1.7.3.12.3. Role of Hybridization of Ring Nitrogen on the Reactivity of N-Containing Cyclic Enediynes

In recent years, various cyclic enediynes with a nitrogen atom replacing a non-enediynyl carbon have been prepared in our laboratory and their thermal reactivities were studied (Scheme 34). The nitrogen atom in all these enediynes was equipped with aryl sulfonamido groups. These groups were so chosen as to perturb the extent of pyramidalization of the ring nitrogen atom. The enediyne, with an electron withdrawing nitro group, was found to undergo cyclization at a faster rate as compared to other enediynes bearing phenyl sulfonyl or para-toluene sulfonyl group. The results were explained on the basis of degree of pyramidalisation of the ring nitrogen atom. A similar effect was also observed in case of amides (Scheme 35).

Scheme 34. Extent of pyramidalization of the amide nitrogen atom on the kinetics of BC.

Scheme 35. Extent of pyramidalization of the enediynyl sulphonamides and amides nitrogen atom on the kinetics of BC.