Stereochemical feature

It is apparent that the reaction of sugars with thiols is influenced by interactions between the numerous functional groups present, as changes in the nature of the reactants are reflected in variations in the rate or course (or both) of the mercaptalation. D-Glucose reacts with an excess of ethanethiol in hydrochloric acid during four hours at 0° to form the diethyl dithioacetal in fair yield,1 whereas it reacts at 

Despite a report11 asserting that, under conditions of mercaptala-tion, deoxyaldoses are insufficiently stable to permit a product to be isolated, a number19,51-64 of deoxyaldose dialkyl dithioacetals have 

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The real world is one of uncertainty. Suppose we are carrying out a reaction. We have obtained a product. In the beginning we observe a total uncertainty regarding the molecule. We have no information about its composition, the constitution of the skeleton, its stereochemical features, its physical properties, its biological activities, etc. Step by step, by routine experiments, we collect data. When the acquisition of the structural information is complete there is no uncertainty, at least about its structure. Well, we may not have perfect experiments, so this will require us to reserve space for the missing relevant information. However, it is rather more noise than genuine uncertainty, which, by the way, will never be eliminated. 

Stereochemical features in the oxidative addition and the elimination of /3-hydrogen of cyclic and acyclic alkenes are different. The insertion (palladation) is syn addition. The syn addition (carbopalladation) of R—Pd—X to an acyclic alkene is followed by the syn elimination of 3-hydrogen to give the trans-a ksne 6, because free rotation of 5 is possible with the acyclic alkene. On the other hand, no rotation of the intermediate 7 is possible with a cyclic alkene and the syn elimination of /3-hydrogen gives the allylic compound 8 rather than a substituted alkene. 

In describing the stereochemical features of chemical reactions, we can distinguish between two types stereospecific reactions and stereoselective reactions. A stereospecific reaction is one in which stereoisomeric starting materials aflFord stereoisomerically different products under the same reaction conditions. A stereoselective reaction is one in which a single reactant has the capacity of forming two or more stereoisomeric products in a particular reaction but one is formed preferentially. 

The replacement of carbon by other elements produces changes in several structural parameters and consequently affects the conformational characteristics of the molecule. In this section, we will first describe some stereochemical features of heterocyclic analogs of cycloalkanes. For the purpose of elaborating conformational principles, the discussion will focus on six-membered rings, so that the properties may be considered in the context of a ring system possessing a limited number of low-energy conformations. 

Give a specific structure, including all stereochemical features, for the product expected for each of the following reactions. 

When an alkyl group migrates, there is an additional stereochemical feature to consider. The shift can occur with retention or inversion at the migrating center. The analysis of sigmatropic shifts of alkyl groups is illustrated in Fig. 11.7. The allowed processes include the suprafacial 1,3-shift with inversion and the suprafacial 1,5-shift with retention. Sigmatropic rearrangements of order [3,3] are very common ... 

The Cope rearrangement usually proceeds through a chairlike transition state. The stereochemical features of the reaction can usually be predicted and analyzed on the basis of a chair transition state that minimizes steric interactions between the substituents. Thus, compound 26 reacts primarily ttuough transition state 27a to give 28 as the major product. Minor product 29 is formed flirough the less sterically favorable transition state 27b. 

The stereochemical features of the Claisen rearrangement are very similar to those described for the Cope rearrangement, and reliable stereochemical predictions can be made on the basis of the preference for a chairlike transition state. The major product has the -configuration at the newly formed double bond because of the preference for placing the larger substituent in the pseudoequatorial position in the transition state. ... 

Another stereochemical feature of the Diels-Alder reaction is addressed by the Alder rule. The empirical observation is that if two isomeric adducts are possible, the one that has an unsaturated substituent(s) on the alkene oriented toward the newly formed cyclohexene double bond is the preferred product. The two alternative transition states are referred to as the endo and exo transition states ... 

The reaction of chlorine and bromine with cycloalkenes illustrates an important stereochemical feature of halogen addition. Anti addition is observed the two bromine atoms of Bi 2 or the two chlorines of CI2 add to opposite faces of the double bond. 

An instructive example on how stereochemical features influence the stereochemical outcome of the elimination is the pyrolysis of xanthates from erythro-and t/zrco-l,2-diphenyl-l-propanol. The erythro-dlcohoX 8 is converted into fi-methylstilbene 9 only, and the threo-dlcohoX 10 is converted into the corresponding Z-isomer 11 only. These results support the assumption of a syn-elimination process through a cyclic transition state ... 

Another stereochemical feature of the Diels-Alder reaction is that the diene and dienophile partners orient so that the endo product, rather than the alternative exo product, is formed. The words endo and exo are used to indicate relative stereochemistry when referring to bicyclic structures like substituted norbornanes (Section 4.9). A substituent on one bridge is said to be exo if it is anti (trans) to the larger of the other two bridges and is said to be endo if it is syn (cis) to the larger of the other two bridges. 

Allylboron compounds have proven to be an exceedingly useful class of allylmetal reagents for the stereoselective synthesis of homoallylic alcohols via reactions with carbonyl compounds, especially aldehydes1. The reactions of allylboron compounds and aldehydes proceed by way of cyclic transition states with predictable transmission of olefinic stereochemistry to anti (from L-alkene precursors) or syn (from Z-alkene precursors) relationships about the newly formed carbon-carbon bond. This stereochemical feature, classified as simple diastereoselection, is general for Type I allylorganometallicslb. 

