Stereoisomers forming

Percentage of p-Carotene Stereoisomers Formed at Different Temperatures in Petroleum Ether-Benzene Solution... 

Resistance to hydration was elucidated with tricyclic model compounds that lack the side chain and, hence, pharmacological activity. In this context, a useful comparison has been made between two meso compounds, namely 5W-dibenz.oja, dIcycloheplene 10,11-oxide (10.130, X = CH2) and d.v-slilbcnc oxide (10.7) [195]. The former compound proved to be a very poor substrate for rabbit liver microsomal EH, with a Km value comparable to that of cis-stilbene oxide, but Emax ca. 100-fold lower. This indicates that the two compounds have a comparable affinity for the enzyme, but that nucleophilic attack in the catalytic step is much less efficient for dibcnzo[ // cycloheplcnc 10,11-oxide than for d.v-slilbcnc oxide. This implies that the former compound acts better as an inhibitor than as a substrate of microsomal EH. Furthermore, there was also a fundamental steric difference in the reaction course of the two substrates, since the predominant stereoisomer formed from dibenzo //]cyclohep(ene 10,11-oxide had the (I OS, 11. -configuration,... 

Fig. 8. Variation, as a function of pressure, of the proportion of saturated stereoisomers formed during the hydrogenation of 4-(eH-butylmethyleneoyclohexane ( ) and i-tert-butyl- -methylcyelohexene (A) PtOj [64).

Although the principal stereoisomer formed at ambient temperatures in the hydrogenation of disubstituted benzenes has the cis configuration, trans isomers are also produced, the amount being a function of the structure of the substrate, the pressure of hydrogen, the temperature, and the catalyst (97-100). Mixtures are formed although the products are virtually unaffected under these conditions consequently the trans isomers result from a kinetically controlled process. 

An important extension of the ketene acetal cycloaddition involved the diazoniapentaphenes. An example is the conversion of 4a,8a-bis(azonia)pentaphene (31) to a mixture of two diadducts of which 32 represents the stereoisomer formed by addition of both mole-... 

Irrespective of the mechanism of resolution, HPLC CSPs work by providing a chiral environment for analyte stereoisomers to interact with. Resolution relies upon the formation of reversible, transient diastereomers on the CSP that have different free energies of interaction and therefore stability. The stereoisomer forming the most stable diastereomer with the CSP will be the most retained and vice versa. Free energy differences are typically small in such systems but may be large enough to produce useable resolutions provided the column efficiency is sufficient [41]. If column efficiency is insufficient to allow complete separation of the stereoisomers, inaccurate integrations can result in erroneous... 

Definitions It is important to define precisely the stereochemical terms that will be employed in this discussion. The term racemization has often been used loosely by chemists to describe any situation in which a mixture of enantiomers or diastereomers is produced as a result of an amide-bond-forming reaction, without regard to the ratio of stereoisomers formed. For the purposes of this discussion though, the term racemization will be used to describe the situation leading toward the formation of an exact 1 1 mixture of stereoisomers. Racemization, therefore, is a process that occurs to a collection of molecules, and can happen to a single residue or to one residue in a peptide sequence (Scheme 1). This is a macroscopic event, as the result is detected subsequent to the amide bond formation. 

Stilbene derivatives can be reduced with alkali metals in liquid ammonia. The reaction is usually performed in a homogeneous medium to give substituted diphenylethane compounds as a mixture of the stereoisomer forms. However, there are compounds (in particular, biologically active ones) for which stereospecificity of the synthesis has decisive importance. A simple modification of the reduction method with an alkali metal in liquid ammonia was found (Collins Hobbs 1983) that makes it possible to perform the process stereoselectively. The metal is not predissolved, as is usual, but is added in small portions without trying to make the reaction medium homogeneous. Stereoselectivity is ensured by having the reduction proceed not in the solution bulk but on the surface of the metal. 

In the case of methylphenylsiloxanes, stereoisomer ring molecules are possible e.g., for the cyclic trimer, there exist a cis and a trans form and for the cyclic tetramer, there are four stereoisomer forms. Based on gas chromatography data, it has been suggested that these are in statistical equilibrium with each other at elevated temperature (117, 210). 

This internal nucleophilic addition introduces a new chiral centre into the molecule. The carbon of the new centre is known as the anomeric carbon and the two new stereoisomers formed are referred to as anomers. The isomer where the new hydroxy group and the CH2OH are on opposite sides of the plane of the ring is known as the alpha (a) anomer. Conversely, the isomer with the new hydroxy group and terminal CH2OH on the same side of the plane of the ring is known as the beta (P) anomer (Figure 1.12). 

We can now let you into the secret of that chemical reaction . A benzocyclobutene was heated with methyl acrylate to give a 1 1 mixture of the two isomers. What is the mechanism of the reaction and why is only one regioisomer but a mixture of stereoisomers formed Isomer B is converted into isomer A on treatment with base. What is the stereochemistry of A and B ... 

At lower temperatures (- 70° C) with lower equilibrium monomer concentrations, unsaturated dimers rapidly form tetramers without producing a significant amount of trimers [295]. This indicates that the unsaturated dimer prefers to dimerize, with depropagation and indan formation less probable. 1,2-Hydride shifts may also occur, as indicated by the large variety of dimer stereoisomers formed, as well as by spectral changes in the stopped-flow studies of the dimerization. Eq. (96) outlines the variety of isomeric indan derivatives formed at later stages of the reaction by a combination of hydride shifts (cf., Section V.A.4) and intramolecular cyclizations. 

What is the major stereoisomer formed in each reaction ... 

Using c/s- and frans-3-hexene, demonstrate that the addition of HCI is not a stereospecific reaction. Draw the structure of the stereoisomers formed from each alkene. 

Given that syn addition of H2 occurs from both sides of a trigonal planar double bond, draw all stereoisomers formed when each alkene is treated with H2. 

Sample Problem 12.2 Draw the stereoisomers formed when cis- and frans-2-butene are epoxidized with mCPBA. 

Problem 12.17 Draw all stereoisomers formed when each alkene is treated with mCPBA. 

Draw the products formed when allylic alcohol B is treated with each reagent. Indicate the stereochemistry of any stereoisomers formed. 

Draw the six products (including stereoisomers) formed when A is treated with NBS + Arv. 

Sample Problem 20.1 Draw all stereoisomers formed in the following reaction. 

Problem 21.14 Draw the products of each reaction, inciude aii stereoisomers formed. 

Draw the products formed in each Wittig reaction. Draw all stereoisomers formed when a mixture of products results. 

Hydroxy aldehydes A and B readily cyclize to form hemiacetals. Draw the stereoisomers formed in this reaction from both A and B. Explain why this process gives an optically inactive product mixture from A, and an optically active product mixture from B. 

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