Stereochemical information loss

On the other hand, the fluorine-induced addition of the diastereomeric silyl-subsliluted sulfides 36 A and 36B to benzaldehyde proceeds without loss of stereochemical information and with retention of configuration32. Since, however, the anionic reagent 35A/35B is known to be configurationally labile, the observed retention of configuration in the fluorine-induced desi-lylative hydroxy alkylation lends experimental evidence to the notion that these reactions proceed via hypervalent silicon species rather than anionic reagents. 

Irradiation of the sodium salt 517 in a solution of maleonitrile in THF gave a 30% yield of the as-spiro[2.6]nonatrienedicarbonitrile 524 (Scheme 6.106). Analogously, fumaronitrile led to the trans-diastereomer 525 exclusively in 29% yield [193b], which was also obtained from (trimethylsilyl)tropylium tetrafluoroborate (519) as precursor of 5 [208]. The interception of 5 by dimethyl fumarate succeeded by generating 5 from 517 by either photolysis [193] or thermolysis [193, 194] and from 519 [208] and afforded the dimethyl trans-dicarboxylate 526 in yields of 50, 40 and 42%, respectively. Liberation of 5 in the presence of dimethyl maleate led also to 526 [194, 214], In this reaction, the isomerization of dimethyl maleate to fumarate could have interfered and hence pretended the loss of the stereochemical information, since... 

An analogous study was carried out38 to investigate the alkyne insertion mechanism. Reacting optically enriched samples of vinylketene complexes 221.c and 221.e with three different alkynes, the expected adducts (247.b, 247.d, 247.k, and 247.n) were isolated, and their e.e. measured by HPLC or NMR methods. It was shown that there had been no loss of stereochemical information, and comparison of CD spectra (along with X-ray... 

This sequence demonstrates that although the stereochemistry is unambiguous in each step, regiochemical uncertainty eventually leads to the loss of the original stereochemical information. Hence, it is essential to control the regiochemistry of both monomesylation (or tosylation) and epoxide opening. 

In addition to the spectroscopic investigations, there have been attempts to obtain structural and stereochemical information about radicals by chemical means.25 The approach generally taken is to generate radicals by one of the methods discussed in the next section at a carbon where stereochemistry can be determined. As an example, we may cite the experiment shown in Equation 9.6, in which an optically active aldehyde is heated in the presence of a source of radicals.26 The reaction follows the chain pathway indicated in Scheme 1 the loss of chirality indicates that the radical is either planar or, if pyramidal, undergoes inversion rapidly with respect to the rate (on the order of 10s sec-1) at which it abstracts a hydrogen atom from another molecule of aldehyde. 

Loss of stereochemical integrity during a chemical reaction is usually taken as good evidence that the products have been formed via an intermediate incapable of sustaining stereochemical information, but there are alternative explanations (competing concerted processes or equilibrations of stereoisomeric products under the reaction conditions) and these should be considered and excluded where possible by other experiments. 

This control, however, can be tested only in acyclic dienes, which lack of the conformational rigidity of cyclic substrates. Loss of the stereochemical information could be due to isomerization of the starting diene [18b,19c], to two-step processes [18], or to the reversible formation of an exciplex which, depending on the substrate structure, may have a zwitterionic and/or diradical character [19]. 

Overall, we have retention of stereochemistry. As you know, Sn2 reactions go with inversion, and SN1 reactions with loss of stereochemical information—so this result is possible only if we have two sequential Sn2 reactions taking place—in other words neighbouring group participation. 

Not so with the other diastereoisomer of this compound Now, the phenonium ion is symmetrical with a plane of symmetry—it is therefore achiral, and the same whichever enantiomer we start from. Attack on each end of the phenonium ion gives a different enantiomer, so whichever enantiomer of starting material we use we get the same racemic mixture of products. You can compare this reaction with the loss of stereochemical information that occurs during an SjnjI reaction of enantiomerically pure compounds. Both reactions pass through an achiral intermediate. 

The same loss of absolute stereochemical information (but retention of relative stereochemistry) occurs in another reaction that you met at the start of this chapter. We then emphasized two features the acceleration in rate and the retention of stereochemistry. 

For the adducts of chiral /V-sulfinyl tnms-2-phenylcyclohexyl carbamates (d.r. >95 5) transfer of stereochemical information from C-l to C-3 is observed, however, the final carbamates are obtained with a lower ratio of diastereomers (d.r. 88 12-92 8)91. The loss of stereochemical integrity might be due to isomerization of the labile ene adducts at room temperature. The level of stereochemical purity of the final carbamates can be regained by recrystallization to give a d.r. of up to >95 591. 

The reaction of singulet states represents a concerted process. Therefore, the absence of cross products indicates that the reaction proceeds stereoselectively for both components with conservation of the relative configuration of the substrates. The formation of cross products is observed when triplet states are involved. This is due to the fact that triplet reactions form 1,4-diradical intermediates, which can undergo single bond rotation with loss of the stereochemical information. ... 

Pasto and Gontarz found, for example, that both the erythro and threo isomers from the acetoxymercuration of trans- and cA-butene gave the same 50 50 mixture of erythro- and threo-3-deutero-2-butanol upon reduction with sodium borondeuteride (Scheme 4.2).3 The complete loss of stereochemical information could only be rationalised by invoking a carbon radical intermediate. 

Photolysis of dicyclopentadienyltitanacyclobutanes in the presence of alkynes gives cyclopropanes and the corresponding metallacyclopentadienes. Similar results are obtained with ethene and phosphanes. With substituted metallacyclobutanes 14, partial loss of stereochemical information depending on the reaction time is observed (15/16 >5 1 to 3 1). Therefore, reversible homolytical cleavage to form a diradical intermediate (with isomerization of the starting material) has been suggested. 

Racemization, epimerization, loss of stereochemical information at C -atom. While the expression racemization is used in organic chemistry for the complete conversion of a single enantiomer into the racemate, it is often used in peptide chemistry for partial or total epimerization at one chiral C , irrespective of whether a mixture of diastereomers or enantiomers is formed. The correct stereochemistry is important for the biological activity of a peptide or protein. Hence, racemization must... 

A similar stereospecific behaviour has also been observed in the stereoisomeric 3-arylcyclohexanols and in their methyl ethers and acetates, but not in the 2-aryl analogues, which undergo cleavage of the C1-C2 bond prior to fragmentation, resulting in the loss of stereochemical information. Many additional systems have been shown to undergo El-induced stereospecific eliminations of water and other neutral molecules, which may be applied in structural studies. 

C2-C3 Disconnections remain attractive retrosynthetic strategies for the construction of THP rings. Prins cycIizatiMi protocols are the predominate method for such disconnectirms in the crmtext of natural product synthesis. The wide utility and scope of this transformation must be balanced by careful selection of substrates, since a variety of side reactions or loss of stereochemical information is possible (vide supra). This method has proved extremely reliable for the installation of... 

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