Methanolysis deacetylation

We shall describe a specific synthetic example for each protective group given above. Regiosdective proteaion is generally only possible if there are hydroxyl groups of different sterical hindrance (prim < sec < tert equatorial < axial). Acetylation has usually been effected with acetic anhydride. The acetylation of less reactive hydroxyl groups is catalyzed by DMAP (see p.l44f.). Acetates are stable toward oxidation with chromium trioxide in pyridine and have been used, for example, for protection of steroids (H.J.E. Loewenthal, 1959), carbohydrates (M.L. Wolfrom, 1963 J.M. Williams, 1967), and nucleosides (A.M. Micbelson, 1963). The most common deacetylation procedures are ammonolysis with NH in CH OH and methanolysis with KjCO, or sodium methoxide. 

Abbreviations All, allyl MP, dimelhylaminopyridine or 2,4,6-trimelhylpyridine MOM, methoxy-methyl Piv, pivaloyl Tr, trityl other abbreviations are the same. Obtained accompanied with the 3,4-0-( methyl orthoacetyl)-a-D-galactoside the 0-deacetyl derivative was also reported. Inserted as a reference reaction. A mixture of two diastereoisomers. On hydrolysis (R = F), 3,6-dideoxy-3,6-difluoro-D-glu-copyranose is formed. Methanolysis gives methyl 3-deoxy-3-fluoro-a-D-galactopyranoside. 

The medicinal chemistry of this period relied on simple modifications of the complex alkaloids. Deacetylation of 1 produced 4-deacetylvinblastine (98), a perfect substrate for these first experiments. Treatment if 1 with hydrogen chloride in anhydrous methanol gives 98 directly (91). Cleavage of the 4-acetyl function can also be effected by methanolysis, but during this period the acid-catalyzed methanolysis method was regarded as the preferred method. 

It is apparent that an increase in the substrate-to-catalyst ratio dramatically decreases the catalytic efficiency for the pNPOAc reactions, but affects to a much lower extent the POAc reaction. This is easily understood with reference to Table 5.3. Since in the reaction of pN POAc the rate-determining step is mainly deacetylation, an increase in ester concentration causes a proportional increase in the rate of background methanolysis, but hardly affects the rate of deacetylation, with the result that catalytic efficiency varies inversely to ester concentration. Conversely, the reaction of POAc approaches a situation in which acetylation of the catalyst is rate determining, which implies that both acetylation and background reactions increase on increasing ester concentration. 

The acetylated, degraded polysaccharide of C. ocellatus thus obtained was deacetylated and methylated by dimethyl sulfate and alkali and then by Purdie s reagents (silver oxide and methyl iodide). Methanolysis of... 

The selenide anion can be used in the heterocyclization of polyhydroxylated selenepane 122 and 124 starting from l,2 5,6-dianhydro-3,4-0-methylidene-L-iditol 121 or from D-mannitol 123 in variable yield (Scheme 10). A general, known method is to combine an acylation for purification and deacetylation by methanolysis but Le Merrer el al. simplified this procedure to obtain directly the crystalline selenepane after flash chromatography. Deprotection of these seleno compounds gave an inextractable mixture <1997T16731>. 

Methanolysis of echitovenaldine gave a deacetyl compound (M+ 384) having spectral properties similar to 11-methoxyminovincinine (9). Acid-catalyzed hydrolysis gave an indolenine (22), which was reduced by sodium borohydride to a quebrachamine derivative 31 having a UV spectrum (Amn 230, 270, and 300 nm) typical of a 6-methoxy-2,3-disubstituted indole... 

Physicochemical studies of the solution properties of amylose benzoate by light-scattering, osmometric, and viscosimetric techniques showed that the molecules behave as coils. Factors affecting the rate of dissolution of cellulose acetate phthalate in aqueous solution are important from the point of view of use as an enteric-coating material. The removal of the benzoyl group during methanolysis is slow and concurrent with the methanolysis, " whereas deacetylation precedes methanolysis. 

FTC tests positive with triphenyltetrazonium chloride (TTC), suggesting the presence of a hydroxylamine group. Methanolysis (0.5% HCl) gives a deacetyl derivative (75), mp 252-253°C, for which the TTC test is still positive and the UV spectrum is unchanged. On the other hand, a carboxyl band of the lactone is shifted from 1790 to 1765 cm , which suggests tryptoquivaline is not the simple acetate of 75. 

D-glucopyranosyl donor, and subsequent displacement of a 2-triflate with azide ion. The crystalhne methyl 3-azido-2,3-dideoxy-a-D-glucopyranoside has been synthesized from D-glucal triacetate by hydration, Michael addition of azide, methanolysis, chromatographic fractionation and deacetylation. ... 

When sodium hydroxide is added to a pure methanol solution of polyvinyl acetate, deacetylation takes place as methanolysis sodium hydroxide is a catalyst. 

The degree of polymerization of polyvinyl acetate drops more or less by the hydrolysis or methanolysis, and PVA of a lower DP is obtained. This is because branches of polyvinyl acetate formed by the chain transfer to acetyl groups of dead polymers are cut at the point of hydrolysis or methanolysis simultaneously with deacetylation, as described in the discussion of chain transfer to dead polymer. 

Continuous methanolysis is adopted for the deacetylation process. Figure 4.6 shows a schematic flow sheet of deacetylation. The first step of the process is the mixing of a concentrated methanol solution of polyvinyl acetate with a methanol solution of alkali. 

Zemplen methanolysis of the triacetate (XXXVII) gives (17) the 2-monoacetate (XXXVIII R = CO CH CHPh). A possible reason (22) for the contrast with the methanolysis of the taxicin-I analog, where the 2-acetyl group is removed also, lies in the anchimeric assistance to deacetylation which is there available from the neighboring free... 

R = H) in warm aqueous solution [79], and 2-O-acctylglyccrol (39 R = H, R = Ac) readily isomerizes to an equilibrium mixture consisting largely of the 1-acetate (39 R = Ac, R = H) in neutral or mildly alkaline solution [58], The use of the acetyl protecting group for alcoholic hydroxyl functions is widespread in glycoside (including nucleoside) synthesis [80], oxidation [81], phosphorylation [82], and other reactions. Acetate esters are most readily solvolyzed under basic conditions, but acid-catalyzed solvolysis has also been used [83]. Perhaps the most commonly used deacetylation procedures are ammonolysis with NHs/MeOH and methoxide ion catalyzed methanolysis. 

Deacetylation by methanolysis. A mixture of 2-acetoxy-2 -iodobiphenyl (prepn. s. 161)... 

Although artifactual peaks are sometimes seen, these are minimized by employing ganglioside samples of at least moderate purity. The use of two columns enables sialic acid identification to be made with a high degree of certainty, although the OV-1 column alone is usually sufficient for routine assays. O-acetylated sialic acids lose some or all of their ester groups despite the mild methanolysis conditions, so that quantification of such species is not yet feasible these may, however, be detected qualitatively when deacetylation is not complete (Yu and Ledeen, 1970). 

Tryptoquivaline (30), C29H30N4O7, afforded a deacetyl methanolysis product (31) [Eq. (10)]. Compound 31 was characterized as the /7-bromo-benzoate and / -bromophenylurethane. Crystals suitable for X-ray crystallography were obtained from the / -bromophenylurethane derivative. The structure of tryptoquivaline as determined by X-ray analysis was shown to be 30. 

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