Rearrangements stereochemical configuration

The obvious advantage of our photochemical procedure is that it allows the formation of 3 under mild conditions at very low temperature. Thereby, thermally rather unstable derivatives of 3 may be generated and their reactions studied.- This enabled us, for example, to show that stereochemical configurations at the diene termini are retained during photolysis, but may be lost by subsequent, rather rapid thermally induced intramolecular rearrangement steps. For example, irradiation of 10 in the presence of pure (Z,Z)-2,4-hexadiene at -60°C yields exclusively s-trans-Tj -(Z,Z)-2,4-hexadiene)zirconocene [3h-(Z,Z)]. At -40°C, the stereochemical information at one terminal diene carbon center is lost. A clean... 

Mechanistic studies on these four rearrangements have revealed that (i) these are concerted intramolecular rearrangements (ii) stereochemical configurations of the migrating groups are completely retained (iii) the kinetics are first order and (iv) neither free carbonium ions nor radical intermediates are formed during the rearrangements. 

Beckmann rearrangement the feature that the migrating group retains its stereochemical configuration. The migration is usually considered to be concerted... 

As has been demonstrated through countless examples of this rearrangement process in chemical synthesis, the migration reaction that converts an intermediate such as 46 into 47 always proceeds with retention of stereochemical configuration. Thus, follow-... 

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. ... 

The stereochemical outcome of the reaction is determined by the geometry of the transition state for the Claisen rearrangement a chairlike conformation is preferred,and it proceeds strictly by an intramolecular pathway. It is therefore possible to predict the stereochemical course of the reaction, and thus the configuration of the stereogenic centers to be generated. This potential can be used for the planning of stereoselective syntheses e.g the synthesis of natural products. 

Evidence for that stereochemical course comes from the rearrangement of meso-3,4-dimethylhexa-1,5-diene 4, which yields the E.Z-configured diene 5 almost quantitatively. With a transition state of boatlike geometry, a Z,Z- or E,E-configured product would be formed." ... 

It may be of interest to note that the stereospecific transformation shown in equation 15 has been cited as the first reported observation of an 1 - 3 chirality transfer. It is evident that on rearrangement of optically active 6d to 7d, the chiral center at C-a is eliminated and a new one created at C-y. The term self-immolative asymmetric synthesis has also been used to describe syntheses of this kind. As pointed out by Hoffmann , quantitative 1 - 3 chirality transfer will follow from the suprafacial - course of rearrangement, provided the reactant has a uniform configuration at the j8, y-double bond. This stereochemical prediction has also been confirmed by the results obtained in several other [2,3]sigmatropic rearrangements, subsequently reported " . 

The stereochemical use of the Beckmann rearrangement in assigning configuration to ketoximes has already been referred to, and it also has a large-scale application in the synthesis of the textile polymer Nylon-6 from cyclohexanone oxime (78) via the cyclic amide (lactam, 79) ... 

The involvement of ion pairs in the addition process has also been related to the stereochemical behavior. The remarkable difference in configuration between the rearranged chlorides and acetates has been rationalized, as shown in equation 113, on the basis of a syn internal attack of Cl- on ion c and anti external attack of AcOH from the solvent pool. 

Discussing the stereochemical outcome of the Claisen rearrangements, two aspects had to be considered. On the one hand, the relative configuration of the new stereogenic centers was found to be exclusively syn in 201 and 202, pointing out the passing of a chair-like transition state c-a and c-jS, respectively, including a Z-acylammonium enolate structure (complete simple diastereo-selectivity/internal asymmetric induction). 

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