N-Heterocyclics s. a. Aza

Chiral N heterocyclic carbenes (NHCs), as Lewis basic organocatalysts, have been synthesized and applied to enantioselective organocatalytic reactions in recent years. Encouraged by Sheehan and Hunneman s first report of chiral thiazolium salts as NHC precursors for organocatalytic reactions [37], Leeper, Enders, Rovis, Glorius, Herrmann, and others have synthesized series of novel chiral NHCs with mono cyclic, bicyclic, or tricyclic backbones [38]. Recently, a series of bifunctional NHCs were synthesized and applied to aza BMH reaction of cyclopent 2 enone with... 

Further substitution of the peripheral carbon atoms of the cyclazines by heteroatoms (N, S, etc.) is indicated in this chapter according to the replacement nomenclature system (aza, thia, etc.). Although, strictly, this runs contrary to the rules,lc since it is a heterocyclic, not a hydrocarbon, system which is replaced, the connection between closely related compounds can more clearly be seen. It should be noted that Chemical Abstracts employs the systematic fusion nomenclature I, for instance, is pyrrolo[2,l,5-cd]indolizine. 

Alternatively, enantioselective nucleophilic addition to imines followed by intramolecular trapping of the transient amino intermediate with creation of a new C-N bond constitutes a powerful strategy for the synthesis of various heterocycles of synthetic and biologic interest. In this field, the formation of six-membered rings largely leads the way, and one of the first examples was reported in 2(X)3 by Ohsawa s group [4a] for the direct synthesis of the 3-carboline core via a Mannich/aza-Michael cascade promoted by (5)-proline and subsequentiy applied to the total synthesis of e t-dihydrocorynantheol (Scheme 16.3) [4b]. 

A. Darque, A. Dumetre, S. Hutter, G. Casano, M. Robin, C. Pannecouque, N. Azas, Synthesis and biological evaluation of new heterocyclic quinolinones as anti-parasite and anti-HIV drug candidates, Bioorg. Med. Chem. Lett. 19 (2009) 5962-5964. 

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