Acetate, selective deacetylation

A. H. Haines, P. Konowicz, and H. F. Jones, Selective deacetylation of sucrose octa-acetate with primary amines to give 2,3,4,6,l, 6 -hexa-0-acetylsucrose, Carbohydr. Res., 205 (1990) 406 109. 

Mansson P (1982) Selective deacetylation of aromatic acetates by aminolysis Tetrahedron Lett 23 1845-1846... 

For the preparation of trichloroacetimidates used in glycoside synthesis (see Section 10.3.2), the anomeric position must be selectively deacetylated. This is possible starting from the j3-anomer by treatment with hydrazine acetate (Excoffier et al. 1975). Thus, compound 5,3 can be prepared from 5.2 by this method. 

Methyl-, hydroxyethyl-, hydroxypropyl-, and carboxymethyl starches, starch acetates, succinates, alkenyl succinates (Fig. 2), adipates, and phosphates, are all well-known products. Furthermore, special derivatives have also been prepared, such as vinyl-, silyl-, ° or propargyl starches, as reactive intermediates for fiirther fime-tionalization. Unusual substitution patterns can also be established by highly selective deacetylation with alkyldiamines and subsequent introduction of such functional groups as sulfates. From die analytical point of view, the most important aspects are stability under alkaline (mediylation) and acidic or Lewis-acidic (depolymerization) conditions, reactivity (such as migration, rearrangement, further substitution or addition reactions, or any intramolecular reaction), and polarity (lipophilic/hydrophilic, ionic/nonionic, acidic/basic). These properties mainly determine the analytical... 

Ytterbium(III) triflate in methanol is an efficient catalyst for cleaving methoxy-acetates (e.g., 23 - 24). The methoxyacetate protecting group was introduced 30 years ago (C.B. Reese and J.C.M. Stewart, Tetrahedron Lett., 1968,4723) but has found little application so far, due to difficulties associated with its removal. Efficient, selective anomeric deacetylation of peracetylated reducing disaccharides has been achieved with hydrazine hydrate (1.2 equivalent) in acetonitrile. It has been observed that the BF3.Et20-catalysed glycosidation of several pento- and hexo-pyranose peracetates with simple alcohols is accompanied by selective deacetylation at the 2-position. ... 

Selective deacetylation of the C-4 acetate of taxol or baccatin III proved to be a challenging task because this acetate group is in a very hindered location. The first clue to a method for selective deacetylation came in studies of the hydrolysis of hexahydrobaccatin III 58) in which it was found that hexahydrobaccatin III itself underwent reasonably selective deacylation at C-10 and C-4 to give the 4,10-bis-deacetyl derivative. A 13-(triethylsilyl)-analog, however, underwent selective hydrolysis at the C-10 and C-2 positions. These results were explained by a neighboring group effect, with intramolecular transfer of the C-4 acetate to the C-13 position. 

Selective deacetylation of the sucrose derivative (14) with methanolic ammonia gave the 3,6 -diacetate (15) and the 3-acetate (16). j8-D-Glucosyl esters have been prepared with high stereospecificity using tributylstannyl derivatives in two ways as outlined in Scheme 4. The method was used to prepare j8-l-0-acetyl and p-nitrobenzyol derivatives from 2,3,4,6-tetra-O-benzyl-a-D-glucose and the plant /3-glucosyl ester, stevioside, from 2,3,4,6-... 

The selective deacetylation of methyl 2,3-di-0-acetyl-a-D-threoside by use of porcine liver esterase afforded the 3-0-acetate exclusively. The P-anomer gave a mixture of the 2- and 3-monoacetates without any trace of fully deacetylated product. Hydrolysis of a series of 1,2-isopropylidene-a-D-hexofuranose 5,6-diacetates 29 by the same enzyme gave initially the 5-acetates 30, which quickly isomerized to the 6-acetates 31. Selective secondary acylation of 2-deoxynucleosides was achieved with Amano PS lipase and oxime esters in pyridine at 60 and... 

Temporary protection of disaccharide 14 [34] at position 4 with levulinic acid under standard conditions (88% yield) and subsequent selective deacetylation of the primary position using [tert-Bu2SnOH(Cl)]2 afforded a free alcohol at position 6, which was further protected as a TBDPS ether (94% yield over the two steps) and finaUy treated with hydrazine acetate in toluene/ethanol [55] to yield glycosyl acceptor 58 (90%). 

Acetates. Because of the significant interest in selective acetylation reactions of sucrose, the need for a convenient and unambiguous method of identification has been recognized (34,35). The position of an acetyl group in a partially acetylated sucrose derivative can be ascertained by comparison of its H-nmr acetyl methyl proton resonances after per-deuterioacetylation with those of the assigned octaacetate spectmm. The synthesis of partially acetylated sucroses has generally been achieved either by way of selectively protected derivatives such as trityl ethers and cychc acetals or by direct selective acetylation and deacetylation reactions. 

The inner disaccharide unit of the trisaccharide hapten of the M. avium serovar 8 GPL148 was assembled in a manner similar to that of the serovar 20, but with reaction of the rhamnosyl trichloroacetimidate (80b) with the benzylidene acetal (81). O-Deacetylation of the product yielded the disaccharide acceptor (84) for the next glycosylation. Incorporation of the pyruvate acetal moiety into the terminal 3-O-methyl-D-glucose residue of 85 was achieved by transacetalation with methyl pyruvate diethyl dithioacetal, with sulfuryl chloride-triflic acid as catalyst. From the mixture of products the desired diastereomer was separated and converted by successive O-debenzylation, acetylation, selective 1-O-deacetylation, and reaction with trichloroacetonitrile into the trichloroacetimidate 86. Reaction of glycosyl donor 86 with acceptor 84, with trimethylsilyl triflate as promoter, afforded fully... 

The model monosaccharides just listed were prepared from common precursor IV.l (Scheme 39), which was readily obtained by azidonitration of 3,4,6-tri-O-acetyl-D-galactal followed by deacetylation with sodium methoxide. Treatment of IV.l with acetone and toluene p-sulfonic acid monohydrate at room temperature led to predominant formation of the thermodynamically favored 3,4-O-isopropylidene (IV.2) in 61% yield while also producing 27% of the 4,6-O-isopropylidene derivative IV.3. The position of the isopropylidene IV.2 was verified by the use of NMR chemical shift analysis to confirm the position of the acetate group in the resultant acetylated adduct IV.4. Synthesis of the 4-O-sulfate derivative (IV.7) from IV.2 utilized a step that differentiated the 3-OH and 4-OH positions after benzylation and de-isopro-pylidination of IV.2, a selective methylation at the 3-OH of diol IV.5 was achieved via a tin procedure [91] to give methyl glycoside IV.6. Conversion of the azide into... 

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