Claisen chair-like transition state

Claisen rearrangement presumably was the boat-like form 156 (bj0) with minimized 1,3 repulsive interactions resulting in the lactams 157. However, the 2,4-as disubstituted pyrrolidine 159 (R =OTBS, R, R =H) gave the expected lactam diastereomer 158 via a chair-like transition state conformation 160 (entry 16, Table 8) (Scheme 31). 

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). 

If the allylic system is substituted, several isomeric products can be formed. Assuming a chair-like transition state, the stereochemical outcome of a metallo-Claisen rearrangement is controlled by the geometries of the vinyl and allyl moieties, so that, if the vinyl part 68 is stereochemically pure, three different products, syn/anti-73 or 74, can be formed, depending on the exact nature of the active allylic part 72 (Scheme 6)38. 

The chair-like transition state of the Claisen rearrangement, with the R group in an equatorial position, adequately accounts for the stereochemical results obtained with allylic alcohols 7,34-35 as illustrated for 7(1 in Scheme 4.35... 

The formation of the allylic amine can involve a thermal [3,3]-sigmatropic rearrangement - comparable to the Claisen Rearrangement - that prefers to proceed via a chair-like transition state ... 

Alkene stereochemistry in the Claisen rearrangement comes from a chair-like transition state... 

These rearrangements involve a six-membered transition state (Scheme 8.5). It is found that a chair-like transition state is usually preferred for the Cope and Claisen rearrangements. 

In acyclic systems, Claisen rearrangements show a well-established prefoence for chair-like transition states. With crotyl propenyl ether, the chair selectivity amounts to 97-98% at 142 C, which corresponds to an approx. 3 kcal mol difference between the fiee energy of activation (AAG ) of chair and boat TS (equation 26). The preference for a chair-like geometry in the TS is even more pronounced in the Cope reaiT ement 99.7% of the 3,4-dimethylhexa-1,5-diene rearranges at 225 C via a chair-like TS, corresponding to a AAG chair-boat of -5.7 kcal mol" . - The latter result closely parallels the difference in energy of the chair and boat conformations of cyclohexane (5-6 kcal mol" ). ... 

In cyclic systems, however, conformational constraints can override the inherent preference for chairlike transition states in Cope as well as Claisen rearrangements and lead to a partial involvement if not a dominance of boat-like TS structures. In the Ireland rearrangement of lactones of type (247), for example, chair-like transition state (249) is accessible only when the diaxial bridging methylene chain becomes sufficient in length (n = 7, Scheme 44). The preference of boat-like transition state (250) over (251) is due to a serious A - -type interaction between the endocyclic oxygen atom and pseudoaxial substituent R in (251). 

The arylsulfonyl carbanion accelerated Claisen rearrangement is completely regioselective and has also been found to be highly diastereoselective (Scheme 2). The stereochemical course of the reaction conforms to the familiar chair-like transition state model usually invoked for the classic thermal process. Recently, high degrees of asymmetric induction have been observed in tlie rearrangements of chiral cyclic phosphoramidate stabilized carbanion derivatives. ... 

This undergoes a vQtro-Claisen rearrangement under the reaction conditions. The mechanism of the [3,3]-sigmatropic rearrangement is shown above. The concerted reaction proceeds via a six-membered, chair-like transition state (36). The driving force of the reaction in this case is the reduced ring strain of the product molecule 24. 

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