E stereoisomers

In the Z stereoisomer the two lower priority substituents—the methyl group and the hydrogen—are on the same side of the double bond In the E stereoisomer these substituents are on opposite sides of the double bond The ring carbons are the higher ranking substituents at each end of the double bond y... 

In the enantioselective synthesis, the asymmetry (i.e., the stereoselectivity) is induced by the external chiral catalyst, while the diastereoselective synthesis does not require a chiral catalyst. The stereogenic center already present in the molecule is able to induce stereoselectivity, assuming that the synthesis starts with a single enantiomer. For instance, imagine that an a,/ -substituted product is formed, and that the reactant already contains a stereogenic carbon at a. If the reaction of (aS) leads, e.g., largely to (aS, / R) and hardly to the (aS, /IS) diastereomer (i.e., stereoisomers that are not mirror-images of each other), the reaction is diastereoselective (Scheme 14.2). 

Free radical addition of HBr to buta-1,2-diene (lb) affords dibromides exo-6b, (E)-6b and (Z)-6b, which consistently originate from Br addition to the central allene carbon atom [37]. The fact that the internal olefins (E)-6b and (Z)-6b dominate among the reaction products points to a thermodynamic control of the termination step (see below). The geometry of the major product (Z)-(6b) has been correlated with that of the preferred structure of intermediate 7b. The latter, in turn, has been deduced from an investigation of the configurational stability of the (Z)-methylallyl radical (Z)-8, which isomerizes with a rate constant of kiso=102s 1 (-130 °C) to the less strained E-stereoisomer (fc)-8 (Scheme 11.4) [38]. 

Syntheses follow a kind of bio-mimetic approach [283, 284] in building up the chain during a sequence of Wittig-type reactions or Horner-Wadsworth-Emmons olefination, adding two carbons to the chain at a time with either methyl- or ethyl-branches. As the final products need to be highly pure (E)-stereoisomers, reaction steps and purification need to be carefully controlled. 

Treatment with water gives the corresponding Z tellurides. Reaction with BuSeBr, iodine and A -bromosuccinimide (NBS) gives respectively Te-Se ketene acetals, a-iodo- and a-bromo vinyl tellurides as mixtures of Z and E stereoisomers (in contrast with the total retention of configuration of the above-discussed Zr/Te exchange reactions). The acylation was effected with acylchlorides in the presence of Cul. ... 

Transformation of [4- F]fluorobenzaldehydes into [4- F]fluorophenyl-alkenes using the Wittig reaction has been relatively unexplored. Examples are shown in Scheme 35. It requires the in situ generation of the ylid [171] by reaction of the phosphonium bromide with propylenoxide [172]. These conditions, successfully used in carbon-11 chemistry [173], have however the drawback of leading to a mixture of Z and E stereoisomers. 

Figure 22.10. Rearrangement products (13), predicted to be formed from 1,1-disilyl-cyclopropanes (11) by conrotatory ring opening, followed by a 1,2-sbift of a trimethylsilyl group in the 1,3-diradicals (12) generated. Pyrolysis of 11b should lead to a mixture of E/2) isomers (13b), but pyrolysis of 11c is predicted to form only the E stereoisomer (13c). ...

Further observations were published concerning 3,4- and 4,5-0-isopropylidene derivatives of D-arabinose and D-fructose.273 Thus, l,2 3,4-di-0-isopropylidene-/J-i>arabinose (266) gave a mixture of (—)-(S)-(Z)-5-butyl-3-nonen-l,2,5-triol (267), its (E) stereoisomer, and 4-... 

Z)-2-Phenyl-2-butene. (Z)-2-pheny 1-2-butene is not as stable as its (E) isomer. When Co >(CO)8 is used, results are the same with the (E) stereoisomer. However, when rhodium is used, isomer selectivity changes (Table VIII). The main isomeric aldehyde is the erythro di-astereoisomer which represents 33.9% of the whole mixture. Aldehyde 27 is formed in nearly the same ratio (32.9% ) while 6.8% and 4.4% of aldehyde 26 and the threo diastereoisomer are formed. 

The other feature of hydroformylation of ( )- and (Z)-2-pheny 1-2-butenes concerns the distribution of aldehydic isomers 26 and 27. With Co2(CO)8, whatever the stereochemistry of the olefin, the distribution of these two aldehydes is exactly the same. On the other hand, with Rh/Al203 the reaction is more or less oriented towards the formation of one of the aldehydic isomers. Moreover, the phenomenon is more striking with (Z) than with (E) stereoisomer. 

Vasvari-Debreczy et al. studied the ring closure of the succinates (77) under various conditions, e.g., in Dowtherm A at 250rC,130-133 in a phos-phoryl chloride-polyphosphoric acid mixture1 32-1 34 at 110-120 C, and in ethanolic sodium ethoxide at room temperature.1 33-1 35 The authors expected various cyclic products pyrido[l,2- ]pyrimidines (78) from the Z stereoisomer of 77 and pyridylpyrrolinones (79) or pyrido[l,2-ci](l,3)diaze-pines (80) from the E stereoisomer (Scheme 3). Experiments were carried out... 

The (R)- and (S)-aldehydes 4 were used to prepare the four (E)-stereoisomers of vitamin K, all of which have essentially identical biological activity. 

OF I-BROMO-1,2-DIPHENYL-PROPANE TO PRODUCE THE (Z) STEREOISOMER AND O THE (E) STEREOISOMER OF I, 2-diphenyl-I-propene. Test yourself on the concepts in this figure at OrganicChemistryNow. 

Mixtures of Z and E stereoisomers have been obtained in most syntheses. This undoubtedly holds in the presence of an acid catalyst. The tendency of the E-enamine to isomerize to the Z-enamine was readily observed on numerous occasions under unexpectedly mild conditions. Munk and Kim357 summarized the requirements for the stereospecific synthesis of enamines. First, if the introduction of the double bond is to be the final step of the synthesis, it must be stereospecific in character. Second, once the enamine is formed it must retain its stereochemical integrity under the conditions employed in the double-bond-forming step. The base-induced bimolecular -elimination reaction fulfills both these requirements. Indeed, treatment of the mesitoate esters of ( )-threo- (99) and ( )-erythro- (100) l-(4-morpholino)-l,2-diphenylethanol with... 

The [(2-alkoxy)ethenyl]carbene complexes are obtained as E/Z mixtures, in which the (E)-stereoisomers predominate. Other carbene complexes, in which carbohydrates are bonded directly to the metal center178 or to the carbene carbon atom,179180 are also known. 

The enthalpy of hydrogenation of the more congested, and hence strained, fZ)-but-2-ene is greater than for the E stereoisomer. 

The method involving a-heterosubstituted cyclopropyllithiums and a-selenenylalde-hydes provides31,3S,77) quite exclusively the (E) stereoisomers of 1-heterosubstituted-1-vinylcyclopropanes after treatment of the l-(l-heterosubstituted)-l -hydroxy-2 -selenoalkyl)cyclopropanes with PI3/NEt3 (Schemes 60, 62b, 63 b). This implies that the two steps involved in the transformations are stereoselective (Schemes 60, 62b, 63b). The stereochemical result of the first step can be rationalized123 on the basis of the Cram-Karabatsos-Felkin rules124,125 and it is well-known that the synthesis of olefins from (P-hydroxyalkyl)selenides occurs by a formal anti elimination of the hydroxyl and selenyl moieties 4 9 11,12 123 163 . 

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