Stereochemistry racemization

Reaction type Substitution (S 1) Stereochemistry Racemization Regiochemistry Not applicable (nucleophile attacks carbon next to LG) ... 

Stereochemistry Racemization (possibly excess inversion) Inversion... 

The important consequence of this is that reactions that involve radicals, like reactions that involve carbocations, result in the loss of stereochemistry (racemization) at the radical carbon. 

For labile metals, the sequence of alkyl dissociation and carbanion inversion could lead to racemization. Elimination, followed by olefin dissociation and coordination of the opposite face results in loss of stereochemistry. Racemizations of Be, Al, and B alkyls proceed by the jS-elimination mechanism... 

Stereochemistry Racemization inversion and retention. Inversion of configuration... 

A novel technique for dating archaeological samples called ammo acid racemiza tion (AAR) IS based on the stereochemistry of ammo acids Over time the configuration at the a carbon atom of a protein s ammo acids is lost m a reaction that follows first order kinetics When the a carbon is the only chirality center this process corresponds to racemization For an ammo acid with two chirality centers changing the configuration of the a carbon from L to D gives a diastereomer In the case of isoleucme for example the diastereomer is an ammo acid not normally present m proteins called alloisoleucme... 

Vitamin K compounds ate yellow solids or viscous liquids. The natural form of vitamin is a single diastereoisomer with 2 (E), 7 (R), ll (R) stereochemistry. The predominant commercial form of vitamin is the racemate and a 2 (E)j (Z) mixture. Table 1 fists some physical and spectral properties of vitamin K. ... 

The stereochemistry of hydrogen-deuterium exchange at the chiral carbon in 2-phenylbutane shows a similar trend. When potassium t-butoxide is used as the base, the exchange occurs with retention of configuration in r-butanol, but racemization occurs in DMSO. The retention of configuration is visualized as occurring through an ion pair in which a solvent molecule coordinated to the metal ion acts as the proton donor... 

There have been many studies aimed at deducing the geometiy of radical sites by examining the stereochemistry of radical reactions. The most direct kind of study involves the generation of a radical at a carbon which is a stereogenic center. A planar or rapidly inverting radical would lead to racemization, whereas a rigid pyramidal structure should... 

Further evidence for a bromine-bridged radical comes from radical substitution of optically active 2-bromobutane. Most of the 2,3-dibromobutane which is formed is racemic, indicating that the stereogenic center is involved in the reaction. A bridged intermediate that can react at either carbon can explain the racemization. When the 3-deuterated reagent is used, it can be shown that the hydrogen (or deuterium) that is abstracted is replaced by bromine with retention of stereochemistry These results are also consistent with a bridged bromine radical. 

Amino acid racemization (Section 27.2) A method for dating archeological samples based on the rate at which the stereochemistry at the a carbon of amino acid components is randomized. It is useful for samples too old to be reliably dated by decay. 

The relative stereostructure of 9-acetyl-7-hydroxy-l,2-dimethyl-7-meth-oxycarbonyl-4-phenyl-6-oxo-l, 4,7,8-tetrahydro-6/7-pyrido[l, 2-u]pyri-midine-3-carboxylate 122 was justified by an X-ray diffraction analysis (97JOC3109). The stereochemistry and solid state structure of racemic trans-6,9-//-l, 6-dimethyl-9 z-ethoxy-9-hydroxy-4-oxo-l,6,7,8,9,9 z-hexahydro-4//-pyrido[l,2- z]pyrimidine-3-carboxylate (123), adopting a cw-fused conformation, were determined by X-ray investigations (97H(45)2175). 

Most of the biochemical reactions that take place in the body, as well as many organic reactions in the laboratory, yield products with chirality centers. Fo example, acid-catalyzed addition of H2O to 1-butene in the laboratory yield 2-butanol, a chiral alcohol. What is the stereochemistry of this chiral product If a single enantiomer is formed, is it R or 5 If a mixture of enantiomers i formed, how much of each In fact, the 2-butanol produced is a racemic mix ture of R and S enantiomers. Let s see why. 

Figure 11.10 Stereochemistry of the S j1 reaction. Because the reaction goes through an achiral intermediate, an enantiomeri-cally pure reactant should give a racemic product.

Desilylation of 20, followed by oxidation of the secondary hydroxyl group furnishes racemic 1 in a yield of 81 %. Spectroscopic and bioassay characteristics of synthetic ( )-1 matched natural peripla-none B in every detail. The constitution and relative stereochemistry of the elusive American cockroach sex excitant periplanone B (1) has been secured in a very elegant way.5... 

The reactions of allylboronates 1 (R = H or CH3) may proceed either by way of transition state 3, in which the a-substituent X adopts an axial position, or 4 in which X occupies an equatorial position. These two pathways are easily distinguished since 3 provides 7 with a Z-olefin, whereas 4 provides 8 with an E-olefinic linkage. There is also a second fundamental stereochemical difference between these two transition states 7 and 8 are heterochirally related from reactions in which 1 is not racemic. That is, 7 and 8 arc enantiomers once the stereochemistry-associated with the double bond is destroyed. Thus, the selectivity for reaction by way of 3 in preference to 4, or via 6 in preference to 5 in reactions of a-subsliluted (Z)-2-butenylboronate 2, is an important factor that determines the suitability of these reagents for applications in enantioselective or acyclic diastereoselective synthesis. 

It should be stressed that this treatment of polymer stereochemistry only deals with relative configurations whether a substituent is "up or down" with respect to that on a neighboring unit. Therefore, the smallest structural unit which contains stereochemical information is the dyad. There are two types of dyad meso (m), where the two chiral centers have like configuration, and racemic /-), where the centers have opposite configuration (Figure 4.1). 

Clear evidence in favor of 6.75 being an intermediate came, however, from stereochemistry. If the indazole cyclization takes place at a chiral carbon atom in the exposition of the alkyl group in 6.78, the stereochemistry of the 3-i/-indazole 6.79 can indicate whether the 5-diazo-6-methylene-l,3-cyclohexadiene 6.75 is an intermediate or whether, on the other hand, deprotonation and cyclization are synchronous. In the first case a racemic indazole 6.79 is expected. In the case of a synchronous reaction, however, a stereospecific product, probably with retention of the chirality at Ca, should be observed. 

In a recent total synthesis of the novel neurotrophic agent merrilactone A (22, Scheme 4) by Inoue and Hirama [24], key intermediate 21 with the cis-bicyclo[3.3.0] octane framework embedded within the caged pentacycle 22 was elaborated from cyclobutane 18 by a sequence of RCM and immediate cleavage of the resulting bicyclic vicinal diol 19 to raeso-diketone 20. Cyclooctenedione 20 then underwent regioselective transannular aldol reaction at low temperature (LHMDS, THF, -100 °C) to produce a 3 1 mixture of isomers in 85% combined yield. The major isomer 21 with the required stereochemistry was then converted into the racemic natural compound ( )-22 in 19 steps. 

The study of optical isomers has shown a similar development. First it was shown that the reduction potentials of several meso and racemic isomers were different (Elving et al., 1965 Feokstistov, 1968 Zavada et al., 1963) and later, studies have been made of the ratio of dljmeso compound isolated from electrolyses which form products capable of showing optical activity. Thus the conformation of the products from the pinacolization of ketones, the reduction of double bonds, the reduction of onium ions and the oxidation of carboxylic acids have been reported by several workers (reviewed by Feokstistov, 1968). Unfortunately, in many of these studies the electrolysis conditions were not controlled and it is therefore too early to draw definite conclusions about the stereochemistry of electrode processes and the possibilities for asymmetric syntheses. 

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