Current Organic Chemistry - Volume 7, Issue 10, 2003

In the Past, the term heterocyclic compounds would invoke in the mind of the average chemist aromatic molecules containing one or more nitrogen atoms. This view undoubtedly derives from the overwhelming biological and pharmaceutical importance of such entities as well as their wide distribution in nature. However, heterocycles can also be aliphatic and can contain, apart from nitrogen, other heteroatoms such as silicon and oxygen. The present issue of Current Organic Chemistry consists of four reviews devoted to a diverse range of heterocycles, most of which are unusual in their structure, provenance, and properties.In the first review, Takatsugu Wakahara, Masahiro Kako, Yutaka Maeda, Takeshi Akasaka, Kaoru Kobayashi, and Shigeru Nagase introduce the reader to what happens at the interface when two markedly different types of chemistry come together. Fullerene and its higher homologues are no longer simply novel as allotropes of carbon, but are able to undergo a variety of reactions. In the same vein, disilanes and polysilanes are remarkable for their versatile synthetic utility. The authors describe the results when the two classes of compound combine as reactants and reagents respectively to produce silylated fullerenes, many of which show promise as new materials.The second review illustrates the strong contribution made by Japanese chemists to the increasingly important field of Marine Chemistry. Shigeki Matsunaga and Nobuhiro Fusetani ably present an up-to-date review on nonribosomal peptides isolated from marine sponges. The authors describe peptides of remarkable structural diversity, as exemplified by peptide lactones, cyclic peptides, and depsipeptides, paying particular attention to their biogenesis and biological activity.The third review by Vadim K. Khlestkin and Dmitrii G. Mazhukin deals with a surprisingly under-represented class of simple N,O-dialkylhydroxylamines and describe how they can be exploited to provide access to acyclic, carbo-and heterocyclic amines, many of which are naturally occurring and biologically active. The fourth review is a solo effort by Wanda Sliwa. She describes in thorough fashion the recent chemistry of quaternary salts of pyridines. Of particular note is the ability of these quaternary azaaromatic compounds to fit inside cyclic ethers to form various types of supramolecular assemblies, and to form host-guest systems, both of which are topics of keen current interest. Application of the salts are as reagents in synthesis and for the design of new materials is also covered. By way of conclusion, I, as Executive Guest Editor, wish to heartily thank the present authors for their perseverance in producing the reviews and sharing with the reader their expert knowledge and the latest research findings. A final word is in order. Good manners decree that guests should never overstay their welcome. I have greatly enjoyed being Guest Executive Editor of the Heterocycles Section of Current Organic Chemistry. It has been a great pleasure and honor to work with my friend and colleague Professor Atta-ur-Rahman as Editor-in-Chief and his team of helpful and efficient assistants in Karachi. However, the time has come for me to quit the editor's desk and make way for another guest. It is therefore appropriate that I extend my thanks to all the previous authors of the Heterocycles editions for their contributions and for making COC such an outstanding success.

Organosilicon compounds are perhaps the most important inter-element compounds, as exemplified by disilanes and polysilanes. Similarly, ever since fullerene C60 became available in preparatively useful quantities, this new allotropic form of carbon has attracted much attention, particularly its chemical functionalization. The present review describes how these two disparate classes of compound can be induced to react together and combine to form novel derivatives endowed with unusual properties. In particular, we describe how silylenes derived from various disilane derivatives under thermal and photochemical conditions can react with fullerenes and endohedral metallofullerenes. Mono- bis- and even multi-silylation of the fullerene framework takes place leading to a new class of fullerene-based organic compounds which at the same time opens a promising new field in materials science.

Marine sponges are shown to be abundant sources of bioactive peptides. Structurally they include linear peptides, peptide lactones, cyclic peptides, and depsipeptides, with number of residues ranging from two to 48. Some of them have highly modified structural features with nonproteinogenic amino acid or hydroxy acid residues, while others have those with minimal alterations from the common ribosomal peptides. The structural characteristics, biogenesis, biological activity, and the issue of producing symbiotic organisms of nonribosomal peptides reported since 1993 are described in the order of structural types.

N,O-Disubstituted hydroxylamines (R1NHOR2 or NODHA), unlike the O-monosubstituted derivatives, have received little attention so far. However, they have recently become important as intermediates for synthesizing complex nitrogen-containing compounds, especially natural products and their analogues.The formation of NODHA by stereoselective reduction, nucleophile and radical additions provides access to acyclic, carbo- and heterocyclic amines, many of which are naturally occurring and biologically active. New methods developed for the acylation of N,O-dimethylhydroxylamine and polymer-bound N,O-disubstituted hydroxylamines are an improvement on the Weinreb amide methodology and provide convenient routes to complex aldehydes and ketones as well as hydroxamic acids. Reductive and base-induced cleavage of the N-O bond of N,O-disubstituted hydroxylamines makes them valuable intermediates for preparing highly functionalized amines.Methods for the synthesis of bis(alkoxyamines) and their use as precursors to alkoxy-substituted heterocycles are described. Their bisnitroso derivatives on intramolecular cyclization provide an entry to hitherto unknown 4,5-dihydro-1,2,3-triazole 2-oxides.Finally, the review describes recent studies of naturally occurring and biologically active N,O-disubstituted hydroxylamines.

Quaternary salts of pyridine and related compounds are a topic of numerous reports. In the present review, eight chapters are devoted to selected examples of these compounds describing their syntheses, physicochemical properties and possible applications, along with their biological activities. The ninth chapter deals with the chemistry of viologens, while the tenth reviews selected work from our research group on benzonaphthyridinium quaternary salts.