Stereoisomers special

In the Sharpless epoxidation of divinylmethanols only one of four possible stereoisomers is selectively formed. In this special case the diastereotopic face selectivity of the Shaipless reagent may result in diastereomeric by-products rather than the enantiomeric one, e.g., for the L -(-(-)-DIPT-catalyzed epoxidation of (E)-a-(l-propenyl)cyclohexaneraethanol to [S(S)-, [R(S)-, [S(R)- and [R(R)-trans]-arate constants is 971 19 6 4 (see above S.L. Schreiber, 1987). This effect may strongly enhance the e.e. in addition to the kinetic resolution effect mentioned above, which finally reduces further the amount of the enantiomer formed. 

In addition to constitution and configuration, there is a third important level of structure, that of conformation. Conformations are discrete molecular arrangements that differ in spatial arrangement as a result of facile rotations about single bonds. Usually, conformers are in thermal equilibrium and cannot be separated. The subject of conformational interconversion will be discussed in detail in Chapter 3. A special case of stereoisomerism arises when rotation about single bonds is sufficiently restricted by steric or other factors that- the different conformations can be separated. The term atropisomer is applied to stereoisomers that result fk m restricted bond rotation. ... 

The threo stereoisomer was the major product obtained by the synthesis in Scheme 13.14. This stereochemistry was established by the conjugate addition in Step A, where a significant (4-6 1) diastereoselectivity was observed. The C(4)-C(7) stereochemical relationship was retained through the remainder of the synthesis. The other special features of this synthesis are in Steps B and C. The mercuric acetate-mediated cyclopropane ring opening was facilitated by the alkoxy substituent.19 The reduction by NaBH4 accomplished both demercuration and reduction of the aldehyde group. 

The related configurations of stereocenters in substituted cyclic nitronates can be determined by analyzing the spin—spin coupling constants between the vicinal protons in the stereoisomer discussed (Chart 3.7) (276). If needed, the results of this analysis are supplemented by special NOE experiments. 

It should be noted that specially purified individual stereoisomers of six-membered cyclic nitronates were used in coupling with silyl ketene acetal. Hence, the mechanistic model of the C,C-coupling reaction can be discussed on the basis of the configurations of the stereocenters of the starting nitronates of intermediate cations (357) (see Section 3.5.2.1), and the resulting tetrahydro-oxazines (358) (for more details, see below). It should be noted that most of C,C-coupling reactions of six-membered cyclic nitronates with silyl ketene acetal are characterized by a very high diastereoselectivity. 

The two forms of mirror images are called enantiomers, or stereoisomers. All amino acids in proteins are left-handed, and all sugars in DNA and RNA are right-handed. Drug molecules with chiral centers when synthesized without special separation steps in the reaction process result in 50/50 mixtures of both the left- and right-handed forms. The mixture is often referred to as a racemic mixture. 

Many drugs are racemates, including 13-blockers, nonsteroidal anti-inflammatory agents, and anticholinergics (e.g benzetimide A). A racemate consists of a molecule and its corresponding mirror image which, like the left and right hand, cannot be superimposed. Such chiral ( handed ) pairs of molecules are referred to as enantiomers. Usually, chirality is due to a carbon atom (C) linked to four different substituents ( asymmetric center ). Enantiomerism is a special case of stereoisomerism. Non-chiral stereoisomers are called diaster-eomers (e.g., quinidine/quinine). 

The flexible, semi-flexible, or anancomeric behavior of tetrahydro-l,3-oxazines 83 spiro-connected to cyclohexane was studied by NMR spectroscopy. The possible stereoisomers were deduced by taking into account the special chirality of the heterocycle and the spiro skeleton <2000M975>. 

Since (Z)- and ( )-stereoisomers of unsaturated oxazolones can be obtained using appropriate isomerization procedures, cis and trans isomers of cyclopropane derivatives can be obtained in a stereoselective manner, although special care must be taken with experimental conditions to obtain the best stereoselectivity. Both racemic cis- and fraui-l-amino-2-phenylcyclopropanecarboxylic acid 641 and 644 have been obtained from the corresponding (Z)- or ( )-4-benzylidene-2-phenyl-5(4//)-oxazolone 621 and 642 using diazomethane. Care was taken to affect the... 

