Stereoselective glycosylation of d -galactals by diethyl phosphorochloridite- and AlCl3-assisted Ferrier rearrangement

Tetrahedron 71 (2015) 350e358

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Stereoselective glycosylation of D-galactals by diethyl phosphorochloridite- and AlCl3-assisted Ferrier rearrangement Yen-Bo Chen a, b, Su-I. Wang b, Zi-Ping Lin b, Chun-Hung Lin c, Min-Tsang Hsieh a, d, Hui-Chang Lin a, b, * a

School of Pharmacy, China Medical University, No. 91, Hsueh-Shih Rd., Taichung 40402, Taiwan Graduate Institute of Pharmaceutical Chemistry, China Medical University, No. 91, Hsueh-Shih Rd., Taichung 40402, Taiwan Institute of Biological Chemistry, Academia Sinica, No. 128, Academia Road, Section 2, Nan-Kang, Taipei 11529, Taiwan d Chinese Medicinal Research and Development Center, China Medical University Hospital, No. 2, Yude Rd., Taichung 40447, Taiwan b c

a r t i c l e i n f o

a b s t r a c t

Article history: Received 12 September 2014 Received in revised form 30 October 2014 Accepted 9 November 2014 Available online 24 November 2014

a-2,3-Unsaturated galactosides were synthesized in good to excellent yields by the initial activation of

Keywords: 2,3-Unsaturated glycopyranosides Ferrier rearrangement Diethyl phosphorochloridite Glycal Lewis acid

D-galactals with diethyl phosphorochloridite and the subsequent glycosyl addition via Ferrier rearrangement with various O-nucleophiles in the presence of AlCl3. The two-step reactions were carried out in one-pot and finished within 60 min in 81e95% yield to give the glycoside products with excellent a-stereoselectivity. Ó 2014 Elsevier Ltd. All rights reserved.

1. Introduction 2,3-Unsaturated-O-glycosides are useful building blocks for the synthesis of various bioactive compounds, such as nucleosides,1 antibiotics,2 glycopeptides,3 oligosaccharides,4 uronic acids,5 and other natural products.6 The double bond in C2 and C3 of the pyranose ring can be further transformed via several types of reactions (e.g., dihydroxylation,8 hydrogenation,9 epoxidation,10 and aminohydroxylation11) to generate structural complexity and diversity. Preparation of 2,3-unsaturated glycosides via Ferrier rearrangement has drawn attention in the past several decades. This rearrangement is believed to involve a cyclic allylic oxocarbonium intermediate that is formed via displacement of the C-3 substituent in an endo-glycal, followed by the attack of a nucleophile to the anomeric carbon preferentially from the quasi-equatorial orientation.7 A wide range of catalysts have been employed to facilitate the syntheses, such as BF3$OEt2,12 FeCl3,13 InCl3,14 Montmorillonite K-10,15 SnCl4,16 K5CoeW12O40,17 BiCl3,18 Dy(OTf)3,19 CeCl3$7H2O,20 ZnCl2,21 Sc(OTf)3,22 IDCP,23 TMSOTf,24 NIS,25 LiBF4,26 trichloroacetimidate,27 Fe(NO3)3$9H2O,28 InBr3,29 ZrCl4,30 I2,31 CAN,32 HClO4eSiO2,33

* Corresponding author. Tel.: þ886 4 22053366x5612; fax: þ886 4 22078083; e-mail addresses: [email protected], [email protected] (H.-C. Lin). http://dx.doi.org/10.1016/j.tet.2014.11.027 0040-4020/Ó 2014 Elsevier Ltd. All rights reserved.

DDQ,34 Bi(NO3)3$5H2O,35 NbCl5,36 DMF-DMA,37 Pd(PhCN)2Cl2,38 Fe(OTf)3,39 Er(OTf)3,40 ZnCl2/Al2O3,41 (S)-Camphorsulfonic acid,42 H3PO4,43 TfOH-SiO2,44 NaHSO4eSiO2,45 and t-BuOK.46 Successful Ferrier rearrangement can be carried out when endoglycals possessing a good leaving group at C-3 position, including trichloroacetamidate,47 tert-butyloxycarbonyl ester,48 n-pentenoyl ester,23 benzoyl ester,12a carbonate,49 propargyl ether,50 and benzyl ether.51 To the best of our knowledge, there is no report to derive from 3-O-diethoxyphosphanyl-D-galactal. However, the diethoxyphosphanyl group can be easily activated in the presence of acid or Lewis acid.53 Herein we developed an efficient two-step, one-pot synthesis to prepare 2,3-unsaturated-O-glycosides. 3-Hydroxy-Dgalactal was reacted with diethyl phosphorochloridite and triethylamine to give in situ the intermediate 3-O-diethoxyphosphanyl-D-galactal, followed by rearrangement and glycosyl addition with various O-nucleophiles in the presence of AlCl3 to give 2,3-unsaturated-O-glycosides. 2. Result and discussion 4,6-Di-O-benzyl-3-hydroxy-D-galactal 1a, prepared from 3,4,6tri-O-acetyl-D-galactal according to previous procedure,52 was reacted with diethyl phosphorochloridite in the presence of Lewis bases (3.0 equiv) to produce the 3-O-diethoxyphosphanyl-D-

Y.-B. Chen et al. / Tetrahedron 71 (2015) 350e358

galactal as the intermediate, and was then followed by C1-addition of n-hexanol to give the glycosyl adduct. Five different Lewis bases (3.0 equiv) were examined, including triethylamine (Et3N), 4dimethylaminopyridine (DMAP), 1,8-diazabicyclo-[5.4.0]undec-7ene (DBU), potassium carbonate (K2CO3), and potassium tert-butoxide (t-BuOK), as shown in entries iev of Table 1. The reactions were all carried out in dichloromethane at 0  C for 1 h. Among these reactions, Et3N was found to provide the desired 2,3-unsaturatedO-glycoside (4) with the highest isolated yield (91%) and moderate stereoselectivity (ratio of a/b-anomers: 78/22).

351

Lewis acid. A CH2Cl2 solution of 4,6-di-O-benzyl-3-diethyl phosphite-D-galactal 3a and n-hexanol was treated with 1.0 equiv of various Lewis acids at 0  C for 30 min, including zinc chloride (ZnCl2), calcium chloride (CaCl2), mercury(II) sulfate (HgSO4), iron chloride (FeCl3), aluminum chloride (AlCl3), and tin(IV) chloride (SnCl4). The use of AlCl3 was concluded to provide the highest isolated yield (94%) and stereoselectivity (ratio of a/b-anomers: 92/ 8) (entry v in Table 2). To our delight, high yields and stereoselectivity were also obtained when the same condition was applied to the reactions with

Table 1 Application of different Lewis bases and solvents for synthesis of 2,3-unsaturated glycosides 4a

Entry

Lewis base

Solvent

Products (yield, a/b ratio)

i ii iii iv v vi vii viii ix

Et3N DMAP DBU K2CO3 t-BuOK Et3N Et3N Et3N Et3N

CH2Cl2 CH2Cl2 CH2Cl2 CH2Cl2 CH2Cl2 THF MeCN DMF Toluene

4 4 4 4 4 4 4 4 4

a b

(91%, 78/22) (13%)b (5%)b (36%)b (11%)b (0%) (0%) (0%) (85%, 75/25)

All reactions were carried out in the presence of diethyl chlorophosphite (1.2 equiv), Lewis base (3.0 equiv) and n-hexanol (3.0 equiv). The a-anomer is predominant with a/b ratio of 2/1e3/1 on the basis of 1H NMR integration.

Table 2 Effect of Lewis acid on the glycosylations of endo-glycal 1a with n-hexanola

Entry

Lewis acid (cat)

Products (yield, a/b ratio)b

i ii iii iv v vi vii

ZnCl2 CaCl2 HgSO4 FeCl3 AlCl3 AlCl3 SnCl4

4 (89%, 73/27) 4 (84%, 81/19) 4 (88%, 80/20) 4 (78%, 90/10) 4 (94%, 92/8) No reactionc 4 (91%, 84/16)

a b c

All reactions were carried out in the presence of Lewis acid (1.0 equiv) and n-hexanol (3.0 equiv). The a-anomer is predominant with a/b ratio on the basis of 1H NMR integration. The reaction was carried out without diethyl phosphorochloridite/Et3N in a CH2Cl2 solution containing AlCl3 (1.0 equiv) and n-hexanol (3.0 equiv) at 0  C for 1 h.

Meanwhile, we investigated the solvent effect by examining the reactions in THF, MeCN, DMF, CH2Cl2 and toluene (entries vi-ix of Table 1). The reactions in toluene and dicholomethane were found to afford higher yields and stereoselectivity than those in other solvents. Among the reactions, the combined use of CH2Cl2 and Et3N gave the desired product (4) in the highest isolated yield (91%) with moderate stereoselectivity (ratio of a/b-anomers: 78/22) (see entry i in Table 1). Interestingly, 3-O-diethoxyphosphanyl-D-galactal (3a) appeared to be more active than glycosyl diethyl phosphites (the phosphite is attached to the anomeric center)53 because of different stability (3a could not be isolated but the latter could be purified and characterized). In order to improve the resulting a-stereoselectivity, addition of Lewis acid was considered in the Ferrier rearrangement-mediated glycosylation. We generated 4,6-di-O-benzyl-3-diethyl phosphiteD-galactal (3a) in situ, followed by the addition of n-hexanol and

other alcohol nucleophiles (entries ievi in Table 3), as well as those with a similar substrate (1b) (entries vii-ix). Furthermore, various sugar nucleophiles were examined. Their reactions gave the desirable 2,3-unsaturated glycoside products 17e22 in 81e86% yields with excellent a-stereoselectivity (Table 4). It is realized that the nucleophile preferentially attacks from the bottom face of the sugar ring, which is not only consistent with the anomeric effect, but also favorable by the less steric hindrance resulting from the C4-substituent. All the product structures, including the new stereogenic configurations, were rigorously determined by using the COSY and NOSY methods, in agreement with previous reports.54 For example, the 1H NMR spectrum of compound 17 showed the characteristic signals of H1 (d 5.43, d), H2 (d 5.90, dd), and H3 (d 6.02, dd). In the corresponding 13C NMR spectrum, the resonances at d 96.30, 129.43, and 126.97 were assigned to C1, C2 and C3 of 17, respectively.

