Kinetic, monoacetalization

The kinetic-acetonation procedure previously reported,107 employing alkyl isopropenyl ethers in N,N-dimethylformamide, and applied to the common pentoses and hexoses, has been extended to various hexuloses, oligosaccharides, and other sugar systems.108 Maltose gave the 4, 6 -monoacetal, isolated as the crystalline hexaacetate, on treatment with isopropenyl methyl ether inN,N-dimethylformamide in the presence of p-toluenesulfonic acid. 

Since R-48 is also an important versatile synthon for prostaglandin synthesis, there has been interest in devising asymmetric methods for its preparation. Japanese workers 7 subjected a 1 1 mixture of cis and trans 44 to esterases from baker s yeast and were able to obtain the optically active (R,R)-45, (R,R)-46 and (S)-predominant 47. Thus a simultaneous kinetic resolution of the dlacetate (44) and asymmetric synthesis of the monoacetate (46) were effected by this hydrolysis. These were converted to prostaglandin synthons.6 ... 

Monoacetates of the corticosteroid dihydroxyacetone side-chain are not ordinarily accessible because of their propensity for acetyl migration to C-21. Hydrolysis of the 17,21-orthoacetate (119) in a phthalate buffer at pH 3, however, gave the 17-acetate (120) in high yield,presumably through kinetically-controlled protonation at the more-exposed C-21 oxygen atom. 

If one plots the contents of the di- and monoacetates as a function of the diol content in the mixture, one finds that the wrong monoacetate disappears quickly, because the enzyme hydrolyses its remaining right acetate group rapidly. If one waits until about 40% of diol is formed, the right monoacetate is essentially pure (e.e. as 100%) and the diacetate has disappeared. The pure enantiomer of the monoacetate now only needs to be separated from the diol. Non-enzymatic reactions follow similar routes from achiral to optically pure chiral products if asymmetric reactions are coupled with kinetic resolution (cf. section 2.2.2). 

The kinetically controlled hydrolysis of acetoxonium ions derived from cyclohexane-1.2-diol analogues gives monoacetates of m-diols, in which the free OH... 

The chiroptical properties of mixed-ligand Co(III) complexes with (S)-aspartic-N-monoacetic add and different amino acids have been described (Colomb and Bernauer, 1977). In addition, kinetic studies of the racemization of optically active (aminoacidato)triethylenetetraammine-Co(III) complexes have been carried out (Ghandehari et ai, 1970 Kawasaki et aU 1970). 

The monoacetate phosphine oxide 158 was isolated in 92% yield and 72% ee. The absolute configuration was shown to be R by chemical correlation after preparing the known phosphine oxide 159. As in the previously described kinetic resolutions of phosphoryl derivatives, the sense of the chiral induction can be explained by the Jones model of the PLE active site. In the same report the enzyme-catalysed preparation of P-stereogenic phosphine oxide 156 from prochiral precursors was described (Scheme 6.62). 

In a polarographic study, the rates of dissociation of some aminopolycarboxylato-zinc(n), -cadmium(ii), and -lead(ii) complexes have been measured, and n.m,r. has been used in an analogous study on the same metals. The kinetics of the dissociation of the cobalt(ii) complex of ethylenediamine monoacetate have also been measured polarographically, and the rate constants for the formation of the cobalt(n) complexes of anthranilate, salicylate, 5-sulphosalicylate, and ligand (5) have been compared with those of the respective nickel(n) analogues. 

High yields and >99% ee were achieved in the lipase-mediated desymmetrization of 2,3-0-cyclohexylidene-erythritol 34, as shown in Scheme 3. The acetonide 35 was similarly asymmetrically acetylated to give the monoacetate 36, which was employed in a synthesis of (+)-em/o-brevicomin (see Chapter 24). The enzymatic kinetic resolution of racemic synthetic glycals is referred to in Chapter 13. 

Isopropylidene-D-glucofuranose, 2,3-O-isopropylidene-D-ribofuranose, and 3,5-0-isopropylidene-D-xylofuranose are readily available by acetonation of the corresponding glycosylamines to give the known intermediates (2)-(4), respectively, and subsequent hydrolysis of the amino function at pH S. 2,3-0-Isopropylidene-D-lyxofuranose was obtained in 68% yield in the p-TsOH-catalysed, kinetic reaction of D-lyxose with 2 molar equivalents of 2-methoxypropene in DMF. Under identical conditions. D-xylose gave a mixture of the monoacetal (5) and the diacetals (6) and (7) (ca. 4 3 2). ... 

Similarly as for prochiral substrates, many of the lipase-catalyzed asymmetrizations of meso compounds are accompanied by a second reaction step that usually enhances the enantiomeric excess of the product. This second step is a kinetic resolution. For example, in the hydrolysis of a m o-diester, die reaction usually does not stop at the monoester stage (Scheme 15). The two enantiomeric monoesters will react further giving the same me o-diol. This second step usually favors the minor monoester enantiomer and therefore leads to an increase of the enantiomeric excess of the major monc ster, but a decrease in the yield. This has been illustrated and described by Wang et al. for the lipase-catalyzed hydrolysis of meso-l,5-diacetoxy-cw-2,4-dimethylpentane [117]. The monoacetate was afforded in 89.7% e.e. 

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