Acetals selective hydrolysis

Lowering the temperature of the reaction would certainly decrease the rate of acetal hydrolysis and thereby partially remove one of the causes of overoxidation. This would simultaneously decrease the rate of oxidation by periodate. Although no comprehensive study of the effect of temperature on oxidation rates has been made, the number of reactions successfully dealt with in the temperature range of 0 to 6°31 39 78 113 126 130 i33, i64, us, 203,210,266,267 indicates that lowered temperatures do not affect the rates unfavorably. In order to obtain the maximum of selective oxidation and the minimum of overoxidation, periodate oxidations should be performed at as low a temperature as is practicable in relation to the solvent system used and the solubility of the reactants therein. 

Fig. 20. Effect of degree of crosslinking (% DVB) of a standard ion exchanger on the diffusivities, Def (cm2 min-1), and the selectivity ratio, S = efs/ efAc ( ef = effective rate coefficient, S = sucrose, Ac = ethyl acetate). Data were obtained by rate measurements and Wheeler—Thiele analysis of simultaneous sucrose and ethyl acetate hydrolysis at 70°C [508],...

A transition-state analogue for an acetal hydrolysis has been used to select and amplify the production of a macrocycle from a dynamic combinatorial library of disulfides in water. The macrocycle gives a modest acceleration of the acetal hydrolysis reaction.6... 

Since the discovery of the illudane-type sesquiterpenoids, a number of related compounds have been isolated, viz., illudalic acid (167), illudinine (168), and dihydroilludin S (169, R = a-OH). The absolute stereochemistry of illudin S (169, R = =0) has been determined. A sequel to the successful stereospecific synthesis of illudin M (170) has been reported by Matsumoto et in which the diacetate (171, R = Ac), which had previously been prepared in the first synthesis, was selectively hydrolysed to the monoacetate (171, R = H). This compound was converted in three steps to the diacetate (172), which, after another selective hydrolysis, Jones oxidation, and acetate hydrolysis, yielded illudin M (170). 

Complications arise when two esters of different acids are present or when acylation occurs on the aglycone. One must then rely on partial hydrolysis. The tactics are examplified hy the Entada saponins which contain C2 and Cjq acids (89). The acetate was selectively removed hy 0.025 % K2CO3 while both adds were removed by 1 % KOH. Comparison of C NMR spectra of the parent compound and of derivatives allowed determination of the points of acylation. The dicrotalic (3-hydroxy 3-methyl glutaric add) esters of the tubeistemosides and related compoimds (29-32) provide more complicated examples where the double anchoring of the diadd transforms a prochiral carbon atom into a center of chirality. 

Epoxidation of the olefin occurs with high diastereofacial selectivity to give carbamoyl-oxirane 945. This epoxide is not extremely stable, and is treated directly with methanesulfonic acid to afford the j5-D- a/o-furanoside 946. The stereocenter at C-2 must be inverted to match the configuration of the natural product. This is accomplished by triflate formation followed by an Sn2 reaction with cesium acetate. Hydrolysis of the OAc group furnishes the desired P D-ga/ac o-furanoside (947). 0-Methylation, benzyl group hydrogenolysis, acidic hydrolysis, and dithioacetal formation completes the synthesis of 948 in 11 steps and 5.7% overall yield from 929 [252]. 

Fig. 39. Electron-diffraction diagrams of microciystals of cellulose at different states of acetylation and after removal of cellulose acetate by selective hydrolysis A, initial B, sample of DS 2.41 C, sample of DS 2.81. Schematic drawing describing the onset of acetylation of a typical crystalline cellulose, showing how chains that are sufficiently acetylated are partially lifted from the crystal. Schematic representation of the change in cross section of the cellulose crystals from Valonia during partial acetylation. CP/MAS C-NMR spectra of the fraction of cellrtlose remaining as insoluble at increasing acetylation ratio, showing disappearance of the la component. Schematic representation of the localization of one part of the la phase in Valonia cellulose...

A number of steric and electronic features underpin the selectivity of acetal hydrolysis but, in this context, it is just the results that are important (1) trans-dioxolanes are more readily hydrolysed than dioxanes than are c/5-dioxolanes (2) acetals derived from a diol in which one of the hydroxyls is primary are hydrolysed more rapidly than those in which both are secondary, particularly if, in the latter... 

Conversion of a d-galactose derivative into the corresponding enopyranoside is depicted in Scheme 12.23. The primary hydroxy group in 95, prepared from methyl 4,6-0-benzylidene-a-d-galactopyranoside 94 by 0-benzylation followed by acid hydrolysis of the benzylidene acetal, was selectively tosylated to give 96. Displacement of the OTs group with iodide followed by O-acetylation of the secondary alcohol provided 97. Treatment of 97 with DBU afforded methyl 4-0-acetyl-2,3-di-0-benzyl-6-deoxy-(3-l-arabino-hex-5-enopyranoside 98. 

In another example developed by Otto et al. [19], macrocycles were selected from a DCL to be used as catalysts for the acetal hydrolysis reaction (Scheme 4.2). The acceleration of the hydrolysis reaction was observed in the presence of the selected macrocycle. However, it remains uncertain whether the reaction acceleration is due to the stabihzation of the transition state, to the shift of the pre-equilibrium towards the protonated acetal, or a combination of both effects. 

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... 

The capsule 397 is reported in [73] to be a reasonably strong Bronsted acid with high affinity toward tertiary amines, their salts, and other guests shown in Scheme 3.74. This allowed performing highly substrate-selective Witting reaction and substrate-selective diethyl acetal hydrolysis within its cavity. 

Real-time ultrafast 2D NMR observations of an acetal hydrolysis at natural abundance have enabled observation of the reactive hemiacetal intermediate. Mutual kinetic enantioselection (MKE) and enantioselective kinetic resolution (KR) have been explored for aldol coupling reactions of ketal- and dithioketal-protected -ketoaldehydes expected to have high Felkin diastereoface selectivity with a chiral ketone enolate. ... 

High purity acetaldehyde is desirable for oxidation. The aldehyde is diluted with solvent to moderate oxidation and to permit safer operation. In the hquid take-off process, acetaldehyde is maintained at 30—40 wt % and when a vapor product is taken, no more than 6 wt % aldehyde is in the reactor solvent. A considerable recycle stream is returned to the oxidation reactor to increase selectivity. Recycle air, chiefly nitrogen, is added to the air introducted to the reactor at 4000—4500 times the reactor volume per hour. The customary catalyst is a mixture of three parts copper acetate to one part cobalt acetate by weight. Either salt alone is less effective than the mixture. Copper acetate may be as high as 2 wt % in the reaction solvent, but cobalt acetate ought not rise above 0.5 wt %. The reaction is carried out at 45—60°C under 100—300 kPa (15—44 psi). The reaction solvent is far above the boiling point of acetaldehyde, but the reaction is so fast that Httle escapes unoxidized. This temperature helps oxygen absorption, reduces acetaldehyde losses, and inhibits anhydride hydrolysis. 

---