Stereochemically plausible

The phase problem and the problem of arbitration. Fibrous structures are usually made up of linear polymers with helical conformations. Direct or experimental solution of the X-ray phase problem is not usually possible. However, the extensive symmetry of helical molecules means that the molecular asymmetric unit is commonly a relatively small chemical unit such as one nucleotide. It is therefore not difficult to fabricate a preliminary model (which incidently provides an approximate solution to the phase problem) and then to refine this model to provide a "best" solution. This process, however, provides no assurance that the solution is unique. Other stereochemically plausible models may have to be considered. Fortunately, the linked-atom least-squares approach provides a very good framework for objective arbitration independent refinements of competing models can provide the best models of each kind the final values of n or its components (eqn. xxiv) provide measures of the acceptability of various models these measures of relative acceptability can be compared using standard statistical tests (4) and the decision made whether or not a particular model is significantly superior to any other. 

These examples indicate that the (Z)-syn,(E)-antt correlation should be considered to be a rule with many exceptions. Two explanations may be given in order to rationalize the manifold stereochemical results in aldol additions. Firstly, it seems plausible that the many different reaction conditions and starting materials (e.g., various types of enolates, counterions, etc.) may cause the aldol addition to follow different reaction mechanisms, so that different types of transition states are involved. Secondly, in a single type of transition state model, the reactants may have different orientations to each other, so that the formation of different stereoisomers may result even for one and the same transition state model. 

Although the chiral propanoates I and 5 are similar and the reaction conditions are almost identical, the stereochemical outcomes arc explained by completely different transition state models. Predominant attack of the ketene acetal 2 to the Ai-face of 2-methylpropanal is interpreted by assuming a Zimmerman-Traxler like model, which minimizes steric hindrance in a plausible way65. 

As outlined in Section D.2.3.5., the stereochemical outcome of the addition of nucleophilic reagents to chiral aldehydes or ketones is rationalized most plausibly by the Cram-Felkin-Anh model. On the other hand, the corresponding reactions of oxygen- or nitrogen-heterosub-stituted aldehydes or ketones may be interpreted either by the same transition state hypothesis or, alternatively, by Cram s cyclic model. 

There is little doubt that Shallenberger s AH,B hypothesis is the most plausible concept in the explanation of the initial stimulation of the sweet-taste receptor. However, it was unfortunate that the evidence was accrued largely with the aid of reducing sugars, which, in solution, equilibrate between many isomers, so that it is not possible to relate total gustatory response to any one particular stereochemical structure It is also not... 

It is not surprising, therefore, that this dynamic technique was chosen to provide information on chiral interactions in compressed films. Given that these stereochemically differentiated systems may have dramatically different HI A isotherm characteristics, and hence different packing arrangements, it is plausible that their flow properties are stereochemically differentiated as well. 

L-Galactose is probably not produced from L-glycerose, since the latter inhibits glycolysis and, even so, reaction with dihydroxyacetone phosphate in the presence of aldolase yields L-sorbose 1-phosphate which, on stereochemical grounds, is also an unlikely precursor. A more plausible route is direct conversion from D-galactose (XXI) by complete reversal of stereo-... 

From a historical standpoint, the Zimmerman-Traxler study on the stereochemical aspects of the Ivanov reaction is of considerable significance (22). Their investigation revealed that the magnesium enediolate 19a, upon condensation with benzaldehyde (Et2 O, reflux 5 hr), afforded the erythro and threo acids 20E and 20T, respectively, in a 24 76 ratio (eq. [13]). In the analysis of plausible transition states, both chair A and boat B geometries were considered. Zimmerman and Traxler concluded that the major diastereomer 20T could... 

Scheme 18 Plausible stereochemical model for antl-diastereoselective iridium-catalyzed allyla-tion of alcohols employing alkynes as allyl donors...

In the absence of more physical data, the authors considered the second hypothesis (equation 32) to be the more plausible one, because it can easily explain the stereochemical outcome of many reactions (see examples below). These reactions could thus be seen as metal variants of the well-known Cope and Claisen rearrangements40. The original... 

