Carbonate interchanges

The structural basis of one of the classic examples of such stereospecificity, that of chymotrypsin for L-amino acid derivatives, is immediately obvious on examination of the crystal structure of the enzyme. D-Amino acid derivatives differ from those of L-amino acids by having the H atom and the side chain attached to the chiral carbon interchanged (structure 8.10). The d derivatives cannot bind because of steric hindrance between the side chain and the walls of enzyme around the position normally occupied by the H atom of L derivatives (Chapter 1). 

The relative rates of hydrogen and carbon interchange have been measured by Saunders et al. (1973c) using a mixture of (1,1,1- H3)- and (2- C)-labelled isopropyl cations at —88°C. The changes in the relative areas of different peaks as well as C-satellites were observed, and the time dependence of the concentrations of different labelled isomers were simulated by computer [using mechanisms (34)-(36)]. A combination of mechanisms (34) and (35) or mechanisms (34) and (36) could match the measurements. The rate for (34)... 

Labelling studies 51> indicate that these rearrangements are intramolecular and occur by ring carbon interchange rather than by alkyl group migrations. 

Shaikh, A.G., Sivaram, S., Puglisi, C., Samperi, R, and Montaudo, G., Poly(aryl-enecarbonate)s oligomers by carbonate interchange reaction of dimethyl carbonate with Bisphenol-A, Polymer Bulletin, 32, 427, 1994. 

Carbonates are rather reactive compounds and thus can be used for syntheses of carbamates and ureas, particularly asymmetrical ureas. Reviews on carbonates are given in [608-611]. Preparations of carbonates described herein are performed with chloroformates [612-632], phosgene [503, 634-649], diphosgene [650, 651], triphosgene [652-657], CDI [658-663], acyl carbonates [664-675], carbonates interchanges [633, 676-701], CO [702, 703], CO2 [704-714], and ureas [715-718], or under enzymes catalysis [719-721]. 

Several books and other publications on the stabilization of polymers have been mentioned in this chapter. In addition, two patents are noteworthy. The one by Hamilton discusses protection of polymer blends against transesteri-fication. For example,blends of PC with semi-crystalline PEST (e.g., PET) finds application in the automotive industry. Since the resistance to solvents depends on the PET crystallinity, which decreases with advancing transreactions, its control is essential. The patent specifies addition of a sUyl phosphate compound for inhibiting the ester-carbonate interchange. The stabilization is achieved by deactivation of the residual metaUic catalyst in the reaction with silyl phosphate stabilizers or their mixture. 

Hamilton, D. G., Polyester-polycarbonate compositions stabilized against ester-carbonate interchange, US Patent, 5,922,816,13.07.1999, to General Electric Co. 

Sodium Chloroacetate Sodium chloroacetate [3926-62-3] mol wt 116.5, C2H2C102Na, is produced by reaction of chloroacetic acid with sodium hydroxide or sodium carbonate. In many appHcations chloroacetic acid or the sodium salt can be used interchangeably. As an industrial intermediate, sodium chloroacetate may be purchased or formed in situ from free acid. The sodium salt is quite stable in dry soHd form, but is hydrolyzed to glycoHc acid in aqueous solutions. The hydrolysis rate is a function of pH and temperature (29). 

The addition of alcohols to form the 3-alkoxypropionates is readily carried out with strongly basic catalyst (25). If the alcohol groups are different, ester interchange gives a mixture of products. Anionic polymerization to oligomeric acrylate esters can be obtained with appropriate control of reaction conditions. The 3-aIkoxypropionates can be cleaved in the presence of acid catalysts to generate acrylates (26). Development of transition-metal catalysts for carbonylation of olefins provides routes to both 3-aIkoxypropionates and 3-acryl-oxypropionates (27,28). Hence these are potential intermediates to acrylates from ethylene and carbon monoxide. 

Alcoholysis (ester interchange) is performed at atmospheric pressure near the boiling point of methanol in carbon steel equipment. Sodium methoxide [124-41 -4] CH ONa, the catalyst, can be prepared in the same reactor by reaction of methanol and metallic sodium, or it can be purchased in methanol solution. Usage is approximately 0.3—1.0 wt % of the triglyceride. 

Like P—O—C linkages, P—O—P linkages are susceptible to hydrolytic degradation. Scrambling or interchange usually occurs for phosphoms oxyesters at temperatures and acidities lower than those required for the carbon esters but greater than those for the sulfur esters. 

Production of carbon electrodes is a capital-intensive business. Two suppHers dominate the prebaked market. Carbon paste producers are more numerous and tend to serve local markets. There is no international standard for the threaded joints on carbon electrodes. Manufacturers of straight pin carbon electrodes have followed the physical specifications adopted for graphite electrodes (37). Unified standards do not exist for pinless joints resulting in limited interchangeability among brands. Electrode diameters are offered in both English and metric sizes with no restrictions on new or unique diameters. 

It is essential to neutralize any strong acid present before distilling lactic esters otherwise, condensation by ester interchange occurs, with liberation of alcohol and production of polylactic acid, a linear polyester. Other neutralizing agents, such as alkali or alkaline-earth hydroxides or carbonates, doubtless could be used satisfactorily instead of sodium acetate. 

The natural product is the asperuloside described in the literature The assignments for the carbon pairs C-1/C-1, C-6 /C-1 and C-11 / CO (acetyl) have been interchanged. Deviations of C chemical shifts (CDCI3-D2O from the values tabulated here [(CD3)2SO] are due mainly to solvent effects. Here the difference between the measurements a and d shows that the use of D2O exchange to locate the OH protons where the CH COSY plot is available is unnecessary since OH... 

In the SS form, the positions of the fluorine and the hydrogen on each carbon are interchanged. This results in a corresponding exchange of their dihedral angles, while... 

This reaction converts ribulose-5-P to another ketose, namely, xylulose-5-P. This reaction also proceeds by an enediol intermediate, but involves an inversion at C-3 (Figure 23.31). In the reaction, an acidic proton located a- to a carbonyl carbon is removed to generate the enediolate, but the proton is added back to the same carbon from the opposite side. Note the distinction in nomenclature here. Interchange of groups on a single carbon is an epimerization, and interchange of groups between carbons is referred to as an isomerization. 

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