Methyl orthoacetate

Orthoesters (e.g., methyl orthoacetate) have also been prepared by acid-catalyzed exchange with trimethyl orthoesters, from 16a,17a-dihydroxy-pregnanes with or without a 21-hydroxyl substituent. 

Preparation—Cortexolone 1,21-Methyl Orthoacetate. A mixture of cortexolone (10 g), methyl orthoacetate (10 ml) dimetliylformamide (10 ml) and /)-toluenesulfonic acid (40 mg) is heated at 110-120° for 4 hr, the alcohol produced during the reaction being removed by a stream of nitrogen. Addition of a few drops of pyridine and evaporation under vacuum affords a residue which yields 7.8 g of product (67%), mp 174-176°, after trituration with methanol [a]o 92° (Diox). 

Tri-O-acetyl-a-D-glucopyranose (fl)-1,2-(methyl orthoacetate) or 3,4,6-tri-0-acetyl-[(fl)-1,2- >(1 -methoxyethylidene)]-a-D-glucopyranose... 

An anomaly appeared in the rhamnose series. When acetobromorhamnose was treated with methanol, and the product was deacetylated with alkali, a methyl rhamnoside acetate was obtained which had retained an acetate group not saponifiable by alkali Fischer named this compound 7-methyl rhamnoside monoacetate. It was not until 1930 that it was shown to be the l,2-(methyl orthoacetate) (25). Today, more orthoacetates of the same kind are known in the sugar series. 

Another type of dehydrating agent, the ortho esters, has also been used for the preparation of processable BMIs. A DMF solution of MDA bismaleamic acid was treated with methyl orthoacetate and triethylamine. A mixture of three products shown in Fig. 3 was obtained with a softening point ranging from 98 to 106 °C[11]. 

Methyl-L-rhamnose (L) has been prepared from the 3,4-diacetyl-L-rhamnose l,2-(methyl orthoacetate), (XLVIII of Haworth, Hirst and Samuels67) which on treatment with methanolic hydrogen chloride gave a mixture of 3,4-diacetyl-L-rhamnopyranose (XLIX) and methyl 2,3,4-triacetyl-/J-L-rhamnopyranoside. Methylation of XLIX followed by... 

Copolymerization of l,4-anhydro-2,3-di-0-(to7-butyldimethylsilyl)-a-D-xylopy-ranose (39) with l,4-anhydro-2,3-di-0-benzyl-a-i>xylopyranose (27), followed by desilylation with tetrabutylammonium fluoride gave a partially benzylated sterero-regular (l -->5)-a-D-xylofuranan. [37] A branched polymer (40) was obtained when it was glycosylated with 3,4,6-tri-0-acetyl-P-D-mannose l,2-(methyl orthoacetate). Debenzylation of the polymer having D-mannosyl branches with sodium in liquid ammonia yielded (1 — 5)-a-i>xylofuranans having 2- or 3-O-a-D-mannopyranosyl branches. 

The palladium(II)-catalyzed oxidative aminocarbonylation of 0-(2-propenyl)-Ar-tosyl carbamates 12 was achieved under acidic buffer conditions, although the reaction was very sluggish, or, preferably in the presence of acetate ion and methyl orthoacetate, which suppresses palladium(II) chloride consuming side reactions114. Substitution at C-l led to the prevalent formation of the trans-4,5-disubstituted 1,3-oxazolidin-2-ones 13 (NMR). 

Pacsu and CrameF treated a chloroform solution of hexaacetylmaltose methyl 1,2-orthoacetate with titanium tetrachloride and obtained the normal heptaacetyl-a-maltosyl chloride. In like manner, when dry hydrogen bromide in glacial acetic acid was employed, a rapid formation of the normal heptaacetyl-a-maltosyl bromide resulted. Obviously, the replacement of the methoxyl group of the methyl orthoacetate by a halogen atom was accompanied by a rearrangement of the orthoester... 

The hexaacetate was also obtained by the addition of water to the solution of the methyl orthoacetate, which had been previously treated with dry hydrogen chloride in methanol. But when the methyl orthoacetate was treated with an aqueous methanol solution of hydrogen chloride, no crystalline hexaacetate was obtained. The reaction was presumably incomplete, since it was claimed that a portion of the methyl orthoacetate was recovered unchanged. 

A study of the action of titanium tetrachloride on the triacetyl-fructose methyl orthoacetate was made by Pacsu and Cramer. The reaction yielded only pure acetylfructosyl chloride. Similarly, rapid formation of acetylfructosyl bromide resulted when the methyl orthoacetate was treated with hydrogen bromide in glacial acetic acid. 

The acetylated neolactose methyl 1,2-orthoacetate was isolated in crystalline form when the original methanol solution was evaporated in air to a sirup and triturated with ethanol. After several recrystallizations the compound (XIV) had m. p. 121-122 and [a]n -t-25.3 . It showed the reactions and properties which characterize the sugar methyl orthoacetates, including stability of the orthoacetate group toward alkaline hydrolysis. When the new compound was treated with an anhydrous 0.1 V solution of hydrogen chloride in chloroform, it was converted into the crystalline heptaacetyl-a-neolactosyl chloride. Alkaline hydrolysis also indicated the presence of six acetyl groups, whereas dilute acid removed seven. 

The problem of the acid-catalyzed hydrolysis of the carbohydrate orthoesters was brought nearer to the final solution by Pacsu s experiments on the hydrolysis of maltose methyl 1,2-orthoacetate. Since two adjacent hydroxyl groups on the same side of the plane are necessary for the formation of orthoester derivatives, the maltose methyl orthoacetate must have an -configuration. Hydrolytic experiments with very dilute hydrochloric acid confirmed this. Two consecutive reactions took place at a pH of 4. In the first reaction, the original specific rotation, Co ]d - -103.7 in pure water, increased to -1-134.6° within two minutes. The latter figure corresponds to the specific rotation of a-maltose 2-acetate. The second reaction k = 0.0095) corresponded to the downward muta-rotation of a-maltose 2-acetate. When the hydrogen-ion concentration,... 

Esterification. Selective acetylation of primary alcohols occurs with methyl orthoacetate as reagent and LaCL/SiOj as catalyst. 

The reaction of ribonucleosides with ethyl orthoformate or methyl orthoacetate under acid-catalyzed conditions affords the 2, 3 -0-(ethoxy-methylidene) (99, R = H, R = CsHs) and the 2, 3 -0-(methoxy-ethylidene) (R = CHs, R = CHs) acetals, respectively. These acetals... 

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