Figure 2.11 Some plausible confoimations of (2/f, 4Y )-l-hydroxy-2,4-dimethylhex-5-ene. How many different torsional isomers might one need to examine, and how would you go about generating them [Note that the notation 2/f, 45 implies that the relative stereochemical configuration at the 2 and 4 centers is R,S - by convention, when the absolute configuration is not known the first center is always assigned to be R. However, the absolute conformations that are drawn here are S,R so as to preserve correspondence with the published illustrations of Stahl and coworkers. Since NMR in an achiral solvent does not distinguish between enantiomers, one can work with either absolute configuration in this instance.]...

Despite mechanistic complications, however, it appears very likely that most, if not all, of the facile and synthetically attractive carbometallation reactions involve, at a critical moment, concerted addition of carbon-metal bonds where the synergistic HOMO-LUMO interactions shown in Scheme 4.3, akin to those for the concerted hydrometallation reactions, provide a plausible common mechanism. This mechanism requires the ready availability of a metal empty orbital. It also requires that addition of carbon-metal bonds be strictly syn, as has generally been observed. Perhaps more important in the present discussion is that concerted syn carbometallation must proceed via a transition state in which a carbon-metal bond and a carbon-carbon bond become coplanar. Under such constraints, one can readily see how chirally discriminated carbon-metal bonds can select either re or si face of alkenes. In principle, the mechanistic and stereochemical considerations presented above are essentially the same as for related concerted syn hydrometalla-tion. In reality, however, carbometallation is generally less facile than the corresponding hydrometallation, which may be largely attributable to more demanding steric and... 

The plausible but somewhat speculative attempts to interpret the stereochemical outcome resembles the previously reported stereoselectivities in glycosylations of glycosyl fluorides [25] many of which could not be confirmed in other studies [26, 27]. 

Ring closure reactions taking place by intramolecular addition of an aromatic radical to a double bond have been widely studied on both their regio- and stereochemical aspects [93]. Aryl halides and diazonium salts substituted at ortho- position with a 0-allyl or TV-allyl chain were used for the preparation of 2,3-dihydrobenzofuranes and 2,3-dihydro-1/7-indoles under different reaction conditions. The reaction pattern involves the generation of an aryl radical 20, which reacts with the double bond in a 5-exo trig fashion to afford the exocyclic radical 21, plausible of reduction by a hydrogen donor to obtain the reduced-cyclized product 22 (Sch. 23) [93d,94]. 

Sprinson and coworkers [30] conducted the methylmalonyl-CoA mutase reaction in deuterium oxide using a crude mitochondrial preparation. The presence of methylmalonyl-CoA epimerase insured that (1) all substrate molecules incorporated one atom of deuterium into position 2, and (2) in the course of the reaction the (2R)-epimer of methylmalonyl-CoA was continuously supplied by epimerization of the (25)-epimer, which was in turn generated by the enzymic carboxylation of propionyl-CoA. Alkaline hydrolysis of the product and subsequent purification furnished succinic acid which was mainly monodeuterated (70% 2H,-, 15% 2H2-labelled and 13% unlabelled species). A positive ORD curve revealed its (5) configuration indicating stereochemical retention for the AdoCbl-dependent rearrangement (Fig. 22). No plausible explanation could be offered for the formation of doubly deuterated and unlabelled species. Essentially the same results were later obtained with a highly purified mutase preparation from Propionibacterium sher-manii (J. Retey, unpublished). 

The absolute configuration of the amine 7 may be explained by a stereochemical model based on the X-ray crystal structure of the chiral BINOL-phosphate (Fig. 4). In the transition state the ketimine is activated by the Brpnsted acid in such a way, that the nucleophile has to approach from the less hindered si face as the re face is effectively shielded by the large aryl substituent of the catalyst (Fig. 4, left). Furthermore, a bifunctional activation seems to be plausible, where next to the ketimine protonation, the dihydropyridine is activated through a hydrogen bond from the Lewis basic oxygen of the phosphoryl group. 

Stereochemical nonrigidity, especially if it is fluxional, seems likely to be consistently characteristic of complexes with coordination numbers of 7 or greater. All 7-coordinate complexes so far investigated by nmr techniques have shown ligand-atom equivalence even though there is no plausible structure for a 7-coordinate complex that would give static or instantaneous equivalence. 

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