Current Organic Chemistry - Volume 8, Issue 8, 2004

The chemistry of heterocyclic betaines, which include several alkaloids like orellanine, is surveyed by Dr A. Schmidt, Technical University of Clausthal, Germany. Heterocycles based on ephedrines have been extensively used in the field of asymmetric synthesis. The chemistry involved is now thoroughly reviewed by Drs A. Cruz and M. Juarez-Juares. 1,3-Diketones have been frequently used for the synthesis of 4,5,6,7-tetrahydro-indoles but also for a large number of other heterocycles. The whole area has now been comprehensively reviewed by a group of authors, including Prof. Viktor Milata, from the Slovak Republic. In the last chapter Prof. R. Lavilla from the University of Barcelona, Spain is discussing the non-conventional redox chemistry of dihydropyridines and how this class of molecules can be used for the preparation of otherwise inaccessible heterocycles.

Betainic pyridinium alkaloids and analogs are members of the four major classes of heterocyclic mesomeric betaines, i.e. conjugated (CMB), cross-conjugated (CCMB), pseudo-cross-conjugated heterocyclic mesomeric betaines (PCCMB), and conjugated Nylides and thus offer the possibility of detailed comparisons of the types of conjugation. The chemistry of these systems is surveyed.

Ephedra heterocycles are well known due to their biological activity, and they have been widely used in asymmetric synthesis during the last decades. To our knowledge no review about ephedra heterocycles has appeared in the literature. In this paper, the synthetic methodology to access chiral heterocycles derived from ephedra alkaloids is discussed. Some of them have displayed biological activity and others have been used as asymmetric inductors, clearly the ephedra alkaloids are potential candidates to be used in both areas.

Owing to their high reactivity and availability 1,3-dicarbonyl compounds occupy an important place in organic synthesis. It is mainly their high reactivity that predetermines them for the synthesis of various types of compounds, particularly heterocycles. Cyclic 1,3-diketones I(n) (where n is the number of ring carbon atoms), which are suitable precursors for preparation of bicyclic or polycyclic condensed heterocycles, respectively, are an important group of 1,3-dicarbonyl compounds. However, at the same time it must be added that in spite of the relatively simple structure of 1,3- cycloalkanediones their syntheses have yet to be satisfactory solved, especially those for medium-sized and large rings. Therefore, currently the chemistry of common cyclic β-diketones, such as 1,3-cyclohexanedione, 5,5-dimethyl-1,3- cyclohexanedione (dimedone), but also that of 1,3-cyclopentanedione remains best known. In the literature one can already find a large scope of heterocycles synthesized on the basis of the above three diketones, and also data about their biological activity. Works dealing with exploitation of other, higher cyclic diketones are scarce. This article is a continuation of two reviews published by Strakov [1,2], and expands them by results published till the end of 2001.

The classical redox chemistry of the NADH-NAD+ cofactors and their synthetic analogues (dihydropyridines and pyridinium salts) has been extended to non-biomimetic processes. These involve reactions that bypass the electron transfer processes and allow the functionalization of the heterocyclic parent systems with a broad variety of carbon- and heteroatom-based functional groups. The scope of the methodology, and its applications to organic synthesis are analyzed.