Transition states, boat-like Diels-Alder reactions

The latter reaction has an activation entropy that must be in error since a positive value should be expected. Nonetheless, the two epimers undergo dramatically different reactions presumably because the 3,3-shift can only occur with the endo isomer, but by a boat-like transition state, while the latter can only undergo the retro Diels-Alder reaction. 

It is noteworthy that, in the corollary intramolecular imino Diels-Alder reaction reported by Weinreb et al where A-acyl imines are employed as heterodienophiles [21], exclusive preference for boat-like transition states is observed, wherein the carbonyl group is restrained in the s-cis conformation within the A-acyl imine and approach is endo to the diene (Scheme 2.8). For example, in the intramolecular acyl imine Diels-Alder reaction en route to the synthesis of the quinolizidine alkaloids epi-lupinine (56) and lupinine (57) ... 

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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