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Table 3 The glycosylations of endo-glycal 1 with varies alcoholsa

Entry

Donor

Acceptor (R2OH)

Products (yield, a/b)b

i ii iii iv v vi vii viii ix

1a 1a 1a 1a 1a 1a 1b 1b 1b

Methanol Benzyl alcohol Allyl alcohol Isopropanol Cyclohexanol p-Methylphenol Benzyl alcohol Isopropanol Cyclohexanol

5 (94%, 93/7) 6 (91%, 92/8) 7 (90%, 93/7) 8 (85%, 92/8) 9 (87%, 93/7) 33 (85%, 91/9) 10 (95%, 93/7) 11 (92%, 95/5) 12 (92%, 93/7)

a b

All reactions were carried out in the presence of AlCl3 (1.0 equiv) and alcohols (3.0 equiv). The a-anomer is predominant with a/b ratio on the basis of 1H NMR integration.

Table 4 The glycosylations of endo-Glycals 1 with various sugar alcoholsa

Acceptor (ROH)a

Products (yield, a/b)b

Entry

Donor

i

1a

17 (82%, 94/6)

ii

1a

18 (83%, 95/5)

iii

1a

19 (86%, 93/7)

iv

1a

20 (81%, 94/6)

v

1c

21 (85%, 95/5)

vi

1c

22 (83%, 94/6)

a b

The glycosyl additions were all catalyzed by AlCl3 (1.0 equiv) with 3 equiv of the acceptor was dissolved in 0.5 mL CH2Cl2. The a-anomer is predominant with a/b ratio on the basis of 1H NMR integration.

Y.-B. Chen et al. / Tetrahedron 71 (2015) 350e358

We previously applied microwave- and AlCl3-assisted glycosylation of C3-substituted endo-galactals (that contains a benzyloxy, acetate or hydroxy group at C3) where these reactions were found to proceed via protonation to produce a-2-deoxyglycosides.48b Those results are different from what we observed here (e.g., formation of 2,3-unsaturated O-glycosides), indicating that the substituent at C3 plays a role in affecting the reactivity and needs to be labile enough to promote Ferrier rearrangement. In addition, exo-glycal 2 was subjected to the similar 2-step procedure. The glycosidations were successful with various alcohols, including methanol, n-butanol, n-hexanol, allyl alcohol, benzyl alcohol, isopropanol, and cyclohexanol. The resulting glycoside products 23e2951c were obtained in 74e86% yields with exclusive a-stereoselectivity (Table 5).

353

We proposed a plausible mechanism to interpret the observed rearrangement and stereoselectivity. As shown in Scheme 1, the first step reaction was expected to result in the formation of 3-Odiethoxyphosphanyl-D-galactal (3a). The subsequent addition of AlCl3 not only led to the possible chelation of C3-, C4- and C6substituents with the Lewis acid, but also facilitated the rearrangement to form the conjugated oxocarbenium ion intermediate 31. At this stage, the blockade of the sugar top face (caused by the chelation) is critical to dominate the nucleophilic attack from the bottom face, which explains the a-stereoselectivity. The final product (4) was obtained by the addition of hexanol (serving as the nucleophile) to the oxocarbenium intermediate. In conclusion, a-2,3-unsaturated glycopyranosides were successfully prepared by glycosyl addition of endo-galactals with

Table 5 The glycosylations of exo-glycal 2 with varies alcoholsa

Entry

Acceptor (ROH)

Products (yield)b

i ii iii iv v vi vii

Methanol n-Butanol n-Hexanol Allyl alcohol Benzyl alcohol Isopropanol Cyclohexanol

23 24 25 26 27 28 29

a b

All reactions were carried out in the presence of AlCl3 (1.0 equiv) and alcohols (3.0 equiv). The a-anomer is predominant with a/b ratio on the basis of 1H NMR integration.

Scheme 1. The possible mechanism proposed to explain the formation of rearranged products.

(85%) (84%) (86%) (86%) (81%) (74%) (77%)

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Y.-B. Chen et al. / Tetrahedron 71 (2015) 350e358

several O-nucleophiles, including simple alcohols and sugar derivatives. The two-step synthesis was operated in one pot via tandem phosphanylation and Ferrier rearrangement-mediated glycosylation. This developed method was also applicable to the addition of exo-glycals. 3. Experimental section 3.1. General procedure All purchased chemicals were of reagent grade. All reactions were carried out under a nitrogen atmosphere and monitored by TLC analysis (layer thickness: 250 mm). Column chromatography was carried out with silica gel 60 (70e230 mesh for gravity column, or 230e400 mesh for flash column). Commercially available reagents were directly used without further purification unless otherwise noted. Dichloromethane, ethyl acetate, hexanes, and methanol were purchased from Mallinckrodt Chemical Co. The following compounds were purchased from Acros Chemical Co, including methanol, benzyl alcohol, n-hexanol, cyclohexanol, isopropanol, allyl alcohol, iodomethane, tri-O-acetyl-D-galactal, benzyl chloride, potassium carbonate, sodium hydride, diethyl chlorophosphite, triethylamine, 1,8-diazabicyclo[5.4.0]undec-7ene, 4-dimethylaminopyridine, potassium tert-butoxide, aluminum chloride, iron chloride, tin(IV) chloride, calcium chloride, zinc chloride, mercury(II) sulfate, 1,2:3,4-di-O-isopropylidene-a-D-galactopyranose. Proton NMR spectra were recorded at a Bruker spectrometer (200 or 400 MHz) with CDCl3 (dH 7.24) and DMSO-d6 (dH 2.50) as the internal standard; Carbon-13 NMR spectra were recorded at 50 or 100 MHz with CDCl3 [dC 77.0 (central line of a triplet)]. Splitting patterns are shown by the abbreviations, such as s (singlet), d (doublet), t (triplet), q (quartet), and m (multiplet). 3.2. Typical procedure of glycosylation for preparing 2,3unsaturated glycopyranoside derivatives 4e12, 33 To a solution of endo-galactal 1 (1.0 equiv) in anhydrous CH2Cl2 (1.5 mL) was added diethyl phosphorochloridite (1.2 equiv) and Et3N (3.0 equiv). The reaction mixture was stirred at 0  C under N2 for 30 min. After the phosphanylation was completed, one equivalent of AlCl3 and three equivalents of an alcohol substrate were added into the reaction mixture and stirred at 0  C for 30 min. Once the reaction was completed, the resulting solution was added to CH2Cl2 (50 mL), washed with H2O (20 mL2), and brine (20 mL2). The organic layer was collected, concentrated under reduced pressure and purified by silica gel column chromatography with 33% EtOAc in hexanes to give the desired product (2,3-unsaturated glycopyranosides 4e12 and 33) in 85e94 % yields. 3.3. Hexyl 4,6-di-O-benzyl-2,3-dideoxy-D-threo-hex-2enopyranoside 4 Pale yellow oil; IR (CHCl3) 2966, 1585, 1398, 1099 cm1; 1H NMR (CDCl3, 400 MHz) d 7.30e7.21 (10H, m, ArH), 6.07 (1H, dd, J3,4¼5.2 Hz, J3,2¼10.0 Hz, H3), 5.95 (1H, dd, J2,1¼3.2 Hz, J2,3¼10.0 Hz, H2), 5.01 (1H, d, J1,2¼3.2 Hz, H1), 4.61e4.50 (4H, m, CH2Ph), 4.26e4.22 (1H, m, H5), 3.81e3.68 (4H, m, H4, H6a, H6b, H1b0 ), 3.46e3.41 (1H, m, H1a0 ), 1.57e1.53 (2H, m, H20 ), 1.31e1.22 (6H, m, H30 , H40 , H50 ), 0.83 (3H, t, J¼6.8 Hz, H60 ); 13C NMR (CDCl3, 100 MHz) d 138.52, 138.36, 129.80, 128.32, 128.31, 128.30, 128.29, 127.74, 127.73, 127.60, 127.54, 127.53, 127.52, 126.87, 90.05, 73.39, 70.98, 69.59, 69.45, 68.36, 67.37, 31.61, 29.72, 25.86, 22.58, 14.01; FABMS m/z (rel intens) 411 (MþHþ, 1), 253 (6), 181 (22), 105 (22), 91 (100), 57 (73); HRMS (FAB) m/z calcd for C26H35O4 (MþHþ) 411.2530, found 411.2538. Anal. Calcd for C26H34O4; C: 76.06; H: 8.35. Found: C: 76.10; H: 8.37.

