Acetals C-atoms

Diastereotopism indicates prochirality, as exemplified by glycerol (25, Fig. 2.23). Other examples of this include diethylacetals, in which the OC//2 protons are diastereotopic on account of the prochiral acetal-C atoms, thus forming AB systems of quartets because of coupling with the methyl protons. 

C. Atomic Charges To di.splay atomic charges for acetic acid, select Atomic Charges. Click on an atom. The charge on that atom is di.splayed at the bottom of the screen. A positive number indicates a deficiency of electrons and a negative number, an excess of electrons. Repeat the process as necessary for different atoms, and click on Done when finished. 

Replacement of the N-l-C-6 moeity by two C-C atoms has been found in the reaction of 4-alkoxypyrimidine 1-oxides with diketene and acetic anhydride in chloroform, yielding a mixture of oxazolopyridines and isoxazolopyridines (Scheme 8a) (88CPB168). 

The diastereoselectivity of the reaction may be rationalized by assuming a chelation model, which has been developed in the addition of Grignard reagents to enantiomerically pure a-keto acetals7,8. Cerium metal is fixed by chelation between the N-atom, the methoxy O-atom and one of the acetal O-atoms leading to a rigid structure in the transition state of the reaction (see below). Hence, nucleophilic attack from the Si-face of the C-N double bond is favored4. 

Write the Lewis structure for acetic acid, CH COOH, the carboxylic acid in vinegar formed when the ethanol in wine is oxidized. In the —COOH group, both O atoms are attached to the same C atom, and one of them is bonded to the final H atom. The two C atoms are bonded to each other. 

Show how resonance can occur in the following organic ions (a) acetate ion, CH,CO, (b) enolate ion, CH,COCH5, which has one resonance structure with a C=C double bond and an —O group on the central carbon atom (c) allyl cation, CH,CHCH,+ (d) amidate ion, CH,CONH (the O and the N atoms are both bonded to the second C atom). 

The snbstitntion proceeds with retention of confignration at the snbstitnted C atom as confirmed by the reaction of snitable isomeric acetates. Retention of confignration has also been observed in analogons Pd-phosphine catalysed reactions [93]. 

Quantitative determination of the products from Haworth methyla-tion of benzyl 4-0-methyl-/3-D-xylopyranoside gave277 the ratio of rate constants k2 k3 as 3.2 1. Satisfactory agreement between predicted and observed product-ratios was found if it was assumed that, after methylation of HO-2, the reactivity of HO-3 increases by a factor of 3, but that methylation of HO-3 does not alter the reactivity of HO-2. The greater reactivity at HO-2 is, presumably, a result of its greater acidity, resulting from the inductive effect of two acetal oxygen atoms on C-l. When this group is ionized, the acidity of HO-3 should be decreased, but methylation at HO-2 removes the effect. Methylation at HO-3 should not, however, similarly affect HO-2. 

In conclusion of this section, it should be emphasized that double silylation of AN is very regio- and stereospecilic. As a mle, the reaction mixtures contain only one structural isomer of the target nitroso acetals. If the starting AN contains the methyl group at the a-C atom, double silylation leads to deprotonation of this group to give terminal BENA regardless of the nature of the substituent R at the p-carbon atom (Scheme 3.201). 

N,C Elimination The reactions of standard BENAs with bases were considered in the previous section. As a rule, these reactions proceed at the silicon atom of the nitroso acetal fragment. However, if a EWG-group is adjacent to the y-C atom of BENA, the ally lie proton (Hy) at this carbon atom becomes so labile that it can be eliminated already in the presence of bases at room temperature (504), thus initiating the transformation of such BENA into conjugated en oximes (Scheme 3.227). 

The process shown in Scheme 3.265 substantially complicates the reaction of cyclic nitroso acetals with nucleophiles at the p-C atom of the enamine fragment. 

Numerous chemical intermediates are oxygen rich. Methanol, acetic acid and ethylene glycol show a O/C atomic ratio of 1, as does biomass. Other major chemicals intermediates show a lower O/C ratio, typically between 1/3 and 2/3. This holds for instance for propene and butene glycols, ethanol, (meth)acrylic acids, adipic acid and many others. The presence of some oxygen atoms is required to confer the desired physical and chemicals properties to the product. Selective and partial deoxygenation of biomass may represent an attractive and competitive route compared with the selective and partial oxidation of hydrocarbon feedstock. 

Monomeric carbohydrates in their cyclic form (furanoses and pyranoses) are hemiacetals, which, to become acetals, form 0-glycosyl conjugates. The C-atom C(l) that bears two O-atoms is the reactive, electrophilic center targeted by glycosidases. Nonenzymatic hydrolysis is also possible, although, as a rule, under physiological conditions of pH and temperature, the reaction is of limited significance. 

The mechanism of the acid-catalyzed hydrolysis of cellulose is based on that normally expected for an acetal (see Scheme 11). This involves formation of a conjugate acid by protonation of either of the acetal oxygen atoms at C-1, and formation of a carbonium ion, followed by stabilization of the product by heterolysis of a participating water molecule. The car-... 

The degradation of the fatty acids occurs in the mitochondrial matrix through an oxidative cycle in which C2 units are successively cleaved off as acetyl CoA activated acetic acid). Before the release of the acetyl groups, each CH2 group at C-3 of the acyl residue (the P-C atom) is oxidized to the keto group— hence the term p-oxidation for this metabolic pathway. Both spatially and functionally, it is closely linked to the tricarboxylic acid cycle (see p. 136) and to the respiratory chain (see p. 140). 

The same synthetic pathway can be used for the synthesis of C-labelled ot-acetates.For metabolic studies, it was possible to place the C-label at different C-atoms by using C lahelled amine or aldehyde (Eq. 5). 

The trifluoroacetate ion CF3CO2 (Fig. 3.5(a)) is similar (Brown 1980 ). F and are both anions so the two C atoms are both formally cations, each with a valence of +4. As before, we treat the C-C unit as a single pseudocation, reserving one electron pair for the C-C bond. It is instructive to compare this with the closely related acetate ion, [H C" — whose... 

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