Stereochemical aspects

Stereochemical Aspects.— The barrier to inversion of phosphiran (140) has been estimated to be 33 kcal mol. The Raman spectrum of gaseous tetra-methylphosphine (141) contained a peak at 426 cm, which disappeared on 

Stereochemical Aspects.—The i.r. and Raman spectra of ethylphosphine and its deuteriated analogue (99) indicate that the fluid phases contain gauche- and trans-conformers, but the solid phase contains the /ra/w-conformer only. The methyl 

Stereochemical Aspects.—The methyl torsional frequencies have been obtained for a series of fluoro- and chloro-methylphosphines and their chalcogenides from the spectra in the solid and gas phases. The spectrum of (129) is the same in all phases, and it is concluded that the molecules take up a gauche conformation, with the PNCa section planar. The far-i.r. and Raman spectra of trimethylphosphine indicate a CPC bond 

The gas- and liquid-phase vibrational spectra and dipole moment of t-butyltetrafluorophosphorane are in accordance with the t.b.p. structure (134), with only slight distortion towards a square-pyramidal geometry.  

The steric course at during the normal y-replacement and abnormal y-ehmina-tion-deamination reactions catalysed by cystathionine-y-synthase has been deduced in an elegant series of experiments described by Walsh and coworkers. An important aid in this stereochemical investigation was the observation that L-vinylglycine [88] 

with deuterium and (ii) vinylglycine containing deuterium at only one of the vinyhc positions. Experiments based on this approach were performed [89] and it was shown that in the reaction catalysed by cystathionine-y-synthase, both (4S)-[4- H]-0-succinyl homoserine and (Z)-(4- H]vinylglycine were converted into (45 )-[4- H]cystathionine (Fig. 38). This result is only possible if both transformations proceed through the same intermediate (Fig. 38, 3). These experiments thus show that in the enzymic conversion of (4S)-[4- H]-0-succinyl homoserine (Fig. 38, 1) to products, the terminal double bond of the complex (Fig. 38, 3) has the (Z) configuration. This observation when considered in conjunction with the earlier demonstration that in the conversion the atom is removed [84-86], allows 

One of the many attractive features of the Ireland-Claisen rearrangement lies in its abiUty to reUably transfer stereochemistry to one or two newly formed sp stereocenters (C2 and/or C3 of the pentenoic acid) as well as to the resulting C4—C5 alkene. It has been found that the stereochemical outcome of the rearrangement may be governed by a variety of influences, some easily rationaUzed and others more subtle. 

The stereochemistry of the reaction catalyzed by the lysine 2,3-aminomutase in C. subterminah SB4 was elucidated in detail by Aberhardt and Gould [32]. Incubation experiments with cell-free extracts of C. subterminah SB4 and (2RS)-[3-13C,2-lsN]lysine and NMR spectroscopy of the isolated /Mysine as the di-N-phthaloyl ethyl ester derivative revealed that the amino group migrates in an intramolecular reaction to position 3S in /Mysine 25. 

Quantitative study of steric effects in heteroaromatic compounds 88AHC(43)173. 

In this Section, no attempt will be made to elucidate the principles governing the formation of ring systems, preferentially, by reaction of sugars with aldehydes and ketones. Instead, a brief outline is presented of developments in this field which have necessitated a reconsideration of some of the earlier concepts and data. 

It had previously been held that the formation of a five-membered 

The earlier interpretation of the difference in the ease of formation of five-membered rings from cis and trans vicinal diols is, with some slight modification, stUl valid. In the cfs-diol, a decrease of the dihedral angle 

It has also been suggested that the greater puckering of 2,2-dimethyl-1,3-dioxolane, relative to that of 1,3-dioxolane, would imply that, as less reduction of the dihedral angle of the hydroxyl groups in the diol would be required for the former, the formation of isopropylidene acetals would therefore be easier than that of methylene derivatives. This is confirmed from our knowledge of acetal condensations. 

