Cycloadditions boat-like transition state

The predominant formation of the endo-isomers 50-A and 50-B can be explained as follows. Based on the concept of "concerted but asynchronous" cycloaddition, Roush stated that a 1,7,9-decatriene system possessing an electron-withdrawing group at the terminal position of the dienophile part can generally adopt a six-membered chair-like transition state while a six-membered boat-like transition state may be occasionally favored depending on the kind of substituents [45, 46]. It seems reasonable to apply the six-member transition state model to our substrate 48. Therefore, four chair-like transition states TS-Chair-A to TS-Chair-D and four boat-like transition states TS-Boat-A to TS-Boat-D are possible (Fig. (5)). All boat-like transition... 

Diels-Alderase antibody 39-All minimizes product inhibition while generating a more conventional Diels-Alder product (Fig. 11) (80). Bicyclo[2.2.2]octane hapten 29 was designed to mimic the proposed boat-like transition state 30 of the [4n -F 2n] cycloaddition between diene 31 and dienophile 32. Product inhibition was circumvented by the structural disparity between the product cycloadduct and the pseudo-boat form of the hapten employed for immunization. X-ray crystallographic data of hapten 29 complexed with the 39-All Fab indicated that the diene and the dienophile are bound in a reactive conformation that reduces... 

The transition state of Diels-Alder reaction of diene 56 with dienophile 57 to give the adduct 58 (Scheme 5.14) is a boat-like type. The antibody 39-Al 1, generated to the bicy-clo [2.2.2 ] octane hapten 59 that mimics the transition state of the reaction, efficaciously catalyzed the cycloaddition, selectively giving 58 in buffered aqueous medium (Scheme 5.14). ° Ribozymes are RNA molecules with catalytic properties. Artificial ribozymes, isolated fi om synthetic combinatorial libraries, accelerate a broad range of reactions including the Diels-Alder cycloaddition. These ribozymes generally require that at least one of the reactants be RNA or be covalently tethered to RNA. 

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