Acetals configuration

Unsaturated acyl derivatives of oxazolidinones can be used as acceptors, and these reactions are enantioselective in the presence of chiral to-oxazoline catalysts.321 Silyl ketene acetals of thiol esters are good reactants and the stereochemistry depends on the ketene acetal configuration. The Z-isomer gives higher diastereoselectivity than the Zf-isomer. 

More reeently, Renaud reported examples of diastereoselective radical cyclization of bromoacetals controlled by the acetal configuration [65] (Scheme 34). 

The 2,3-> 3>5- and 5 6-benzylidene acetals of 1,4-anhydro-D-mannitol with the ( R)- and ( )-acetal configurations have all been obtained. Interconversion of the exo- and endo-2.3-0-benzylidene acetal in methyl 2,3 4>>6-di- -benzylidene— (-D-mannopyranoside using catalytic aluminium(III) chloride has been described. The exo-... 

A great many polymers appear to form films having a flat molecular configuration. Thus various polyesters [7] gave extrapolated areas of about 2.5 m /mg corresponding to about the calculated 60-70 area per segment, or mono-layer Sickness of 3-5 A. A similar behavior was noted for poly(vinyl acetate)... 

The chemist can try to separate the two isomers by careful fractional distillation but it will be next to impossible to do because both the cis and the trans have practically the same boiling point. There are a few things that the chemist can do or hope for to get rid of that cis isomer. The cis configuration is less stable than the trans and may switch over to the trans configuration with a little help. The chemist can gently heat the isosafrole oil to about 150°C for an hour or so. She can also try the same heating except have the oil mixed with some acetic acid. Also, the isosafrole can be fractionally distilled to ultra purity and then be allowed to simply sit for a couple of days. Trans isosafrole may spontaneously crystalize out while the cis form remains as an oil. They can then be separated by filtration. 

Furthermore, the catalytic allylation of malonate with optically active (S)-( )-3-acetoxy-l-phenyl-1-butene (4) yields the (S)-( )-malonates 7 and 8 in a ratio of 92 8. Thus overall retention is observed in the catalytic reaction[23]. The intermediate complex 6 is formed by inversion. Then in the catalytic reaction of (5 )-(Z)-3-acetoxy-l-phenyl-l-butene (9) with malonate, the oxidative addition generates the complex 10, which has the sterically disfavored anti form. Then the n-a ir rearrangement (rotation) of the complex 10 moves the Pd from front to the rear side to give the favored syn complex 6, which has the same configuration as that from the (5 )-( )-acetate 4. Finally the (S)-( )-mal-onates 7 and 8 are obtained in a ratio of 90 10. Thus the reaction of (Z)-acetate 9 proceeds by inversion, n-a-ir rearrangement and inversion of configuration accompanied by Z to isomerization[24]. 

The reaction of phenylzinc reagent proceeds with opposite stereochemistry, namely by retention of configuration at the final step via transmetallation. Both the (S)-( )- and (i )-(Z)-allylic acetates 4 and 9 afford the (/ )-( )-phe-nylated product II by overall inversion[23]. 

Notice that the eclipsed conformation of d ribose derived directly from the Fischer pro jection does not have its C 4 hydroxyl group properly oriented for furanose ring forma tion We must redraw it m a conformation that permits the five membered cyclic hemi acetal to form This is accomplished by rotation about the C(3)—C(4) bond taking care that the configuration at C 4 is not changed... 

The anomenc carbon m a cyclic acetal is the one attached to two oxy gens It IS the carbon that corresponds to the carbonyl carbon m the open chain form The symbols a and (3 refer to the configuration at the anomenc carbon... 

A particular carbohydrate can mterconvert between furanose and pyra nose forms and between the a and (3 configuration of each form The change from one form to an equilibrium mixture of all the possible hemi acetals causes a change m optical rotation called mutarotation... 

Absolute configuration (Section 7 5) The three dimensional arrangement of atoms or groups at a chirality center Acetal (Section 17 8) Product of the reaction of an aldehyde or a ketone with two moles of an alcohol according to the equation... 

The configuration of the glucoside linkage is different in the two, however. Structures [I] and [II], respectively, illustrate that the linkage is a /3-acetal-hydrolyzable to an equitorial hydroxide—in cellulose, and an a-acetal-hydrolyz-able to an axial hydroxide—in amylose, a starch ... 

Fig. 14. Section of cellulose acetate cigarette filter tow showing crimp configuration.

Protonolysis. Simple trialkylboranes are resistant to protonolysis by alcohols, water, aqueous bases, and mineral acids. In contrast, carboxyUc acids react readily with trialkylboranes, removing the first alkyl group at room temperature and the third one at elevated temperatures. Acetic and propionic acids are most often used. The reaction proceeds with retention of configuration of the alkyl group via a cycHc, six-membered transition state (206). 

