The Polycyclization Approach

Harrington-Frost and Pattenden have reported a new synthesis of pental-ene, a natural angular triqiunane, using a novel tandem cychzation involving a ketene radical intermediate [26]. 

Joshi et al. have reported a diastereospecific synthesis of pentacyclic /3-lactams by 6-exo-trig/7-endo-dig tandem radical cyclization [33]. On the occasion of the total synthesis of new sargine-type alkaloids, Takayama et al. have disclosed a formation of pentacyclic derivatives from a tryptophan derivative [34]. 

N-centered radicals such as iminyl radicals can also be generated from a radical cyclization step onto nitrile groups. The fate of the iminyl radical depends on the nature of the a-substituent. When this latter is an alkyl group, further cychzation can take place [45]. A combination of both processes has been achieved addition of a staimyl radical onto an azide moiety generated a N-stannylaminyl radical that cyclized onto a nitrile and underwent further cyclization, thus opening access to pyrrolopyrroles and pyrrolopyridines derivatives [46]. 

Tokuda and Senboku have studied the radical cychzation of aminyl radicals resulting from the reaction of tributyltin hydride with N-chloro pre- 

Stereoselective polycyclization is a highly stimulating aim. Among several contributors in that field, Yang et al. focused on Lewis-acid- pro- 

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