Racemization Stereoisomers

The racemic stereoisomer (16b) of monomorine I has been stereoselectively synthesized (416). Thus, successive bis-alkylations of l-methoxycarbonyl-3-pyrroline (379) gave tranj-2,5-diaIkylpyrroIine (381) as a 1 1 mixture of 4 -bromopentane isomers. N-Decarbomethoxylation gave the bicyclic compound... 

Determination of the absolute configuration of 27 (a constituent of certain peptide antibiotics, the relative configuration for all racemic stereoisomers 27 - 30 was determined as summarized on p 472 and 485) by a correlation-type synthesis commencing from (S)-vinylglycine (26, R = H)203. Note that this synthesis serves only to determine the absolute configuration at C-2 of 27 (and of 29). 

Another (4+2)-cycloaddition, not accompanied with fragmentation, was observed by Martin et al.I60> when 1-formyl hexahelicene (87) was treated with the ylid of (Et0)2P0CH2C02Et in boiling benzene for 12 h. Two racemic stereoisomers (88 a) and (88 b) were formed in 80 and 12%, respectively. At room temperature only one isomer (88a) was obtained in 90%. The obvious intermediate P-(l-hexahelicycl)acrylic ester, could not be isolated (Scheme 23). 

Diethyl ethylsuccinate and ethyl formate added to a suspension of Na-methoxide prepared from Na and methanol in anhydrous toluene, kept 2 days at 10-15° diethyl a-ethyl-a -formylsuccinate (Y 90.2% based on startg. m. consumed) hydrogenated 3-4 hrs. at 110-120° with Raney-Ni in anhydrous alcohol at an initial pressure of 80-100 atm., the catalyst separated, the solvent distilled off, and the residue distilled at 10 mm ethyl a-ethylparaconate (mixture of racemic stereoisomers Y 96.8%). F. e. s. N. A. Preobrazhenskii et al., X. 30, 2250 (1960) G. A. 55, 9368a. 

Trisdimethylamino phosphine turned out to be the preference for this type of reaction [383], when the halogen was less reactive, as found in the polyfluorine-containing halocarbons. This seems to be the method of choice to synthesize vinylfluorides from aldehydes. The next table hsts a number of pyrethroid acids prepared by the Homer-Wittig reaction from caronaldehyde, mostly as mixtures of stereoisomers, others as single racemic stereoisomers and some as single isomers. 

The hydrogenation of thymol leads to an equihbrium mixture of 60% ( )-menthol, 30% ( )-neomenthol, and 10% ( )-isomenthol. ( )-Neoisomenthol is formed to a negligible extent. Thus, 40% of undesired racemic stereoisomers have to be separated by distillation and are then converted on a Niotalyst by epimerization in the presence of about 10 MPa H2 hydrogen is not needed according to the stoichiometry, but suppresses dehydrogenation to menthone (Scheme 6.17.1). The resulting menthol rich mixture is recycled to the distillation. Subsequently, only the epimerization as a central part of the process is discussed. 

The diimine of benzaldehyde with // j-(9-l,2-cyclohexanediamine reacts with [PtMe2(//-SMe2)]2 to form a methyl complex with the cyclometallated 6 ,A, iV-tridentate ligand 636. The complex exists as a mixture of racemic stereoisomers. Protonation by [H(OEt)2] in the presence of ethylene produces a cationic complex with 7r-coordinated ethylene 637 (Equation (136)). Complexes with styrene and with propene 638 exhibit high stereoselectivity toward coordination of enantiofaces of the prochiral olefms. ... 

Ferslew, KE, Acuff, RV, Daigneault, EA, Woolley, TW and Stanton, PE (1993) Pharmacokinetics and hioavailability of the RRR and all racemic stereoisomers of alpha-tocopherol in humans after single oral administration. J. Clin. Pharmacol., 33, 84-88. 

It is also of interest to consider the effect of stereoregularity. Longworth [28] found ( j)/for isotactic PS to be 10.0 while atactic PS had a value of 2.3. Ishii [29] obtained a similar result with the isotactic PS having a ratio of 15.4 and the atactic a ratio of 4.6. Similar findings have been reported for PS model compounds. Longworth and Bovey [30] were the first to note qualitatively that meso 2,4-diphenylpentane (DPP) had more excimer emission than the racemic stereoisomer. Finally, Bokobza [13] found... 

The 9 — 15 fragment was prepared by a similar route. Once again Sharpless kinetic resolution method was applied, but in the opposite sense, i.e., at 29% conversion a mixture of the racemic olefin educt with the virtually pure epoxide stereoisomer was obtained. On acid-catalysed epoxide opening and lactonization the stereocentre C-12 was inverted, and the pure dihydroxy lactone was isolated. This was methylated, protected as the acetonide, reduced to the lactol, protected by Wittig olefination and silylation, and finally ozonolysed to give the desired aldehyde. 

