Current Organic Chemistry - Volume 10, Issue 17, 2006

The 2006 Natural Product Chemistry and Heterogeneous Catalysis issue of Current Organic Chemistry contains five contributions from eminent scientists in different fileds. The first subject deals with nitroalkanes and their recent synthetic applications. Their chemistry is reviewed by Ballini and coworkers. Mohamad Ali Zolfigol and coworkers provide insights abouth the use of Silica Sulfuric Acid and Silica Chloride as effficient catalysts in organic synthesis. Both reviews represent excellent contributions in the field of heterogeneous catalysis. The other three reviews are focused on Natural Product Chemistry. Hong and coworkers provide a novel overview of recent results concerning catalytic carbon-carbon bond formation in natural product synthesis covering years from 2000 to 2005. Catalytic oxidative processes in steroid chemistry is the topic of review by Salvador and coworkers. Finally Vidari and coworkers review what is currently known with regard to biomimetic cyclizations of functionalized isoprenoid polyenes. I would like to express my sincere thanks to all the authors for providing these articles and for the fascinating results described.

Solid catalysis is crucial to chemical technology. Innumerable chemical reactions are facilitated by this catalytic system and, in this context, nitroalkanes have been demonstrated a great reactivity under solid catalysis, especially in the formation of new carbon-carbon single bond. In fact, aliphatic nitrocompounds are a valuable source of stabilized carbanions since the high electron-withdrawing power of the nitro group provides an outstanding enhancement of the hydrogen acidity at the α-position (cf. pKa MeNO2 = 10). Nitronate anions, that can be generated from nitroalkanes, using a wide range of bases, act as carbon nucleophiles with common electrophiles such as aldehydes (Nitroaldol-Henryreaction), and electron-poor alkenes (Michael reaction), leading to carbon-carbon bond formation. In the last few decades several solid catalyses have been efficiently proposed for the employment of the nitroalkanes in the latter reactions. Very often the use of nitroalkanes, combined with solid catalysis, offers the opportunity to perform the reactions more efficiently, with enhanced chemoselectivity, high purity, improved yields and without the use of any solvent.

Silica sulfuric acid and silica chloride, two silica based solid acids have been used for various organic functional group transformations either as reagent or as catalyst. All reactions have been carried out under mild and heterogeneous conditions. These reagents were used for C-C, C-N, and C-O bond formation and cleavage and also deprotection of different protecting groups. Silica sulfuric acid was recycled in many cases, and reused. Silica chloride has been used as a starting material for preparation of some new silica based reagents with special properties.

The development of new catalytic reactions for C-C bond formation has played a significant role in natural products synthesis. In this article we review catalytic C-C bond formation with respect to the carbon atoms' hybridization and the products formed. The reactions of unusual substrates and the development of new reaction conditions and catalyst systems are included. This survey highlights the use of catalysts in natural products synthesis during the years 2000-2005.

The preparation of steroids containing oxygenated functions in suitable positions of the steroid nucleus is of great importance and can be achieved by means of several oxidative processes. In this paper allylic oxidation, β-selective epoxidation, alcohol oxidation and remote functionalization reactions in steroid substrates are reviewed. Focus has been given to catalytic processes because of their major importance from the viewpoint of synthetic organic chemistry.

The synthetic aspects of biomimetic cyclizations of isoprenoid polyenes, simulating one of the key steps in the biosynthesis of terpenoids in living organisms, have been reviewed, with emphasis on transformations carried out on carbocyclic systems. These reactions are made possible by a clever choice of initiating and terminating units of cyclization, and by introducing suitable functional groups on the acyclic precursor which can be manipulated after cyclization. Complex functionalized polycyclic structures are thus assembled with an efficiency paralleling or even superior that of purely biochemical processes. In particular, synthetic strategies towards alkyl and hydroxyl-substituted compounds have been examined in detail. The last part of the review has been dedicated to a thorough illustration of carbenium ion initiated polyene cyclizations, with their synthetic applications and a discussion of the different mechanisms proposed.