The stereochemical features of the reactions of racemic 1-substituted (Z)-2-butenyl-boronates 2 are considerably different from those of the 1-substituted 2-propenyl- and 1-substituted ( )-2-butenylboronates discussed above. Transition state 5 (see p 1470) is destabilized by allylic interactions between X and the (Z)-methyl substituent26, and consequently diastcrcomcr 10 is the major product via transition state 6 (sec the following table)4,15. 

Some selected examples from the more recent literature, in which no information on stereochemical features is given, are listed18-23. 

Bis(cyclopentadienyl)zirconium 1,3-alkadiene complexes19-20 show interesting stepwise double insertion reactions to carbonyl compounds, exploration with respect to their stereochemical features has only just begun21-23. 

Disubstituted allyl derivatives, bearing hetero substituents have also been investigated86. As observed for the titanium(IV) counterparts, the regio- and stereochemical features depend largely on the type of substituents. 

Anti TT-facial selectivity with respect to the sterically demanded substituent in the Diels-Alder reactions of dienes having unsymmetrical tt-plane has been straightforwardly explained and predicted on the basis of the repulsive interaction between the substituent and a dienophile. However, there have been many counter examples, which have prompted many chemists to develop new theories on the origin of 7t-facial selectivity. We have reviewed some theories in this chapter. Most of them successfully explained the stereochemical feature of particular reactions. We believe that the orbital theory will give us a powerful way of understanding and designing of organic reactions. 

According to Ref. [12], template for synthesis of nanomaterials is defined as a central structure within which a network forms in such a way that removal of this template creates a filled cavity with morphological or stereochemical features related to those of the template. The template synthesis was applied for preparation of various nanostructures inside different three-dimensional nanoporous structures. Chemically, these materials are presented by polymers, metals, oxides, carbides and other substances. Synthetic methods include electrochemical deposition, electroless deposition, chemical polymerization, sol-gel deposition and chemical vapor deposition. These works were reviewed in Refs. [12,20]. An essential feature of this... 

Hydrolysis of an optically active form of a chiral halide presents some interesting stereochemical features. Thus considering each pathway in turn ... 

Figure 8 Stereochemical features of [1,2,3]oxathiazino[4,3-a]isoquinoline derivatives.

Ethyl 2-phenylcyclopropanecarboxylate, obtained in the presence of 207a, has S configuration at C-l in both the cis- and trans-isomer. As that carbon has been furnished by the diazo ester, this result indicates enantiofacial selection at the carbenoid. In contrast, hardly any discrimination between the enantiofaces of the prochiral olefin occurs. Only when the ester substitutents become bulkier, does this additional stereochemical feature gain importance, and the S configuration at C-2 of the cyclopropane is favored. 

Another barnacle species, Elminius modestus, was found to produce mono and trihydroxy fatty acids [146]. Analysis of the extract of whole animal homogenates by TLC provided two hatching factor active bands. The more polar band was tentatively identified as a trihydroxy fatty acid (THFA) band. The less polar band had an / r value similar to a 5-HETE standard. The compounds from this latter band were eluted from the TLC plate, methylated, and trimethylsilylated. GC-MS analysis detected several HEPE s and small amounts of monohydroxy derivatives of Ci8 1, C18 2, and C22 fatty acids. Hydrogenation and subsequent GC-MS analysis allowed identification of the major compound as 8-HEPE (ca. 70%). Five to ten percent of 9-, 11-, and 15-HEPE and minor amounts of 5-, 6-, 12-, and 13-HEPE were also detected. No stereochemical features of these oxylipins were determined. 

The unusually facile formation of a disulfonium dication from sulfide 10 is the result of stereochemical features of the eight-membered ring, which favor the formation of a transannular bond.31 According to X-ray data (see in Chapter 7.1 Table 1), the distance between the two sulfur atoms in 1,5-dithiacyclooctane 10 is smaller than the sum of their van der Waals radii (3.75 A), which results in a strong non-bonded interaction between the atoms confirmed by photoelectron spectroscopy and mass spectrometry.32 33 This interaction and the sulfur-sulfur distance can be decreased as a result of bond formation with an electronegative substituent as in sulfoxide 13 or sulfoximine 14.34,35... 

Stereochemical features of 130 favor formation of a sulfurane dication 131 by treatment with concentrated sulfuric acid.132,133 The sulfuranyl dication 133 is postulated to be involved in the 1,5-oxygen migration observed in the reaction of monosulfoxide 132 with trifluoroacetic acid (Scheme 50).82... 

The use of Et3B as a radical initiator makes it possible to carry out the addition of other alkyl radicals to nitrone (286) using alkyl iodides. Good yields have been obtained of products (288b-d) when an excess of the appropriate alkyl iodide was used (Scheme 2.110). It has been established that the yield of alkyl by-products (288a) tends to decrease with the increase of the reaction temperature. The stereochemical features of this reaction are explained by the alkyl radical addition taking place predominantly from the less hindered re-face of (286) to avoid steric interaction with the phenyl group (525). 

OPC manifest toxic effects as a result of certain structural similarity with natural ChE substrate, acetylcholine (ACh), by both stereochemical features and reactivity. 

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