Another aspect of the stereochemistry of cyclic acetals which has received attention is the phenomenon of diastereoisomerism. It had hitherto been presumed that, if the condensation of an aldehyde with a diol gives a six-membered acetal ring, this would have an equatorially disposed alkyl or aryl group. For a five-membered acetal ring, the isomer expected is not so readily defined. The absolute configurations of some cyclic benzylidene acetals have been determined by Foster and coworkers from proton magnetic resonance spectral data. They established that the phenyl substituent in 4,6-0-benzylidene-D-glucopyranose occupies an equatorial position, as in (12). 

Chelated peptide ester enolates [35] and, more recently, peptide amide enolates [36] have also been shown to be suitable nucleophiles in Pd-catalyzed allylations. The absolute configuration of the newly formed stereogenic center is directed 

Asymmetric Induction on Prochiral or Racemic Allylic Substrates The most common type of asymmetric allylations starts from prochiral or racemic allyhc substrates. The difference in the reactivity of the diastereomeric Pd-allyl complexes leads to asymmetric induction on the newly formed stereogenic centers. 

Enantiomeric substitution products can be obtained with the corresponding enantiomeric ligands with the same rate of selectivity. In general, this is true in most cases, but sometimes different enantiomeric excess values are observed, depending on the ligand or the leaving group used. This memory effect can only be explained if the substitution does not proceed via a fully symmetrical it-allyl complex, but via a close ion pair [45]. 

1 Allylic Esters Mainly allylic esters are used as substrates for palladium-catalyzed allylic alkylations. Among these substrates, acetates play a dominant role. 

Reactions that proceed under neutral conditions are highly desirable, and several allylic substrates meet this requirement 

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Enolate geometry (E- or Z-) is an important stereochemical aspect. Z-Enolates usually give a higher degree of stereoselection than E-enolates. 

Electron deficient carbon-carbon double bonds are resistant to attack by the electrophilic reagents of Section 5.05.4.2.2(t), and are usually converted to oxiranes by nucleophilic oxidants. The most widely used of these is the hydroperoxide ion (Scheme 79). Since epoxidation by hydroperoxide ion proceeds through an intermediate ct-carbonyl anion, the reaction of acyclic alkenes is not necessarily stereospecific (Scheme 80) (unlike the case of epoxidation with electrophilic agents (Section 5.05.4.2.2(f)) the stereochemical aspects of this and other epoxidations are reviewed at length in (B-73MI50500)). 

Up to this point, we have emphasized the stereochemical properties of molecules as objects, without concern for processes which affect the molecular shape. The term dynamic stereochemistry applies to die topology of processes which effect a structural change. The cases that are most important in organic chemistry are chemical reactions, conformational changes, and noncovalent complex formation. In order to understand the stereochemical aspects of a dynamic process, it is essential not only that the stereochemical relationship between starting and product states be established, but also that the spatial features of proposed intermediates and transition states must account for the observed stereochemical transformations. 

In Section 6.21 we listed three main methods for polymerizing alkenes cationic, free-radical, and coordination polymerization. In Section 7.15 we extended our knowledge of polymers to their stereochemical aspects by noting that although free-radical polymerization of propene gives atactic polypropylene, coordination polymerization produces a stereoregulai polymer with superior physical properties. Because the catalysts responsible for coordination polymerization ar e organometallic compounds, we aie now in a position to examine coordination polymerization in more detail, especially with respect to how the catalyst works. 

Describe the stereochemical aspects of the structure of cystine, the structure that is a disulfide-linked pair of cysteines. 

Stereochemical aspects of epoxidation of substituted norbomenes and accompanying intramolecular transformations 98UK299. 

Previous work on the reactions of allylzinc reagents, which usually does not include stereochemical aspects, is collected in a review4. 

