A light-initiated process for rapid debenzylation of carbohydrates

Abstract

were determined with the following spectrometers: UCB-200 and UCB-250 (super-conducting, FT instruments operating at 200 and 250 MHz). Chemical shifts are expressed in ppm downfield from tetramethylsilane using tetramethylsilane as an internal standard for 'H NMR and chloroform (77.0) for 13C NMR. ' H NMR data are tabulated in the order of: multiplicity (s, singlet; d, doublet; t, triplet, q, quartet; m, multiplet), number of protons, coupling constants in hertz. The ultrasonic waves were produced by a 250-W high-intensity ultrasonic processor, Vibra Cell from Sonics & Materials Inc. During our reactions the power meter showed an average power of 23% corresponding to a power of about 45 W cm-2. Ultrasonic irradiation was carried out with the tip of the horn immersed directly in the solution.General Procedure for the Decomposition of Esters by Sonication. The ester was dissolved in the solvent system listed and irradiated with ultrasonic waves under an argon atmosphere until thin-layer chromatography indicated complete consumption of starting ester.1° A 20 "C water bath was kept around the reaction vessel during sonication, and this maintained the internal reaction temperature a t 20 "C up to a maximum of 35 "C. Aluminum foil was used to surround the reaction vessel to completely exclude light from the reaction mixture. There was no need to stir the reaction mixture during the sonication process. The solvent was then evaporated under reduced pressure, and the residue was purified by chromatography on silica gel using a solvent gradient, generally pentane, followed by ethyl acetatepentane mixtures. The compounds obtained by this method are listed below.Pentadecane (2). Sonication of a solution of ester 1 (0.105 g, 0.287 mmol), carbon tetrachloride (10 mL), and thiophenol (0.088 mL, 0.857 mmol, 3 equiv) for 20 min gave, after chromatography (pentane), fractions A and B.Fraction A contained 2 as a clear liquid (0.051 g, 85%): identical with an authentic sample.Fraction B contained 5 as yellow-white crystals (0.088 g, 70%): identical with an authentic sample.The column was further eluted (ethyl acetate) to give 4 as a yellowish solid (0.028 g, 87%): identical with an authentic sample.Pentadecylselenobenzene (6) a n d 1-Chloropentadecane (7). Sonication of a solution of ester 1 (0.26 mmol, 1.0 equiv), carbon tetrachloride (10 mL), and diphenyl diselenide (0.26 mmol, 1.0 equiv or 0.52 mmol, 2.0 equiv) for 30 min gave, after chromatography (pentane), 6 (77% or 80%, see table) ['H NMR (200 MHz, CDC13) 6 0.90 (t, 3, J = 7.0 Hz), 1.2-1.4 (m, 24), 1.72 (tt, 2, J = 7.0, 7.0 Hz), 2.95 (t, 2, J = 7.0 Hz), 7.26 (m, 3), 7.53 (m, 2); identical with the published spectrumss] and 7 (10% or 0%, see table), identical with an authentic ample.^ The column was further eluted (pentane 90%, ethyl acetate 10%) to give 8 (68-88%, see table): 'H NMR (200 MHz, CDCl,) 6 7.02-7.08 (m, l), 7.26-7.29 (m, 3), 7.57-7.69 (m, 4 ) , 8.45-8.47 (m, 1); I3C NMR (200 MHz, CDCl,) 6 149.40,137.11,131.61, 129.82, 129.24, 127.73, 121.27, 120.62; decomposed sl...

Side-bridged cyclam transition metal complexes bearing a phenolic ether or a phenolate pendent arm

“…57 A light-initiated process was also attempted. The rapid debenzylation of sterically hindered benzyl ethers is reported by Binkley and Hehemann, 63 which has been further developed by Riley and Grindley. 64 However, the conditions described did not result in the desired product being isolated.…”

Introduction of 2,2,2‐Trichloroethyl‐Protected Sulfates into Monosaccharides with a Sulfuryl Imidazolium Salt and Application to the Synthesis of Sulfated Carbohydrates

“…However, the benzyl group in 19 was selectively cleaved to give 34 in 91 % yield by irradiation with a 250-W lamp in the presence of NBS and CaCO 3 . [11] As mentioned earlier, Karst et al removed the TFE moiety from sulfate groups in fully protected disaccharides with low to moderate yields. [5] We anticipated that this would not be an issue with the TCE group as it is removed under very mild conditions.…”

Debenzylation of Benzyl-Protected Methylcellulose

“…There have been improvements in the benzylation of β-glucans [19][20][21][22][23] and some examples of debenzylation of low molecular weight (M w ) synthetic cellulose (with a degree of polymerization (DP) around 20) [12,[24][25][26][27], as well as of regioselectively synthesized cellulose derivatives [28][29][30] by heterogeneous catalytic hydrogenation. However, facile debenzylation of high M w benzylated β-glucans remained an issue mainly due to their low solubility and the tendency of deprotected products to adsorb on the catalysts (which rendered the catalysts inefficient) in the case of hydrogenation in the presence of Pd/C [31,32]. In particular, the linear β-1,4-glucan cellulose is a critical example in this regard.…”

“…Alkaline treatment of the esters formed will be the last step of debenzylation. To suppress the local concentration of hydrobromic acid (HBr)-which is a side-product of free radical bromination and promotes hydrolysis of glycosidic linkages of the carbohydrates-Binkley et al [31] performed debenzylation of several monosaccharide derivatives using NBS as a source of bromine radicals instead of elemental bromine, an incandescent lamp, and a vigorously stirred aqueous suspension of calcium carbonate in carbon tetrachloride to neutralize the formed HBr.…”