3.4. Methyl 4,6-di-O-benzyl-2,3-dideoxy-D-threo-hex-2enopyranoside 5 Pale yellow oil; IR (CHCl3) 2945, 1585, 1398, 1099 cm1; 1H NMR (CDCl3, 400 MHz) d 7.27e7.17 (10H, m, ArH), 6.03 (1H, dd, J3,4¼5.0 Hz, J3,2¼10.0 Hz, H3), 5.91 (1H, dd, J2,1¼2.5 Hz, J2,3¼10.0 Hz, H2), 4.88 (1H, d, J1,2¼2.5 Hz, H1), 4.56 (1H, d, J¼12.0 Hz, CH2Ph), 4.55 (1H, d, J¼12.0 Hz, CH2Ph), 4.50 (1H, d, J¼12.0 Hz, CH2Ph), 4.47 (1H, d, J¼12.0 Hz, CH2Ph), 4.17e4.16 (1H, m, H5), 3.75 (1H, dd, J6a,5¼6.5 Hz, J6a,6b¼10.0 Hz, H6a), 3.69 (1H, dd, J6b,5¼7.0 Hz, J6b,6a¼10.0 Hz, H6b), 3.65e3.64 (1H, m, H4), 3.36 (3H, s, OCH3); 13C NMR (CDCl3, 100 MHz) d 138.40, 138.27, 129.46, 128.34, 128.33, 128.32, 128.31, 127.75, 127.74, 127.63, 127.57, 127.56, 127.55, 127.04, 95.07, 73.40, 70.99, 69.54, 69.46, 67.21, 55.45; FABMS m/z (rel intens) 341 (MþHþ, 1), 91 (100); HRMS (FAB) m/z calcd for C21H25O4 (MþHþ) 341.1747, found 341.1756. Anal. Calcd for C21H24O4; C: 74.09; H: 7.11. Found: C: 74.11; H: 7.14. 3.5. Benzyl 4,6-di-O-benzyl-2,3-dideoxy-D-threo-hex-2enopyranoside 6 Pale yellow oil; IR (CHCl3) 2960, 1496, 1100 cm1; 1H NMR (CDCl3, 400 MHz) d 7.27e7.15 (15H, m, ArH), 6.04 (1H, dd, J3,4¼5.2 Hz, J3,2¼10.0 Hz, H3), 5.91 (1H, dd, J2,1¼3.2 Hz, J2,3¼10.0 Hz, H2), 5.08 (1H, d, J1,2¼3.2 Hz, H1), 4.73 (1H, d, Ja,b¼11.6 Hz, CH2Ph), 4.56e4.45 (5H, m, CH2Ph), 4.27e4.23 (1H, m, H5), 3.75 (1H, dd, J6a,5¼6.4 Hz, J6a,6b¼10.0 Hz, H6a), 3.67e3.63 (2H, m, H4, H6b); 13C NMR (CDCl3, 100 MHz) d 138.45, 138.33, 137.87, 129.62, 128.35, 128.34, 128.32, 128.31, 128.30, 128.29, 128.20, 128.19, 127.74, 127.73, 127.66, 127.65, 127.56, 127.54, 127.53, 127.12, 93.23, 73.41, 70.98, 69.68, 69.58, 69.55, 67.33; FABMS m/z (rel intens) 417 (MþHþ, 1), 91 (100); HRMS (FAB) m/z calcd for C27H29O4 (MþHþ) 417.2060, found 417.2064. Anal. Calcd for C27H28O4; C: 77.86; H: 6.78. Found: C: 77.88; H: 6.82. 3.6. Allyl 4,6-di-O-benzyl-2,3-dideoxy-D-threo-hex-2enopyranoside 7 Pale yellow oil; IR (CHCl3) 2985, 1585, 1454, 1101 cm1; 1H NMR (CDCl3, 400 MHz) d 7.27e7.17 (10H, m, ArH), 6.05 (1H, dd, J3,4¼5.6 Hz, J3,2¼10.0 Hz, H3), 5.93e5.81 (2H, m, H2, H20 ), 5.19 (1H, dd, J3a0 ,3b0 ¼1.6 Hz, J3a0 ,20 ¼17.2 Hz, H3a0 ), 5.09 (1H, dd, J3b0 ,3a0 ¼1.6 Hz, J3b0 ,3a0 ¼10.4 Hz, H3b0 ), 5.03 (1H, d, J1,2¼3.2 Hz, H1), 4.55 (1H, d, Ja,b¼12.0 Hz, CH2Ph), 4.54 (1H, d, Ja,b¼12.0 Hz, CH2Ph), 4.49 (1H, d, Ja,b¼11.6 Hz, CH2Ph), 4.47 (1H, d, J¼11.6 Hz, CH2Ph), 4.23e4.17 (1H, m, H10 ), 4.02e3.97 (1H, m, H5), 3.75 (1H, dd, J6a,5¼6.0 Hz, J6a,6b¼10.0 Hz, H6a), 3.70e3.65 (2H, m, H4, H6b); 13C NMR (CDCl3, 100 MHz) d 138.46, 138.33, 134.42, 129.60, 128.33, 128.32, 128.31, 128.30, 127.75, 127.74, 127.73, 127.55, 127.54, 127.53, 127.11, 117.37, 93.31, 73.39, 71.00, 69.54, 69.53, 68.71, 67.31; FABMS m/z (rel intens) 367 (MþHþ, 5), 91 (100); HRMS (FAB) m/z calcd for C23H27O4 (MþHþ) 367.1904, found 367.1910. Anal. Calcd for C23H26O4; C: 75.38; H: 7.15. Found: C: 75.39; H: 7.17. 3.7. Isopropyl 4,6-di-O-benzyl-2,3-dideoxy-D-threo-hex-2enopyranoside 8 Pale yellow oil; IR (CHCl3) 2972, 1642, 1493, 1094 cm1; 1H NMR (CDCl3, 400 MHz) d 7.26e7.17 (10H, m, ArH), 6.03 (1H, dd, J3,4¼5.2 Hz, J3,2¼10.0 Hz, H3), 5.87 (1H, dd, J2,1¼3.2 Hz, J2,3¼10.0 Hz, H2), 5.13 (1H, d, J1,2¼3.2 Hz, H1), 4.57e4.46 (4H, m, CH2Ph), 4.25e4.21 (1H, m, H5), 3.97e3.91 (1H, m, H10 ), 3.75 (1H, dd, J6a,5¼6.0 Hz, J6a,6b¼10.0 Hz, H6a), 3.69e3.64 (2H, m, H4, H6b), 1.15 (3H, d, J¼6.4 Hz, H20 ), 1.09 (3H, d, J¼6.4 Hz, H30 ); 13C NMR (CDCl3, 100 MHz) d 138.53, 138.36, 130.19, 128.29, 128.28, 128.27, 128.26, 127.72, 127.71, 127.57, 127.49, 127.48, 127.47, 126.77, 92.32, 73.37,

Y.-B. Chen et al. / Tetrahedron 71 (2015) 350e358

355

70.96, 69.69, 69.58, 69.27, 67.43, 23.58, 21.84; FABMS m/z (rel intens) 369 (MþHþ, 6), 91 (100); HRMS (FAB) m/z calcd for C23H29O4 (MþHþ) 369.2060, found 369.2069. Anal. Calcd for C23H28O4; C: 74.97; H: 7.66. Found: C: 74.95; H: 7.62.

71.79, 69.11, 68.90, 59.18, 56.71, 33.77, 32.10, 25.64, 24.44, 24.24; FABMS m/z (rel intens) 257 (MþHþ, 6), 91 (100); HRMS (FAB) m/z calcd for C14H25O4 (MþHþ) 257.1747, found 257.1749. Anal. Calcd for C14H24O4; C: 65.60; H: 9.44. Found: C: 65.64; H: 9.45.

3.8. Cyclohexyl 4,6-di-O-benzyl-2,3-dideoxy-D-threo-hex-2enopyranoside 9

3.12. p-Methylphenyl-4,6-di-O-benzyl-2,3-dideoxy-D-threohex-2-enopyranoside 33

Pale yellow oil; IR (CHCl3) 2970, 1510, 1105 cm1; 1H NMR (CDCl3, 400 MHz) d 7.26e7.17 (10H, m, ArH), 6.03 (1H, dd, J3,4¼5.2 Hz, J3,2¼10.0 Hz, H3), 5.89 (1H, dd, J2,1¼2.8 Hz, J2,3¼10.0 Hz, H2), 5.13 (1H, d, J1,2¼2.8 Hz, H1), 4.57e4.46 (4H, m, CH2Ph), 4.28e4.24 (1H, m, H5), 3.77e3.57 (4H, m, H4, H6, H10 ), 1.96e1.83 (2H, m, H20 ), 1.65e1.61 (2H, m, H60 ), 1.30e1.09 (6H, m, H30 , H40 , H50 ); 13C NMR (CDCl3, 100 MHz) d 138.55, 138.40, 130.35, 128.29, 128.28, 128.27, 128.26, 128.25, 127.70, 127.69, 127.57, 127.44, 127.43, 126.69, 92.24, 75.75, 73.33, 70.88, 69.71, 69.32, 67.50, 33.80, 32.09, 25.63, 24.42, 24.27; FABMS m/z (rel intens) 409 (MþHþ, 4), 91 (100); HRMS (FAB) m/z calcd for C26H33O4 (MþHþ) 409.2373, found 409.2382. Anal. Calcd for C26H32O4; C: 76.44; H: 7.90. Found: C: 76.48; H: 7.94.