Epoxidation of alkenes is a stereospecific syn addition Which stereoisomer of 2 butene reacts with peroxyacetic acid to give meso 2 3 epoxybu tane Which one gives a racemic mixture of (2/ 3/ ) and (25 35) 2 3 epoxybutane ... 

The separation of a racemic mixture into its enantiomeric components is termed resolution The first resolution that of tartaric acid was carried out by Louis Pasteur m 1848 Tartaric acid IS a byproduct of wine making and is almost always found as its dextrorotatory 2R 3R stereoisomer shown here m a perspective drawing and m a Fischer projection... 

Enantiomers (Section 7 1) Stereoisomers that are related as an object and its nonsupenmposable mirror image Enantioselective synthesis (Section 27 4) Reaction that converts an achiral or racemic starting material to a chiral product in which one enantiomer is present in excess of the other... 

DUactide (5) exists as three stereoisomers, depending on the configurations of the lactic acid monomer used. The enantiomeric forms whereia the methyl groups are cis are formed from two identical lactic acid molecules, D- or L-, whereas the dilactide formed from a racemic mixture of lactic acid is the opticaUy iaactive meso form, with methyl groups trans. The physical properties of the enantiomeric dilactide differ from those of the meso form (6), as do the properties of the polymers and copolymers produced from the respective dilactide (23,24). 

When additional substituents ate bonded to other ahcycHc carbons, geometric isomers result. Table 2 fists primary (1°), secondary (2°), and tertiary (3°) amine derivatives of cyclohexane and includes CAS Registry Numbers for cis and trans isomers of the 2-, 3-, and 4-methylcyclohexylamines in addition to identification of the isomer mixtures usually sold commercially. For the 1,2- and 1,3-isomers, the racemic mixture of optical isomers is specified ultimate identification by CAS Registry Number is fisted for the (+) and (—) enantiomers of /n t-2-methylcyclohexylamine. The 1,4-isomer has a plane of symmetry and hence no chiral centers and no stereoisomers. The methylcyclohexylamine geometric isomers have different physical properties and are interconvertible by dehydrogenation—hydrogenation through the imine. 

The structure of a natural product is shown without any specification of stereochem-istiy. It is a pure substance which gives no indication of being a mixture of stereoisomers and has zero optical rotation. It is not a racemic mixture because it does not yield separate peaks on a chiral HPLC column. When the material is completely hydrolyzed, it gives a racemic sample of the product shown. Deduce the complete stereochemical structure of the natural product fiom this information. 

Asymmetric epoxidation of racemic unsaturated fluoro alcohols by the chiral Sharpless reagent can be exploited for kmetic resolution of enantiomers The recovered stereoisomer has 14-98% enantiomeric excess [55] (equation 50)... 

Recall from Section 7.13 that a stereospecific reaction is one in which each stereoisomer of a particular starting material yields a different stereoisomeric form of the reaction product. In the examples shown, the product from Diels-Alder cycloaddition of 1,3-butadiene to c/s-cinnamic acid is a stereoisomer of the product from trans-cinnamic acid. Each product, although chiral, is formed as a racemic mixture. 

It is understood that a-amino acids occur as their l stereoisomers unless otherwise indicated. The d notation is explicitly shown when a D amino acid is present, and a racemic amino acid is identified by the prefix dl. 

In the desulfurization of 3-substituted thiophenes several stereoisomers may be formed in certain cases. Both meso and racemic compounds have been obtained from the desulfurization of 3,4-diaryl-substituted thiophenes. It is claimed, however, that only meso, -diphenyladipic acid is obtained upon desulfurization of 3,4-di-phenyl-2,5-thiophenedicarboxylic acid and only di-isoleucin from 3-thienylglycine. The formation of small amounts of dimeric products in the desulfurization has been discussed with reference to the mechanism of this reaction. ... 

Schemes 28 and 29 illustrate Curran s synthesis of ( )-hirsutene [( )-1]. Luche reduction58 of 2-methylcyclopentenone (137), followed by acetylation of the resulting allylic alcohol, furnishes allylic acetate 138. Although only one allylic acetate stereoisomer is illustrated in Scheme 28, compound 138 is, of course, produced in racemic form. By way of the powerful Ireland ester enolate Clai-sen rearrangement,59 compound 138 can be transformed to y,S-unsaturated tm-butyldimethylsilyl ester 140 via the silyl ketene acetal intermediate 139. In 140, the silyl ester function and the methyl-substituted ring double bond occupy neighboring regions of space, a circumstance that favors a phenylselenolactonization reac-...

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