Current Organic Chemistry - Volume 13, Issue 8, 2009

When I started to organize this Special Issue on “Nitrogen heterocycles and their acyclic precursors: new routes and methods”, my major aim was to present some new and interesting results regarding the synthesis of some exotic ring systems and to demonstrate the usefulness of azides and their derivatives in the synthesis of various nitrogen heterocycles. The literature of nitrogen heterocyles and the synthetic methodologies leading to them is extremely wide but I do hope that these four reviews piled up in the issue will supply new and relevant information for many chemists working in the field of synthetic organic chemistry. The first review was written by Gyorgy Hajos and Zsuzsanna Riedl (Chemical Research Center of HAS, Budapest, Hungary) who present an overview on the applicability of nitrenes generated from azide species for the synthesis of different nitrogen-containing heterocycles. In their contribution they are discussing the generation of nitrenes, the insertions of these reactive intermediates, their addition to the double bond of alkenes and the formation of N-N bonds by the attack of the electrophilic nitrene on nucleophilic nitrogen center. This latter methodology offers a new and efficient approach to various natural products. In the final part, the 1,3-dipolar cycloaddition of acylnitrens to double bond and its synthetic aspects are surveyed. The review of the Hungarian authors covers the literature of the past 15 years and includes 61 references. The second review came from Spain, it was written by Francisco Palacios, Domitila Aparicio, Gloria Rubiales, Concepcion Alonso and Jesus M. de los Santos (University País Vasco, Vitoria-Gasteiz, Spain). They provide a detailed overview on the role of phosphazenes (phosphinimines, iminophosphoranes) obtained from azides in the synthesis of nitrogen-containing heterocycles focusing on the intramolecular aza-Wittig reaction, i.e. the reaction of an intermediate with N=P double bond with an internal oxo, ester, thioester or amide functionality. The first examples of this methodology go back to the first half of the 20th century but in the last decades this reaction became an important and widely used tool in the synthesis of the five-, sixmembered, or even larger, nitrogen-containing heterocyles. The usefulness of this technique in the synthesis of multifunctionalized polycyclic systems and natural products is demonstrated in the contribution including 71 references. This and the previous review underline the synthetic importance of the azides, a frequently underestimated group of compounds.

This paper provides a review on ring closure reactions proceeding via intermediate nitrene formation published during the past 15 years. These cyclisations are classified according to the reaction types taking place. These are nitrene aromatic C-H insertions to form indoles, pyridine rings, nitrene - alicyclic CH insertions to fused oxazolidones, various other insertions on CH bonds in diverse saturated ring and aliphatic chains, nitrene insertion on a methoxy group, addition of the nitrene to a double bond, reactions with intermediate ring formation and subsequent transformation, N-N bond formation by addition of an electrophilic nitrene, and 1,3-dipolar cycloaddition of acyl nitrenes and C=C double bonds.

A review focused on recent advances in intramolecular aza-Wittig reaction of phosphazenes with several carbonyl or analogous compounds is reported. Phosphazenes afford intramolecular aza-Wittig reaction with different groups within the molecule as aldehydes, ketones, esters, thioesters, amides, anhydrides and sulfimides. One of the most important applications of this reaction is the synthesis of a wide range of heterocyclic compounds, ranging from simple monocyclic compounds to complex polycyclic and macrocyclic systems.

A broad overview is given concerning the synthesis of 3-haloazetidines and 3-sulfonyloxyazetidines. The importance of these compounds is highlighted by numerous examples from different scientific fields. Furthermore, the reactivity and application potential of the title compounds is thoroughly documented.

Syntheses of 2-alkylideneaziridines and 2-alkylideneazetidines are reviewed. Since their increasing availability, these strained cyclic enamines, 2-alkylideneaziridines in particular, are used in several ring expansion and ring opening reactions. In addition, their use in organic chemistry will be covered.