Programming organic molecules: Design and management of organic syntheses through free-radical cascade processes

Abstract

Cascade, domino, or tandem processes, that link together two or more transformations in one pot, are increasing in popularity because they lead to improvements in synthetic efficiency and decreases in environmental impact. Not only do these cascades contain choice mechanistic gems but they also deliver compact and elegant syntheses of complex natural products. Longer cascades require more functional groups precisely configured within carefully designed initial molecular architectures. Such "purposeful" molecules can be thought of as chemical algorithms. This article surveys the phenomenal range of unimolecular free-radical cascades. A convenient system for classifying free-radical cascades is described that is useful for evaluating and comparing cascades and aids the design of synthetic routes to polycyclic structures. Double cyclization cascades lead to cyclopentylcyclopentane or bicyclo[3.3.0]octane derivatives. Precursors that contain a ring as a template have been used to control stereochemistry in syntheses of triquinanes and many related compounds. Of the cascades containing ring-cleavage steps, the most useful are the ring expansions which have opened up new synthetic routes to medium ring polycycles. The key design features of three-stage unimolecular free-radical cascades that yielded steroid structures, are linear arrays of radical acceptor units associated with methyl groups distributed every fifth C-atom in the precursor polyenes. Ring cleavage is the reverse of cyclization. In special, symmetrical structures, therefore, this led to sequences that were reversible, thus launching endlessly repeating cascades supported by delightfully fluxional structures. The science of "programming" organic molecules to achieve particular target structures is maturing rapidly. Coordination and classification of the welter of information in this area is intended to facilitate design and hence to extend the range and complexity of attainable structures.