Pale yellow oil; IR (CHCl3) 2945, 1605, 1408, 1099 cm1; 1H NMR (CDCl3, 500 MHz) d 7.25e7.14 (10H, m, ArH), 6.99e6.93 (4H, m, ArH), 6.18e6.15 (1H, m, H3), 6.04 (1H, dd, J2,1¼3.0 Hz, J2,3¼10.0 Hz, H2), 5.63 (1H, d, J1,2¼3.0 Hz, H1), 4.58 (1H, d, J¼12.0 Hz, CH2Ph), 4.52 (1H, d, J¼12.0 Hz, CH2Ph), 4.45 (1H, d, J¼11.5 Hz, CH2Ph), 4.41 (1H, d, J¼11.5 Hz, CH2Ph), 4.36e4.33 (1H, m, H5), 3.79 (1H, dd, J6a,5¼6.5 Hz, J6a,6b¼10.0 Hz, H6a), 3.74 (1H, dd, J4,5¼2.5 Hz, J4,3¼5.5 Hz, H4), 3.65 (1H, dd, J6b,5¼6.5 Hz, J6b,6a¼10.0 Hz, H6b), 2.21 (3H, s, CH3); 13C NMR (CDCl3, 125 MHz) d 155.19, 138.35, 138.22, 131.56, 129.88, 129.87, 128.77, 128.38, 128.37, 128.26, 128.25, 127.85, 127.83, 127.82, 127.72, 127.63, 127.62, 127.49, 127.23, 127.22, 93.22, 73.35, 71.28, 70.28, 69.25, 67.06, 20.56.

3.9. Benzyl 4,6-di-O-methyl-2,3-dideoxy-D-threo-hex-2enopyranoside 10 Pale yellow oil; IR (CHCl3) 2989, 1450, 1105 cm1; 1H NMR (CDCl3, 500 MHz) d 7.30e7.18 (5H, m, ArH), 6.13 (1H, dd, J3,4¼5.0 Hz, J3,2¼10.0 Hz, H3), 5.95 (1H, dd, J2,1¼3.0 Hz, J2,3¼10.0 Hz, H2), 5.06 (1H, d, J1,2¼3.0 Hz, H1), 4.74 (1H, d, J¼11.5 Hz, CH2Ph), 4.50 (1H, d, J¼11.5 Hz, CH2Ph), 4.20e4.17 (1H, m, H5), 3.60 (1H, dd, J6a,5¼5.5 Hz, J6a,6b¼10.0 Hz, H6a), 3.51 (1H, dd, J6b,5¼7.0 Hz, J6b,6a¼10.0 Hz, H6b), 3.44 (1H, dd, J4,5¼2.5 Hz, J4,3¼5.0 Hz, H4), 3.36 (3H, s, OCH3), 3.32 (1H, s, OCH3); 13C NMR (CDCl3, 125 MHz) d 137.88, 129.84, 128.39, 128.38, 128.20, 128.19, 127.69, 126.62, 93.24, 71.79, 69.63, 69.36, 69.04, 59.24, 56.71; FABMS m/z (rel intens) 265 (MþHþ, 5), 91 (100); HRMS (FAB) m/z calcd for C15H21O4 (MþHþ) 265.1434, found 265.1441. Anal. Calcd for C15H20O4; C: 68.16; H: 7.63. Found: C: 68.18; H: 7.66. 3.10. Isopropyl 4,6-di-O-methyl-2,3-dideoxy-D-threo-hex-2enopyranoside 11 Pale yellow oil; IR (CHCl3) 2968, 1490, 1105 cm1; 1H NMR (CDCl3, 500 MHz) d 6.11 (1H, dd, J3,4¼5.0 Hz, J3,2¼10.0 Hz, H3), 5.90 (1H, dd, J2,1¼3.0 Hz, J2,3¼10.0 Hz, H2), 5.07 (1H, d, J1,2¼3.0 Hz, H1), 4.17e4.14 (1H, m, H5), 3.96e3.91 (1H, m, H1), 3.58 (1H, dd, J6a,5¼5.5 Hz, J6a,6b¼10.0 Hz, H6a), 3.52 (1H, dd, J6b,5¼7.0 Hz, J6b,6a¼10.0 Hz, H6b), 3.43 (1H, dd, J4,5¼2.5 Hz, J4,3¼5.0 Hz, H4), 3.34 (3H, s, OCH3), 3.32 (3H, s, OCH3), 1.17 (3H, d, J¼6.5 Hz, CH3), 1.10 (3H, d, J¼6.5 Hz, CH3); 13C NMR (CDCl3, 125 MHz) d 130.38, 126.28, 92.30, 71.85, 69.75, 69.11, 68.91, 59.19, 56.71, 23.56, 21.87; FABMS m/z (rel intens) 217 (MþHþ, 3), 91 (100); HRMS (FAB) m/z calcd for C11H21O4 (MþHþ) 217.1434, found 217.1440. Anal. Calcd for C11H20O4; C: 61.09; H: 9.32. Found: C: 61.12; H: 9.35. 3.11. Cyclohexyl 4,6-di-O-methyl-2,3-dideoxy-D-threo-hex-2enopyranoside 12 Pale yellow oil; IR (CHCl3) 2959, 1550, 1105 cm1; 1H NMR (CDCl3, 500 MHz) d 6.10 (1H, dd, J3,4¼5.0 Hz, J3,2¼10.0 Hz, H3), 5.90 (1H, dd, J2,1¼3.0 Hz, J2,3¼10.0 Hz, H2), 5.11 (1H, d, J1,2¼3.0 Hz, H1), 4.19e4.16 (1H, m, H5), 3.61e3.57 (2H, m, H6a, H10 ), 3.52 (1H, dd, J6b,5¼7.0 Hz, J6b,6a¼10.0 Hz, H6b), 3.43 (1H, dd, J4,5¼2.5 Hz, J4,3¼5.0 Hz, H4), 3.34 (3H, s, OCH3), 3.32 (3H, s, OCH3), 1.90e1.82 (2H, m, H20 ), 1.68e1.66 (2H, m, H60 ), 1.33e1.09 (6H, m, H30 , H40 , H50 ); 13C NMR (CDCl3, 125 MHz) d 130.48, 126.22, 92.21, 75.74,

3.13. Typical procedure of glycosylation for preparing 2,3unsaturated glycopyranoside derivatives 17e23 To a solution of endo-galactal 1 (1.0 equiv) in anhydrous CH2Cl2 (1.5 mL) was added diethyl phosphorochloridite (1.2 equiv) and Et3N (3.0 equiv). The reaction mixture was stirred at 0  C under N2 for 30 min. After the phosphanylation was completed, one equivalent of AlCl3 and three equivalents of a sugar alcohol substrate were added into the reaction mixture and stirred at 0  C for 30 min. Once the reaction was completed, the resulting solution was added to CH2Cl2 (50 mL), washed with H2O (20 mL2), and brine (20 mL2). The organic layer was concentrated under reduced pressure and purified by column chromatography on silica gel (33% EtOAc in hexanes as eluant) to give 2,3-unsaturated glycopyranosides 17e23 in 81e86 % yields. 3.14. 4,6-Di-O-benzyl-2,3-dideoxy-D-threo-hex-2enopyranoside-(1/6)-1,2:3,4-di-O-isopropylidene-D-galactopyranoside 17 Pale yellow oil; IR (CHCl3) 2922, 1643, 1463, 1168 cm1; 1H NMR (CDCl3, 400 MHz) d 7.26e7.18 (10H, m, ArH), 6.02 (1H, dd, J3,4¼5.2 Hz, J3,2¼10.0 Hz, H3), 5.90 (1H, dd, J2,1¼2.8 Hz, J2,3¼10.0 Hz, H2), 5.43 (1H, d, J10,20 ¼5.2 Hz, H10 ), 5.04 (1H, d, J1,2¼2.8 Hz, H1), 4.57e4.45 (5H, m, H30 , CH2Ph), 4.23e4.18 (3H, m, H5, H20 , H40 ), 3.99e3.95 (1H, m, H50 ), 3.80e3.61 (5H, m, H4, H6, H60 ), 1.44 (3H, s, CH3), 1.35 (3H, s, CH3), 1.26 (3H, s, CH3), 1.24 (3H, s, CH3); 13C NMR (CDCl3, 100 MHz) d 138.46, 138.26, 129.43, 128.29, 128.28, 128.26, 128.25, 127.72, 127.71, 127.61, 127.60, 127.55, 127.47, 126.97, 109.18, 108.48, 96.30, 94.48, 73.35, 71.11, 70.83, 70.63, 70.60, 69.45, 69.14, 67.11, 66.60, 65.91, 26.04, 25.93, 24.89, 24.49; FABMS m/z (rel intens) 569 (MþHþ, 1), 253 (6), 181 (22), 91 (100); HRMS (FAB) m/z calcd for C32H41O9 (MþHþ) 569.2745, found 569.2755. Anal. Calcd for C32H40O9; C: 67.59; H: 7.09. Found: C: 67.63; H: 7.12. 3.15. 4,6-Di-O-benzyl-2,3-dideoxy-D-threo-hex-2enopyranoside-(1/6)-1,2,3,4-tetra-O-methyl-D-glucopyranoside 18 Pale yellow oil; IR (CHCl3) 2918, 1641, 1453, 1100 cm1; 1H NMR (CDCl3, 400 MHz) d 7.26e7.18 (10H, m, ArH), 6.03 (1H, dd, J3,4¼5.2 Hz, J3,2¼10.0 Hz, H3), 5.90 (1H, dd, J2,1¼2.8 Hz, J2,3¼10.0 Hz, H2), 5.07 (1H, d, J1,2¼2.8 Hz, H1), 4.70 (1H, d, J10,20 ¼3.6 Hz, H10 ), 4.57e4.47 (4H, m, CH2Ph), 4.20e4.16 (1H, m, H5), 3.92 (1H, dd, J6a0 ,50 ¼4.0 Hz, J6a0 ,6b0 ¼11.2 Hz, H6a0 ), 3.75 (1H, dd, J6a,5¼6.4 Hz,