Because each chiral center added to a chain doubles the number of possible configurations, we expect eight different stereoisomers with three chiral carbons, sixteen with four, and so on. The simple rule then is 2n possible different stereoisomers for n chiral centers. As we shall see later, this rule has to be modified in some special cases. 

Optical isomers are special kinds of stereoisomers they are non-superimposable mirror images of each other (Fig. 16.27). Both geometrical and optical isomerism can occur in an octahedral complex, as in [CoCI2(en)2]+ the trans isomer is green (14a) and the two alternative cis isomers (14b) and (14c), which are optical isomers of one another, are violet. 

These findings precipitated the widespread use of a fenfluramine-phentermine combination therapy that came to be known as Fen-Phen. The combined use of the two anorexigens had never been approved by the FDA, nor had the longterm safety of the therapy ever been established. Nevertheless, weight loss clinics specializing in Fen-Phen therapy were established throughout the country. In 1996, the FDA narrowly approved the more potent, less adverse-effect-prone (+)stereoisomer of fenfluramine, dexfenfluramine, for less-than-l-yr use in the treatment of obesity. The New York Times reported that in 1996, 18 million prescriptions had been written for fenfluramine alone or in combination with phentermine and that about 6 million Americans took the drug (37). 

Ketone 2 and its metabolite chloro-acetone (16) are alkylating agents. This property is of special importance, when the substrate is being reduced stereoselectively by reductases with an essential sulfhydryl function. Because the sulfhydryl groups are inhibited irreversibly by alkylation [69], stereoisomer distribution in the product is considerably affected. 

The cis- and trans-stereoisomers of cyclic compounds that were presented previously are actually just special cases of the type of stereoisomers that we have just discussed. For example, consider the case of 1,2-dimethylcyclohexane ... 

Differences in spatial orientation might seem unimportant, but stereoisomers often have remarkably different physical, chemical, and biological properties. For example, the cis and trans isomers of butenedioic acid are a special type of stereoisomers called cis-trans isomers (or geometric isomers). Both compounds have the formula HOOC—CH=CH—COOH, but they differ in how these atoms are arranged in space. The cis isomer is called maleic acid, and the trans isomer is called fumaric acid. Fumaric acid is an essential metabolic intermediate in both plants and animals, but maleic acid is toxic and irritating to tissues. 

A special situation applies to compound 30. The exo-endo structure was identified from the two allyl vicinal proton coupling constants33. Only one stereoisomer could be detected. With increasing temperature above 210 K the latter two coupling constants average, the result of presumably sequential rotations around both allyl carbon-carbon... 

The acid-catalysed reaction of resorcinol with aldehydes leads to cyclic tetra-mers which are grouped in the class of resorc[4]arenes (exo-calixarenes).3 This reaction is quite general with respect to the aldehyde and to the starting resorcinol, which can bear several substituents in the 2-position.15 Niederl and Hogberg showed in a typical example, the synthetic procedure by which an ethanol solution of resorcinol, acetaldehyde and 37% hydrochloric acid maintained at 80 °C for 16 h, afforded 75% of the all cis C-methylresorc[4]arene. A shorter reaction time leads to the kinetic stereoisomer (trans,cis,trans,cis).16 This procedure is general for many aldehydes (Scheme 7.4), but the use of formaldehyde is limited to some special cases.17... 

The reduction of yff-ketoesters to aldols is one of the most important applications of Ru(II)-BlNAP catalysts [7]. As a special bonus, the chirally labile C2 stereogenic center can be exploited in a dynamic kinetic resolution such that racemic reactants yield only one of the four conceivable stereoisomers in high diastereomeric and enantiomeric excess. This strategy has been extended to the reduction of -ketophosphonates 10. The 3-hydroxyphosphonic acids 7 which are accessible by this route constitute promising starting materials for the synthesis of peptide analog and antibiotics [8]. 

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