Nevertheless, there are promising strategies which may help to overcome these problems, for example, the use of less basic organometallics and/or the activation of the C-N double bond by Lewis acids. Therefore, it is not particularly surprising that the number of publications dealing with the stereochemical aspects of these reactions increases continuously. 

Stereochemical aspects of reactions of complexes of d° transition metals with multiply bonded ligands. Y. V. Kokunov and Y. A. Buslaev, Coord. Chem. Rev., 1982, 47,15-40 (120). 

For a recent discussion on the stereochemical aspects of the Diels-Alder reaction with vinyl sulphoxides see References 662, 663. It should be pointed out that vinyl sulphoxides can be considered in [2 + 4]-cycloadditions as acetylene synthons since the sulphinyl moiety may be removed from the product by sulphenic acid elimination. Paquette and coworkers took advantage of this fact in the synthesis of properly substituted anthracenes 562664, (equation 360). 

Before we leave the enhancement of the + R effect of S02Me by octet expansion, it is appropriate to mention its stereochemical aspect. This was examined by Kloosterziel and Backer in the later of their two papers to which reference has already been made78. They measured the pKa value of 4-methylsulfonyl-3,5-xylenol 25 and compared its acidity with that of 4-methylsulfonylphenol. Parallel measurements were made with 4-CN and 4-N02... 

Details of the conditions used in these reductions are discussed, especially the fact that this is apparently the first example in which the stereochemical aspects of an electrochemical desulphonylation reaction on a complex molecule have been examined. It is likely that further work will be profitable, given suitable substituents on a molecule, since sulphones (especially vinyl and aryl sulphones) should be good candidates for this type of reduction. 

Cyclopentadienes and cyclohexadienes are versatile dienes that are commonly used to study mechanistic, regio- and stereochemical aspects of the Diels-Alder reaction and for synthetic purposes. 

There are other stereochemical aspects to the reduction of aldehydes and ketones. If there is a chiral center to the carbonyl group, even an achiral reducing agent can give more of one diastereomer than of the other. Such diastereoselective reductions have been carried out with considerable success. In most such cases Cram s rule (p. 147) is followed, but exceptions are known. ... 

In a series of three papers, Noguchi and co-workers have reported their continuing studies on the formation of heterocycle-fused azepine systems <96X13081, 96X13097, 96X13111>. A typical example is the conversion of the aldehyde 15 into the azepines 16 and 17 (Scheme 3). Xhe reaction also proceeds with imines when the dihydroazepine prior to bridging can be isolated. Mechanistic and stereochemical aspects of the reaction have been explored. 

The structure-reactivity relationship between a 19-Me- and 19-nor-5,10-seco-steroid has been investigated using lOOC and intramolecular nitrone cycloaddition taking into account various stereochemical aspects (Schemes 27 and 28) [67]. The E-19-nor-5,10-seco-ketone 255 a, on treatment with hydroxylamine hydrochloride (R = H), undergoes lOOC via 256a to a single isoxazolidine 257... 

It can be used both in polar and non-polar solvents and complements the Mala-prade oxidation with periodate, with which it has been compared and contrasted. Much has been published on the mechanism of its action, particularly on the stereochemical aspects of its reactivity, and only a summary of the main features can be presented. 

The addition reaction of enolates and enols with carbonyl compounds is of broad scope and of great synthetic importance. Essentially all of the stabilized carbanions mentioned in Section 1.1 are capable of adding to carbonyl groups, in what is known as the generalized aldol reaction. Enolates of aldehydes, ketones, esters, and amides, the carbanions of nitriles and nitro compounds, as well as phosphoms- and sulfur-stabilized carbanions and ylides undergo this reaction. In the next section we emphasize the fundamental regiochemical and stereochemical aspects of the reactions of ketones and aldehydes. 

Ralph, J. Young, R. A. Stereochemical aspects of addition reactions involving lignin model quinone methides. J. Wood Chem. Technol. 1983, 3, 161-181. 

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