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J6a,6b¼10.0 Hz, H6a), 3.67e3.60 (3H, m, H4, H6b, H6b0 ), 3.54e3.50 (4H, m, H50 , OCH3), 3.44e3.39 (7H, m, H30 , 2OCH3), 3.31 (3H, s, OCH3), 3.12e3.07 (2H, m, H20 , H40 ); 13C NMR (CDCl3, 50 MHz) d 138.46, 138.25, 129.43, 128.29, 128.28, 128.26, 128.25, 127.72, 127.71, 127.61, 127.60, 127.55, 127.47, 126.97, 109.18, 108.48, 96.30, 94.48, 73.35, 71.11, 70.83, 70.63, 70.60, 69.45, 69.14, 67.11, 66.60, 65.91, 26.04, 25.93, 24.89, 24.49; LRMS (EI) m/z (rel intens) 544 (Mþ, 1), 394 (11), 91 (100); HRMS (EI) m/z calcd for C30H40O9 (Mþ) 544.2672, found 544.2676. Anal. Calcd for C30H40O9; C: 66.16; H: 7.40. Found: C: 66.19; H: 7.45. 3.16. 4,6-Di-O-benzyl-2,3-dideoxy-D-threo-hex-2enopyranoside-(1/6)-1-methyl-2,3,4-tri-O-benzyl-D-glucopyranoside 19 Pale yellow oil; IR (CHCl3) 2920, 1643, 1454, 1099 cm1; 1H NMR (CDCl3, 400 MHz) d 7.28e7.14 (25H, m, ArH), 6.01 (1H, dd, J3,4¼5.2 Hz, J3,2¼10.0 Hz, H3), 5.91 (1H, dd, J2,1¼2.8 Hz, J2,3¼10.0 Hz, H2), 5.06 (1H, d, J1,2¼2.8 Hz, H1), 4.88 (1H, d, Ja,b¼10.8 Hz, CH2Ph), 4.76 (1H, d, Ja,b¼10.8 Hz, CH2Ph), 4.71 (1H, d, Ja,b¼10.8 Hz, CH2Ph), 4.69 (1H, d, Ja,b¼12.0 Hz, CH2Ph), 4.58 (1H, d, Ja,b¼10.8 Hz, CH2Ph), 4.57 (1H, d, Ja,b¼12.0 Hz, CH2Ph), 4.53 (1H, d, J10,20 ¼3.6 Hz, H10 ), 4.51 (1H, d, Ja,b¼10.8 Hz, CH2Ph), 4.45 (1H, d, Ja,b¼10.8 Hz, CH2Ph), 4.37 (1H, d, Ja,b¼12.0 Hz, CH2Ph), 4.31 (1H, d, Ja,b¼12.0 Hz, CH2Ph), 4.17e4.14 (1H, m, H5), 3.98 (1H, dd, J6a0 ,50 ¼3.6 Hz, J6a0 ,6b0 ¼11.2 Hz, H6a0 ), 3.90 (1H, dd, J30 ,40 ¼9.6 Hz, J30 ,20 ¼9.6 Hz, H30 ), 3.72e3.62 (4H, m, H4, H6a, H50 , H6b0 ), 3.57e3.48 (2H, m, H40 , H6b), 3.43 (1H, dd, J20 ,10 ¼3.6 Hz, J20 ,30 ¼9.6 Hz, H20 ), 3.28 (3H, s, OCH3); 13C NMR (CDCl3, 50 MHz) d 138.39, 138.34, 138.15, 138.14, 129.45, 128.41, 128.40, 128.39, 128.36, 128.35, 128.34, 128.33, 128.29, 128.28, 128.27, 128.02, 128.01, 127.89, 127.88, 127.87, 127.74, 127.73, 127.72, 127.71, 127.59, 127.52, 127.51, 127.50, 126.87, 98.01, 94.53, 81.97, 79.86, 77.77, 75.69, 74.94, 73.28, 73.27, 71.07, 70.00, 69.57, 69.08, 66.97, 66.64, 55.11; FABMS m/z (rel intens) 795 (MþNaþ, 2), 105 (32), 191 (100); HRMS (FAB) m/z calcd for C48H52NaO9 (MþNaþ) 795.3504, found 795.3507. Anal. Calcd for C48H52O9; C: 74.59; H: 6.78. Found: C: 74.66; H: 6.84. 3.17. 4,6-Di-O-benzyl-2,3-dideoxy-D-threo-hex-2enopyranoside-(1/3)-1,2:5,6-di-O-isopropylidene-D-glucofuranoside 20 Pale yellow oil; IR (CHCl3) 2926, 1646, 1463, 1095 cm1; 1H NMR (CDCl3, 400 MHz) d 7.27e7.17 (10H, m, ArH), 6.05 (1H, dd, J3,4¼5.2 Hz, J3,2¼10.0 Hz, H3), 5.89 (1H, dd, J2,1¼2.4 Hz, J2,3¼10.0 Hz, H2), 5.76 (1H, d, J10,20 ¼3.6 Hz, H10 ), 5.23 (1H, d, J1,2¼2.4 Hz, H1), 4.73 (1H, d, J20 ,10 ¼3.6 Hz, H20 ), 4.56 (1H, d, Ja,b¼12.0 Hz, CH2Ph), 4.54 (1H, d, Ja,b¼12.0 Hz, CH2Ph), 4.48 (1H, d, Ja,b¼12.0 Hz, CH2Ph), 4.44 (1H, d, Ja,b¼12.0 Hz, CH2Ph), 4.23 (1H, d, J30 ,40 ¼2.8 Hz, H30 ), 4.18e4.12 (2H, m, H5, H50 ), 4.04e3.99 (2H, m, H40 , H6b0 ), 3.89 (1H, dd, J6a0 ,50 ¼5.6 Hz, J6a0 ,6b0 ¼8.4 Hz, H6a0 ), 3.71 (1H, d, J6,5¼6.0 Hz, H6), 3.63 (1H, dd, J4,5¼2.4 Hz, J4,3¼5.2 Hz, H4), 1.39 (3H, s, CH3), 1.33 (3H, s, CH3), 1.25 (3H, s, CH3), 1.09 (3H, s, CH3); 13C NMR (CDCl3, 50 MHz) d 138.23, 138.22, 129.16, 128.38, 128.37, 128.36, 128.35, 127.75, 127.74, 127.73, 127.60, 127.59, 127.58, 127.04, 111.76, 109.03, 105.38, 95.71, 83.92, 81.41, 81.23, 73.66, 72.70, 70.90, 70.37, 69.96, 67.70, 67.29, 26.88, 26.82, 26.09, 25.39; FABMS m/z (rel intens) 569 (MþHþ, 3), 91 (100); HRMS (FAB) m/z calcd for C32H41O9 (MþHþ) 569.2745, found 569.2746. Anal. Calcd for C32H40O9; C: 67.59; H: 7.09. Found: C: 67.61; H: 7.12. 3.18. 4,6-Di-O-p-mthoxybenzyl-2,3-dideoxy-D-threo-hex-2enopyranoside-(1/6)-1,2:3,4-di-O-isopropylidene-D-galactopyranoside 21 Pale yellow oil; IR (CHCl3) 2952, 1645, 1464, 1170 cm1; 1H NMR (CDCl3, 500 MHz) d 7.18 (2H, d, J¼7.5 Hz, ArH), 7.12 (2H, d, J¼7.5 Hz,

