Mini-Reviews in Organic Chemistry - Volume 3, Issue 4, 2006

Niobium catalysts generally possess a good Lewis acidity and have received increased attention in recent years. The applications of niobium pentachloride as Lewis acid in organic synthesis have been recently reviewed. Since then, there has been an increasing number of examples on the use of niobium catalysts in organic reactions such as Biginelli reactions, Friedel-Crafts acylation and Sakurai-Hosomi reactions of acetals, Knoevenagel condensation, acetylation of alcohols and phenols, among others. Also, chiral niobium complexes have been reported to give high enantiomeric excesses in asymmetric Mannich-type reactions. In this review, recent applications of these versatile reagents in organic synthesis, including our own results, will be disclosed.

A straightforward and versatile method for the determination of the absolute configuration of amino acids is presented. The proposed method involves the in situ formation of chiral complexes of optically active amino acids with the achiral dimolybdenum tetraacetate [Mo2(OAc)4] acting as an auxiliary chromophore. Optically active amino acids in reaction with [Mo2(OAc)4] in DMSO exchange in situ its acetate ligand to form chiral Mo-complexes. These complexes display several CD bands in the spectral range from 600 to 250 nm. The resulting CD spectra are suitable for the assignment of absolute configuration, since the observed sign of Cotton effects arising within the d-d absorption bands of the metal core depends solely upon the chirality of the amino acid ligands. It is shown that two Cotton effects at around 300 nm and 400 nm are especially useful for the correlation between their sign and the stereostructure of amino acids by means of the hexadecant rule. The present study demonstrates that the hexadecant rule can be extended to differently N-protected amino acids. The dimolybdenum method can also be applied for determination of the absolute configuration of cyclic β9-amino acids representing precursors of peptidomimetics.

Cyclopropanols have found an increasing number of uses as synthetic intermediates and as functional groups in the design of enzyme inhibitors. This mini-review will discuss some of the most recent and successful methodologies for the synthesis of these entities, focusing mainly on the stereoselective transformations. The new procedure developed in our group for the preparation of cyclopropanols in chiral form is also reviewed.

The guanidinium group is a typical functional group in molecules with physiological or biological activity. Its geometry and physiochemical properties are of importance for molecular recognition processes. We discuss in this review modern synthetic methods for the introduction of the guanidinium group and focus our survey on examples from natural product synthesis and drug synthesis. The presented methods are sufficiently mild and selective to allow guanidylation of more complex structures.

This review covers the synthesis, conformational analysis, mass spectrometry, and reactivity and supramolecular chemistry of azolium cyclophanes. Included is a comprehensive coverage of the structural diversity of azolium cyclophanes. Aspects of reactivity examined include anion binding, carbene formation and metal coordination chemistry and H/D exchange.

The syntheses and fascinating properties of calix[4]arene derivatives in the 1,3-alternate conformation are reviewed. Particular attention is given to the supramolecular chemistry and applications in nanochemistry of such calixarenes.