Mercuration. Mercury(II) salts react with alkyl-, alkenyl-, and arylboranes to yield organomercurials, which are usehil synthetic intermediates (263). For example, dialkyhnercury and alkyhnercury acetates can be prepared from primary trialkylboranes by treatment with mercury(II) chloride in the presence of sodium hydroxide or with mercury(II) acetate in tetrahydrofuran (3,264). Mercuration of 3 -alkylboranes is sluggish and requires prolonged heating. Alkenyl groups are transferred from boron to mercury with retention of configuration (243,265). 

The most common oxidatiou states and corresponding electronic configurations of rhodium are +1 which is usually square planar although some five coordinate complexes are known, and +3 (t7 ) which is usually octahedral. Dimeric rhodium carboxylates are +2 (t/) complexes. Compounds iu oxidatiou states —1 to +6 (t5 ) exist. Significant iudustrial appHcatious iuclude rhodium-catalyzed carbouylatiou of methanol to acetic acid and acetic anhydride, and hydroformylation of propene to -butyraldehyde. Enantioselective catalytic reduction has also been demonstrated. 

Isomer separation beyond physical fractional crystallization has been accompHshed by derivatization using methyl formate to make /V-formyl derivatives and acetic anhydride to prepare the corresponding acetamides (1). Alkaline hydrolysis regenerates the analytically pure amine configurational isomers. 

Biolog ic lRemoval. Certain organisms normally present in activated sludge have the abiUty to store phosphoms. The process configuration for bio-P removal involves an anaerobic step in which phosphoms is released and acetate taken up by the bio-P organisms. This is followed by an aerobic step in which phosphoms is rapidly taken up by the bio-P. Under proper operating conditions, soluble effluent phosphoms levels of 0.1 mg/L are achievable from municipal wastewater. 

With these kinetic data and a knowledge of the reactor configuration, the development of a computer simulation model of the esterification reaction is iavaluable for optimising esterification reaction operation (25—28). However, all esterification reactions do not necessarily permit straightforward mathematical treatment. In a study of the esterification of 2,3-butanediol and acetic acid usiag sulfuric acid catalyst, it was found that the reaction occurs through two pairs of consecutive reversible reactions of approximately equal speeds. These reactions do not conform to any simple first-, second-, or third-order equation, even ia the early stages (29). 

Furfural — see Furan-2-oarbaldehyde, 532 Furfuryl acetate, o -(butoxycarbonyl)-anodic oxidation, 1, 424 Furfuryi acrylate polymerization, 1, 279 Furfuryl alcohol configuration, 4, 544 2-Furfuryl alcohol polyoondensation, 1, 278 reactions, 4, 70-71 Furfuryl alcohol, dihydro-pyran-4-one synthesis from, 3, 815 Furfuryl alcohol, tetrahydro-polymers, 1, 276 rearrangement, 3, 773 Furfuryl chloride reactions... 

Entry 4 shows that reaction of a secondary 2-octyl system with the moderately good nucleophile acetate ion occurs wifii complete inversion. The results cited in entry 5 serve to illustrate the importance of solvation of ion-pair intermediates in reactions of secondary substrates. The data show fiiat partial racemization occurs in aqueous dioxane but that an added nucleophile (azide ion) results in complete inversion, both in the product resulting from reaction with azide ion and in the alcohol resulting from reaction with water. The alcohol of retained configuration is attributed to an intermediate oxonium ion resulting from reaction of the ion pair with the dioxane solvent. This would react until water to give product of retained configuratioiL When azide ion is present, dioxane does not efiTectively conqiete for tiie ion-p intermediate, and all of the alcohol arises from tiie inversion mechanism. ... 

Nucleophilic substitution in cyclohexyl systems is quite slow and is often accompanied by extensive elimination. The stereochemistry of substitution has been determined with the use of a deuterium-labeled substrate (entry 6). In the example shown, the substitution process occurs with complete inversion of configuration. By NMR amdysis, it can be determined that there is about 15% of rearrangement by hydride shift accon any-ing solvolysis in acetic acid. This increases to 35% in formic acid and 75% in trifiuoroacetic acid. The extent of rearrangement increases with decreasing solvent... 

Evidently, since there is no appreciable rate acceleration, this participatimi is not very strong at the transition state. Nevertheless, the participation is strong enough to control stereochemistry. When mote nucleophilic solvents are used (e.g., acetic acid), participation is not observed, and the product is 100% of inverted configuration. 

When bicyclo[2.2.2]octyl brosylate was solvolyzed in acetic acid containing sodium acetate, the products were a mixture of bicyclo[2.2.2]octyl acetate and bicyclo[3.2.1]octyl acetate, each of which was optically active. The formation of bicyclo[2.2.2]octyl acetate was found to proceed with 82 15% retention of configuration, a result which is in... 

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