ArH), 6.79 (2H, d, J¼7.5 Hz, ArH), 6.75 (2H, d, J¼7.5 Hz, ArH), 5.98 (1H, dd, J3,4¼5.5 Hz, J3,2¼10.0 Hz, H3), 5.88 (1H, dd, J2,1¼4.0 Hz, J2,3¼10.0 Hz, H2), 5.43 (1H, d, J1,2¼4.0 Hz, H1), 5.03 (1H, brs, H10 ), 4.50e4.39 (5H, m, CH2Ph, H40 ), 4.23e4.16 (3H, m, H20 , H30 , H5), 3.97e3.95 (1H, m, H50 ), 3.79e3.68 (9H, m, H60 , H6a, OCH3), 3.60e3.57 (2H, m, H4, H6b), 1.44 (3H, s, CH3), 1.36 (3H, s, CH3), 1.26 (3H, s, CH3), 1.24 (3H, s, CH3); 13C NMR (CDCl3, 125 MHz) d 159.19, 159.15, 130.59, 130.38, 129.40, 129.39, 129.33, 129.32, 129.21, 127.19, 113.77, 113.76, 113.71, 113.70, 109.21, 108.53, 96.33, 94.53, 73.03, 70.88, 70.84, 70.65, 70.61, 69.46, 68.82, 66.75, 66.60, 65.92, 55.23, 55.22, 26.08, 25.97, 24.93, 24.51; FABMS m/z (rel intens) 629 (MþHþ, 3), 91 (100); HRMS (FAB) m/z calcd for C34H45O11 (MþHþ) 629.2956, found 629.2961. Anal. Calcd for C34H44O11; C: 64.95; H: 7.05. Found: C: 64.98; H: 7.07. 3.19. 4,6-Di-O-p-mthoxybenzyl-2,3-dideoxy-D-threo-hex-2enopyranoside-(1/6)-1-methyl-2,3,4-tri-O-benzyl-D-glucopyranoside 22 Pale yellow oil; IR (CHCl3) 2920, 1640, 1501, 1109 cm1; 1H NMR (CDCl3, 500 MHz) d 7.26e7.17 (15H, m, ArH), 7.11e7.09 (2H, m, ArH), 7.07e7.05 (2H, m, ArH), 6.75e6.72 (4H, m, ArH), 5.98e5.96 (1H, m, H3), 5.90e5.87 (1H, m, H2), 5.04 (1H, brs, H1), 4.87 (1H, d, Ja,b¼11.0 Hz, CH2Ph), 4.75 (1H, d, Ja,b¼11.0 Hz, CH2Ph), 4.71 (1H, d, Ja,b¼11.0 Hz, CH2Ph), 4.69 (1H, d, Ja,b¼11.0 Hz, CH2Ph), 4.59e4.56 (2H, m, CH2Ph), 4.53 (1H, brs, H10 ), 4.44 (1H, d, Ja,b¼11.0 Hz, CH2Ph), 4.38 (1H, d, Ja,b¼11.0 Hz, CH2Ph), 4.30 (1H, d, Ja,b¼12.0 Hz, CH2Ph), 4.24 (1H, d, Ja,b¼12.0 Hz, CH2Ph), 4.11 (1H, brs, H5), 3.98e3.96 (1H, m, H50 ), 3.91e3.88 (1H, m, H30 ), 3.70e3.64 (9H, m, H60 , H6a, OCH3), 3.58 (1H, brs, H4), 3.51e3.42 (3H, m, H40 , H20 , H6b), 3.28 (3H, s, OCH3); 13C NMR (CDCl3, 125 MHz) d 159.18, 159.09, 138.81, 138.39, 138.19, 130.56, 130.31, 129.38, 129.37, 129.25, 129.20, 129.19, 128.42, 128.41, 128.40, 128.37, 128.36, 128.34, 128.33, 128.32, 128.04, 128.03, 127.90, 127.89, 127.85, 127.77, 127.76, 127.63, 127.51, 127.05, 113.70, 113.69, 98.04, 94.57, 82.00, 79.92, 77.81, 75.70, 74.96, 73.31, 72.97, 70.82, 70.03, 69.61, 68.81, 66.66, 66.65, 55.23, 55.22, 55.13; FABMS m/z (rel intens) 833 (MþHþ, 4), 91 (100); HRMS (FAB) m/z calcd for C50H57O11 (MþHþ) 833.3895, found 8.33.3897. Anal. Calcd for C50H56O11; C: 72.10; H: 6.78. Found: C: 72.15; H: 6.84. 3.20. Typical glycosylation procedure (synthesis of 23) To a solution of exo-galactal 2 (1.0 equiv) in anhydrous CH2Cl2 (1.5 mL) was added diethyl phosphorochloridite (1.2 equiv) and Et3N (3.0 equiv). The reaction mixture was stirred at 0  C under N2 for 30 min. After the phosphanylation was completed, one equivalent of AlCl3 and three equivalents of methanol were added into the reaction mixture and stirred at 0  C for 30 min. Once the reaction was completed, the resulting solution was added to CH2Cl2 (50 mL), washed with H2O (20 mL2), and brine (20 mL2). The organic layer was concentrated under reduced pressure and purified by column chromatography on silica gel (15% EtOAc in hexanes as eluant) to give 23 in 85% yields. 3.21. Methyl 4,5,6,8-tetra-O-benzyl-1,2-dideoxy-a-D-galactooct-3-ulo-1-enopyranoside 23 IR (CHCl3) 2960, 1642, 1493, 1094 cm1; 1H NMR (CDCl3, 400 MHz) d 7.28e7.17 (20H, m, ArH), 5.77 (1H, dd, J10,2b0 ¼10.8 Hz, J10,2a0 ¼17.6 Hz, H10 ), 5.43 (1H, dd, J2a0 ,2b0 ¼2.0 Hz, J2a0 ,10 ¼17.6 Hz, H2a0 ), 5.21 (1H, dd, J2b0 ,2a0 ¼2.0 Hz, J2b0 ,10 ¼10.8 Hz, H2b0 ), 4.88 (1H, d, Ja,b¼11.6 Hz, CH2Ph), 4.77 (1H, d, Ja,b¼10.8 Hz, CH2Ph), 4.69 (1H, d, Ja,b¼11.6 Hz, CH2Ph), 4.64 (1H, d, Ja,b¼11.6 Hz, CH2Ph), 4.54 (1H, d, Ja,b¼10.8 Hz, CH2Ph), 4.53 (1H, d, Ja,b¼11.6 Hz, CH2Ph), 4.43 (1H, d, Ja,b¼12.0 Hz, CH2Ph), 4.37 (1H, d, Ja,b¼12.0 Hz, CH2Ph), 3.97 (1H, dd, J3,4¼2.8 Hz, J3,2¼10.0 Hz, H3), 3.90 (1H, dd, J4,5¼0.8 Hz, J4,3¼2.8 Hz, H4), 4.06e4.03 (1H, m, H5), 3.94 (1H, brs, H4), 3.72 (1H, d,

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J2,3¼9.6 Hz, H2), 3.79e3.72 (2H, m, H2, H5), 3.56 (1H, dd, J6a,5¼6.8 Hz, J6a,6b¼9.6 Hz, H6a), 3.51 (1H, dd, J6b,5¼6.0 Hz, J6b,6a¼9.6 Hz, H6b), 3.07 (3H, s, OCH3); 13C NMR (CDCl3, 100 MHz) d 138.93, 138.71, 138.18, 138.09, 134.64, 128.64, 128.63, 128.62, 128.35, 128.34, 128.33, 128.32, 128.14, 128.13, 128.12, 128.11, 127.98, 127.97, 127.96, 127.69, 127.68, 127.67, 127.64, 127.63, 127.55, 127.48, 127.47, 127.46, 127.45, 118.99, 100.01, 80.49, 80.31, 76.05, 75.09, 74.47, 73.46, 73.01, 70.36, 69.12, 49.07; FABMS m/z (rel intens) 581 (MþHþ, 4), 91 (100); HRMS (FAB) m/z calcd for C37H41O6 (MþHþ) 581.2898, found 581.2902. Anal. Calcd for C37H40O6; C: 76.53; H: 6.94. Found: C: 76.59; H: 6.97. 3.22. n-Butyl 4,5,6,8-tetra-O-benzyl-1,2-dideoxy-a-D-galactooct-3-ulo-1-enopyranoside 24 IR (CHCl3) 2950, 1641, 1453,1090 cm1; 1H NMR (CDCl3, 400 MHz)

d 7.27e7.18 (20H, m, ArH), 5.82 (1H, dd, J10,2b0 ¼10.8 Hz, J10,2a0 ¼17.6 Hz,

H10 ), 5.41 (1H, dd, J2a0 ,2b0 ¼2.0 Hz, J2a0 ,10 ¼17.6 Hz, H2a0 ), 5.14 (1H, dd, J2b0 ,2a0 ¼2.0 Hz, J2b0 ,10 ¼10.8 Hz, H2b0 ), 4.87 (1H, d, Ja,b¼11.6 Hz, CH2Ph), 4.79 (1H, d, Ja,b¼11.6 Hz, CH2Ph), 4.68 (1H, d, Ja,b¼11.6 Hz, CH2Ph), 4.64 (1H, d, Ja,b¼11.6 Hz, CH2Ph), 4.55 (1H, d, Ja,b¼11.6 Hz, CH2Ph), 4.53 (1H, d, Ja,b¼11.6 Hz, CH2Ph), 4.43 (1H, d, Ja,b¼12.0 Hz, CH2Ph), 4.38 (1H, d, Ja,b¼12.0 Hz, CH2Ph), 4.00 (1H, dd, J3,4¼2.8 Hz, J3,2¼10.0 Hz, H3), 3.92 (1H, dd, J4,5¼1.2 Hz, J4,3¼2.8 Hz, H4), 3.82e3.79 (1H, m, H5), 3.75 (1H, d, J2,3¼10.0 Hz, H2), 3.56 (1H, dd, J6a,5¼7.2 Hz, J6a,6b¼9.6 Hz, H6a), 3.50 (1H, dd, J6b,5¼6.0 Hz, J6b,6a¼9.6 Hz, H6b), 3.33e3.19 (2H, m, H100 ), 1.50e1.45 (2H, m, H200 ), 1.26e1.18 (2H, m, H300 ), 0.81 (3H, t, J400 ,30 ¼7.2 Hz, H400 ); 13C NMR (CDCl3,100 MHz) d 139.02,138.80,138.57, 138.19, 135.71, 128.37, 128.36, 128.35, 128.31, 128.30, 128.29, 128.17, 128.16, 128.15, 128.11, 128.10, 127.99, 127.98, 127.74, 127.73, 127.72, 127.47, 127.46, 127.44, 127.43, 118.30, 99.87, 80.64, 80.35, 75.65, 75.06, 74.47, 73.44, 72.86, 70.30, 69.15, 61.60, 31.83, 19.57, 13.97; FABMS m/z (rel intens) 623 (MþHþ, 3), 91 (100); HRMS (FAB) m/z calcd for C40H47O6 (MþHþ) 623.3367, found 623.3373. Anal. Calcd for C40H46O6; C: 77.14; H: 7.44. Found: C: 77.19; H: 7.48. 3.23. n-Hexyl 4,5,6,8-tetra-O-benzyl-1,2-dideoxy-a-D-galactooct-3-ulo-1-enopyranoside 25

