Na]+

385.2. Anal. Calcd for C17H30O8: C, 56.34; H, 8.34.Found: C, 56.49; H, 8.06.

(b) The solution of methyl 3,6-di-O-pivaloyl-�-D-galactopyranoside (3; 40 mg, 0.1 mmol) in PBS (pH7.2) was stirred at 60 °C for 5 days. Column chro-matography of the residue (solvent A, 1:1) gave first thestarting compound 3 (3 mg, 8%) followed by 2 (9 mg,23%). Eluted last was 10 as an oil (13 mg, 34%).

(c) Conventional acetylation of 10 (19 mg, 0.5 mmol)with Ac2O–pyridine gave methyl 6-O-acetyl-4-O-pival-oyl-[2,3-O-(1-acetoxy)-2,2-dimethylpropylidene]-�-D-galactopyranoside (11) as an oil (21 mg, 98%); [� ]D+100° (c 1.04); Rf �0.4 (solvent A, 5:1). 1H NMR(CDCl3): � 5.47 (m, 1 H, H-4), 5.37 (dd, 1 H, J2,3 10.99,J3,4 3.30 Hz, H-3), 5.16 (dd, 1 H, J2,3 10.71, J1,2 3.57Hz, H-2), 5.00 (d, 1 H, J1,2 3.57 Hz, H-1), 4.23 (m, 1 H,H-6b), 4.09 (m, 2 H, H-5, H-6a), 3.40 (s, 3 H, MeO),2.11 (s, 3 H, AcOCO-2,3), 1.97 (s, 3 H, AcO-6), 1.27 (s,9 H, PivO-4), 1.20 (s, 9 H, Me3CO-2,3); 13C NMR(CDCl3): � 178.11 (C�OH, CO, Piv), 170.21 (2 CO,Ac), 97.09 (C-1), 68.00 (C-6), 67.53(C-5), 67.45 (C-4),66.18(C-3), 61.25 (C-2), 55.30 (OMe), 38.61 (2 Me3C),26.99 (2 Me3C), 20.76, 20.48 (2 MeC). ESMS m/z[M+Na]+ 469.5. Anal. Calcd for C21H34O10: C, 56.49;H, 7.67. Found: C, 56.59; H, 7.54.

V. Petro�ic et al. / Carbohydrate Research 337 (2002) 863–867 867

Acknowledgements

We wish to thank the Ministry of Science and Tech-nology of the Republic of Croatia for support of thiswork (119401). We also thank Dr Vesna Gabelica,Director of Applied Research, PLIVA Pharm. Ind. Inc.for the MS spectra.

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