Radical Anion-Mediated Cyclizations

The chemistry of radical anions, generated by PET processes, is less developed than that of radical cations, possibly due to the nonavailability of suitable photosystems to initiate photosensitized one-electron redox reactions. 

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Kariv-Miller and coworkers have developed indirect electroreductive cyclizations with the dimethyl-pyrrolidinium ion (DMP") as a mediator. Preparative electrolysis of 6-hepten-2-one (9) at a graphite cathode afforded cu-dimethylcyclopentanol (10) in 90% yield (equation 5). The reduction is believed to occur via the ketyl radical anion, which cyclizes onto the alkenic bond. In the absence of DMP simple reduction to 6-hepten-2-ol takes place.Very recently it was shown that instead of DMP several aromatic hydrocarbons can be used as mediators to initiate the cyclization reaction. The carbonyl group can also be cyclized onto an alkynic bond and even an aromatic ring. - ... 

On the other hand, since oxime ethers were electrochemically more inert than ketones under the electroreduction conditions, the electroreductive intra- and inter-molecular coupling of ketones with oxime ethers proceeded via anion radicals in good yields (equations 5 and 6) °4i. Moreover, cobaloxime-mediated intramolecular radical addition onto oxime functions in the electrolysis media proceeded to afford the cyclized aminoethers (equation 7). ... 

Many methods for the preparation of both the a- and )8-C-glycosides have been developed, and new approaches are still appearing in the literature. This chapter covers the literature from the end of 1994 to early 1999, with other inclusions as deemed appropriate. Following the organization of earlier work [1], we select the mode reactivity of the anomeric center to define the category. The chapter discusses free radical chemistry. Cl anions, Wittig chemistry-cyclization chemistry, transition metal mediated chemistry, sigmatropic chemistry, and approaches based on cationic chemistry. The chapter focuses on the most recent and novel developments in the field, but all the relevant references are included whenever possible. 

A radical cyclization has been used by Hart, at The Ohio State University, to prepare the manzamine tricyclic core (139). Utilizing tri-w-butyltin hydride to mediate the cyclization of selenide 171, the desired stereochemistry of the octahydroisoquinoline ring (172) was established (Scheme 13) electrophilic catalysis generated the tricyclic intermediate 174. An anionic cyclization leading to a tricyclic intermediate is being developed by Marko, at the University of Sheffield, toward which he has recently reported a model study (140). 

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