357

J2a0 ,10 ¼17.6 Hz, J2a0 ,2b0 ¼1.6 Hz, H2a0 ), 5.25 (1H, dd, J3a00 ,200 ¼11.6 Hz, J3a00 ,3b00 ¼1.6 Hz, H3a00 ), 5.22 (1H, dd, J2b0 ,10 ¼17.6 Hz, J2b0 ,2a0 ¼1.6 Hz, H2b0 ), 5.08 (1H, dd, J3b00 ,200 ¼10.4 Hz, H3b00 ), 4.95 (1H, d, Ja,b¼11.6 Hz, CH2Ph), 4.87 (1H, d, Ja,b¼11.2 Hz, CH2Ph), 4.75 (1H, d, Ja,b¼11.6 Hz, CH2Ph), 4.72 (1H, d, Ja,b¼11.6 Hz, CH2Ph), 4.63 (1H, d, Ja,b¼11.2 Hz, CH2Ph), 4.60 (1H, d, Ja,b¼11.6 Hz, CH2Ph), 4.49 (1H, d, Ja,b¼12.0 Hz, CH2Ph), 4.45 (1H, d, Ja,b¼12.0 Hz, CH2Ph), 3.99e3.83 (5H, m, H2, H4, H5, H6a, H6b), 3.64e3.55 (2H, m, H1a00 , H1b00 ); 13C NMR (CDCl3, 100 MHz) d 138.99, 138.77, 138.45, 138.17, 135.26, 135.11, 128.40, 128,39, 128.38, 128.37, 128.36, 128.19, 128.18, 128.17, 128.16, 128.15, 128.02, 128.01, 127.69, 127.68, 127.67, 127.49, 127.48, 127.47, 127.46, 127.45, 118.65, 116.30, 100.25, 80.39, 75.76, 75.04, 74.51, 73.41, 72.91, 70.45, 69.02, 63.06; FABMS m/z (rel intens) 607 (MþHþ, 7), 91 (100); HRMS (FAB) m/z calcd for C39H43O6 (MþHþ) 607.3054, found 607.3061. Anal. Calcd for C39H42O6; C: 77.20; H: 6.98. Found: C: 77.24; H: 6.73. 3.25. Benzyl 4,5,6,8-tetra-O-benzyl-1,2-dideoxy-a-D-galactooct-3-ulo-1-enopyranoside 27 IR (CHCl3) 2950, 1651, 1503, 1102 cm1; 1H NMR (CDCl3, 400 MHz) d 7.25e7.16 (25H, m, ArH), 5.91 (1H, dd, J10,2b0 ¼10.8 Hz, J10,2a0 ¼17.6 Hz, H10 ), 5.51 (1H, dd, J2a0 ,2b0 ¼2.0 Hz, J2a0 ,10 ¼17.6 Hz, H2a0 ), 5.21 (1H, dd, J2b0 ,2a0 ¼2.0 Hz, J2b0 ,10 ¼10.8 Hz, H2b0 ), 4.86 (1H, d, Ja,b¼11.6 Hz, CH2Ph), 4.82 (1H, d, Ja,b¼11.2 Hz, CH2Ph), 4.66 (1H, d, Ja,b¼11.6 Hz, CH2Ph), 4.63 (1H, d, Ja,b¼11.6 Hz, CH2Ph), 4.57 (1H, d, Ja,b¼11.2 Hz, CH2Ph), 4.52 (1H, d, Ja,b¼11.6 Hz, CH2Ph), 4.44 (1H, d, Ja,b¼12.8 Hz, CH2Ph), 4.38e4.32 (3H, m, CH2Ph), 4.04 (1H, dd, J3,4¼2.8 Hz, J3,2¼10.0 Hz, H3), 3.90 (1H, dd, J4,5¼1.2 Hz, J4,3¼2.8 Hz, H4), 3.81 (1H, d, J2,3¼10.0 Hz, H2), 3.79e3.76 (1H, m, H5), 3.53 (1H, dd, J6a,5¼7.2 Hz, J6a,6b¼9.6 Hz, H6a), 3.43 (1H, dd, J6b,5¼6.0 Hz, J6b,6a¼9.6 Hz, H6b); 13C NMR (CDCl3, 100 MHz) d 139.02, 138.89, 138.74, 138.68, 138.18, 135.41, 128.35, 128,34, 128.29, 128.28, 128.17, 128.16, 128.15, 128.14, 128.13, 128.12, 128.11, 128.10, 127.98, 127.97, 127.69, 127.68, 127.61, 127.49, 127.48, 127.47, 127.46, 127.42, 127.41, 127.40, 127.01, 118.84, 100.54, 80.62, 80.20, 75.56, 74.98, 74.48, 73.36, 72.80, 70.53, 68.99, 63.78; FABMS m/ z (rel intens) 657 (MþHþ, 3), 91 (100); HRMS (FAB) m/z calcd for C43H45O6 (MþHþ) 657.3211, found 657.3214. Anal. Calcd for C43H44O6; C: 78.63; H: 6.75. Found: C: 78.65; H: 6.78.

IR (CHCl3) 2958, 1640, 1498, 1100 cm1; 1H NMR (CDCl3, 400 MHz) d 7.26e7.17 (20H, m, ArH), 5.77 (1H, dd, J10,2b0 ¼10.8 Hz, J10,2a0 ¼17.6 Hz, H10 ), 5.41 (1H, dd, J2a0 ,2b0 ¼2.0 Hz, J2a0 ,10 ¼17.6 Hz, H2a0 ), 5.14 (1H, dd, J2b0 ,2a0 ¼2.0 Hz, J2b0 ,10 ¼10.8 Hz, H2b0 ), 4.87 (1H, d, Ja,b¼11.6 Hz, CH2Ph), 4.79 (1H, d, Ja,b¼11.6 Hz, CH2Ph), 4.68 (1H, d, Ja,b¼11.6 Hz, CH2Ph), 4.64 (1H, d, Ja,b¼11.6 Hz, CH2Ph), 4.55 (1H, d, Ja,b¼11.6 Hz, CH2Ph), 4.53 (1H, d, Ja,b¼11.6 Hz, CH2Ph), 4.43 (1H, d, Ja,b¼12.0 Hz, CH2Ph), 4.37 (1H, d, Ja,b¼12.0 Hz, CH2Ph), 4.00 (1H, dd, J3,4¼2.8 Hz, J3,2¼10.0 Hz, H3), 3.92 (1H, dd, J4,5¼1.6 Hz, J4,3¼2.8 Hz, H4), 3.82e3.79 (1H, m, H5), 3.75 (1H, d, J2,3¼10.0 Hz, H2), 3.56 (1H, dd, J6a,5¼7.2 Hz, J6a,6b¼9.6 Hz, H6a), 3.50 (1H, dd, J6b,5¼6.0 Hz, J6b,6a¼9.6 Hz, H6b), 3.32e3.18 (2H, m, H100 ), 1.51e1.46 (2H, m, H200 ), 1.25e1.17 (6H, m, H300 , H400 , H500 ), 0.79 (3H, t, J600 ,50 ¼7.2 Hz, H600 ); 13C NMR (CDCl3, 100 MHz) d 139.02, 138.81, 138.77, 138.20, 135.74, 128.28, 128,27, 128.26, 128.25, 128.14, 128.13, 128.12, 128.11, 128.10, 128.09, 127.99, 127.98, 127.97, 127.66, 127.65, 127.43, 127.42, 127.41, 127.40, 127.39, 118.28, 99.90, 80.64, 80.35, 75.62, 75.09, 74.46, 73.44, 72.85, 70.32, 69.18, 61.91, 31.66, 29.68, 25.98, 22.61, 14.04; FABMS m/z (rel intens) 651 (MþHþ, 5), 91 (100); HRMS (FAB) m/z calcd for C42H51O6 (MþHþ) 651.3680, found 651.3682. Anal. Calcd for C42H50O6; C: 77.51; H: 7.74. Found: C: 77.54; H: 7.79.

3.26. Isopropyl 4,5,6,8-tetra-O-benzyl-1,2-dideoxy-a-D-galacto-oct-3-ulo-1-enopyranoside 28

3.24. Allyl 4,5,6,8-tetra-O-benzyl-1,2-dideoxy-a-D-galacto-oct3-ulo-1-enopyranoside 26

3.27. Cyclohexyl 4,5,6,8-tetra-O-benzyl-1,2-dideoxy-a-D-galacto-oct-3-ulo-1-enopyranoside 29

IR (CHCl3) 2960,1642, 1493,1094 cm1; 1H NMR (CDCl3, 400 MHz) d 7.36e7.25 (20H, m, ArH), 5.98e5.87 (2H, m, H10, H200 ), 4.52 (1H, dd,

IR (CHCl3) 2945, 1632, 1499, 1104 cm1; 1H NMR (CDCl3, 400 MHz) d 7.28e7.16 (20H, m, ArH), 6.96 (1H, dd, J10,2b0 ¼11.2 Hz,

IR (CHCl3) 2935, 1635, 1498, 1090 cm1; 1H NMR (CDCl3, 400 MHz) d 7.26e7.17 (20H, m, ArH), 5.95 (1H, dd, J10,2b0 ¼10.8 Hz, J10,2a0 ¼17.6 Hz, H10 ), 5.44 (1H, dd, J2a0 ,2b0 ¼2.0 Hz, J2a0 ,10 ¼17.6 Hz, H2a0 ), 5.11 (1H, dd, J2b0 ,2a0 ¼2.0 Hz, J2b0 ,10 ¼10.8 Hz, H2b0 ), 4.86 (1H, d, Ja,b¼11.6 Hz, CH2Ph), 4.83 (1H, d, Ja,b¼11.2 Hz, CH2Ph), 4.68 (2H, brs, CH2Ph), 4.54 (1H, d, Ja,b¼11.6 Hz, CH2Ph), 4.52 (1H, d, Ja,b¼11.6 Hz, CH2Ph), 4.43 (1H, d, Ja,b¼12.0 Hz, CH2Ph), 4.39 (1H, d, Ja,b¼12.0 Hz, CH2Ph), 4.02e3.95 (3H, m, H3, H4, H5), 3.93e3.85 (1H, m, H100 ), 3.71 (1H, d, J2,3¼10.0 Hz, H2), 3.58 (1H, dd, J6a,5¼7.6 Hz, J6a,6b¼9.6 Hz, H6a), 3.50 (1H, dd, J6b,5¼6.0 Hz, J6b,6a¼9.6 Hz, H6b), 1.09 (1H, d, J¼2.4 Hz, H200 ),1.07 (1H, d, J¼2.4 Hz, H300 ); 13C NMR (CDCl3,100 MHz) d 139.03,138.78,138.70,138.15,136.47,128.37,128,36,128.29,128.28, 128.15, 128.14, 128.13, 128.12, 127.94, 127.93, 127.90, 127.89, 127.64, 127.63, 127.62, 127.39, 127.38, 127.37, 127.36, 127.33, 117.45, 100.17, 81.08, 80.45, 75.56, 74.65, 74.34, 73.41, 72.59, 70.22, 69.00, 65.27, 24.17, 23.06; FABMS m/z (rel intens) 609 (MþHþ, 4), 91 (100); HRMS (FAB) m/z calcd for C39H45O6 (MþHþ) 609.3211, found 609.3215. Anal. Calcd for C39H44O6; C: 76.95; H: 7.29. Found: C: 76.98; H: 7.31.

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J10,2a0 ¼17.6 Hz, H10 ), 5.46 (1H, dd, J2a0 ,2b0 ¼2.4 Hz, J2a0 ,10 ¼17.6 Hz, H2a0 ), 5.10 (1H, dd, J2b0 ,2a0 ¼2.4 Hz, J2b0 ,10 ¼11.2 Hz, H2b0 ), 4.87 (1H, d, Ja,b¼12.0 Hz, CH2Ph), 4.84 (1H, d, Ja,b¼12.0 Hz, CH2Ph), 4.67 (1H, d, Ja,b¼12.0 Hz, CH2Ph), 4.64 (1H, d, Ja,b¼12.0 Hz, CH2Ph), 4.54 (1H, d, Ja,b¼11.6 Hz, CH2Ph), 4.53 (1H, d, Ja,b¼11.6 Hz, CH2Ph), 4.44 (1H, d, Ja,b¼12.0 Hz, CH2Ph), 4.39 (1H, d, Ja,b¼12.0 Hz, CH2Ph), 4.05e4.02 (1H, m, H5), 3.98 (1H, dd, J3,4¼1.6 Hz, J3,2¼9.6 Hz, H3), 3.94 (1H, dd, J4,5¼0.8 Hz, J4,3¼2.4 Hz, H4), 3.70 (1H, d, J2,3¼9.6 Hz, H2), 3.58 (1H, dd, J6a,5¼7.2 Hz, J6a,6b¼9.6 Hz, H6a), 3.55e3.47 (2H, m, H6b, H100 ), 1.78e1.71 (2H, m, H200 ), 1.63e1.59 (2H, m, H600 ), 1.35e1.05 (6H, m, H6b, H300 , H400 , H500 ); 13C NMR (CDCl3, 50 MHz) d 139.14, 138.85, 138.28, 136.70, 128.29, 128.28, 128.27, 128.14, 128.13, 128.12, 128.11, 128.10, 127.87, 127.81, 127.80, 127.61, 127.60, 127.59, 127.58, 127.39, 127.38, 127.37, 127.36, 127.35, 117.39, 100.30, 81.31, 80.37, 75.37, 74.90, 74.37, 73.41, 72.63, 71.48, 70.34, 69.14, 34.18, 33.88, 25.56, 24.87, 24.65; FABMS m/z (rel intens) 649 (MþHþ, 3), 91 (100); HRMS (FAB) m/z calcd for C42H49O6 (MþHþ) 649.3524, found 649.3527. Anal. Calcd for C42H48O6; C: 77.75; H: 7.46. Found: C: 77.78; H: 7.48. Acknowledgements The authors thank the Ministry of Science and Technology of Taiwan (MOST 103-2113-M-039-004) and China Medical University, Taiwan (CMU95-194) for their financial support. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.tet.2014.11.027. References and notes 1. (a) Borrachero-Moya, P.; Cabrera-Escribano, F.; Gomez-Guillen, M.; ParadesLeon, M. D. Carbohydr. Res. 1998, 308, 181e190; (b) Schmidt, R. R.; Angerbauer, R. Carbohydr. Res. 1979, 72, 272e275. 2. Williams, N. R.; Wander, J. D. The Carbohydrates in Chemistry and Biochemistry; Academic: New York, NY, 1980; p 761. 3. (a) Chambers, D. J.; Evans, G. R.; Fairbanks, A. J. Tetrahedron: Asymmetry 2005, 16, 45e55; (b) Dorgan, B. J.; Jackson, R. F. W. Synlett 1996, 859e861. 4. (a) Seeberger, P. H.; Bilodeau, M. T.; Danishefsky, S. J. Aldrichim. Acta 1977, 30, 75e92; (b) Danishefsky, S. J.; Bilodeau, M. T. Angew. Chem., Int. Ed. 1996, 35, 1380e1419. 5. Schmidt, R. R.; Angerbauer, R. Carbohydr. Res. 1981, 89, 159e162. 6. (a) Zhang, G.; Shi, L.; Liu, Q.; Wang, J.; Li, L.; Liu, X. Tetrahedron 2007, 63, 9705e9711; (b) Domon, D.; Fujiwara, K.; Ohtaniuchi, Y.; Takezawa, A.; Takeda, S.; Kawaski, H.; Murai, A.; Kawai, H.; Suzuki, T. Tetrahedron Lett. 2005, 46, 8279e8283; (c) Krohn, K.; Gehle, D.; Kamp, O.; Ree, T. V. J. Carbohydr. Chem. 2003, 22, 377e383; (d) Reddy, B. G.; Vankar, Y. D. Tetrahedron Lett. 2003, 44, 4765e4767; (e) Durham, T. B.; Miller, M. J. Org. Lett. 2002, 4, 135e138; (f) Lewis, A.; Stefanuti, I.; Swain, S. A.; Smith, S. A.; Taylor, R. J. K. Tetrahedron Lett. 2001, 42, 5549e5552; (g) Patterson, I.; Keown, L. E. Tetrahedron Lett. 1997, 38, 5727e5730. 7. (a) Ferrier, R. J.; Sankey, G. H. J. Chem. Soc. C 1966, 2345e2349; (b) Wieczorek, B.; Thiem, J. J. Carbohydr. Chem. 1998, 17, 785e809; (c) Csuk, R.; Shaad, M.; Krieger, C. Tetrahedron 1996, 52, 6397e6408; (d) Ferrier, R. J.; Zubkov, O. A. Org. React. 2004, 62, 569e736. 8. (a) Nicolaou, K. C.; Snyder, S. A.; Nalbandian, A. Z.; Longbottom, D. A. J. Am. Chem. Soc. 2004, 126, 6234e6235; (b) Fuerstner, A.; Jeanjean, F.; Razon, P. Angew. Chem., Int. Ed. 2002, 41, 2097e2101; (c) Tiwari, P.; Misra, A. K. J. Org. Chem. 2006, 71, 2911e2913; (d) Kim, J. Y.; Bussolo, V. D.; Gin, D. Y. Org. Lett. 2001, 3, 303e306. rez-Pe rez, M. J.; Doboszewski, B.; Rozenski, J.; Herdewijn, P. Tetrahedron: 9. (a) Pe Asymmetry 1995, 6, 973e984; (b) Meng, X. B.; Han, D.; Zhang, S. N.; Guo, W.; Cui, J. R.; Li, Z. J. Carbohydr. Res. 2007, 342, 1169e1174. 10. (a) Lafont, D.; D’Attoma, J.; Gomez, R.; Goekjian, P. G. Tetrahedron: Asymmetry 2011, 22, 1197e1204; (b) Schimmel, J.; Eleuterio, M. I. P.; Ritter, G.; Schmidt, R. R. Eur. J. Org. Chem. 2006, 1701e1721; (c) Cheshev, P.; Marra, A.; Dondoni, A. Carbohydr. Res. 2006, 341, 2714e2716; (d) Upreti, M.; Ruhela, D.; Vishwakarma, R. A. Tetrahedron 2000, 56, 6577e6584. 11. (a) Honda, E.; Gin, D. Y. J. Am. Chem. Soc. 2002, 124, 7343e7352; (b) Yang, J.; Mercer, G. J.; Nguyen, H. M. Org. Lett. 2007, 9, 4231e4234; (c) Bussolo, V. D.; Kim, Y. J.; Gin, D. Y. J. Am. Chem. Soc. 1998, 120, 13515e13516. 12. (a) Descotes, G.; Martin, J. C. Carbohydr. Res. 1977, 56, 168e172; (b) Klaffke, W.; Pudlo, P.; Springer, D.; Thiem, J. Liebigs Ann. Chem. 1991, 6, 509e512; (c) Ferrier, R. J. Top. Curr. Chem. 2001, 215, 153e175; (d) Ferrier, R. J.; Hoberg, J. O. Adv. Carbohydr. Chem. Biochem. 2003, 58, 55e119.

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