Current Organic Synthesis - Volume 1, Issue 3, 2004

The last fifteen years have seen the Pd-catalysed cross-coupling of organosilanes and organogermanes developed to the point where organosilane cross-couplings in particular are emerging as viable and attractive alternatives to the Stille and Suzuki reactions. In this review, the development of these group 14 cross-coupling reactions is overviewed and the current state-of-the-art assessed. Moreover, some mechanistic considerations are presented that, it is hoped, may encourage efforts to overcome remaining challenges in this important area.

Indium metal and selected indium(III) compounds have emerged as an alternative tool in organic synthesis, but the use of compounds with the metal at the oxidation state (I) has been hardly explored. This account discusses the major routes to indium(I) compounds, their chemical properties and applications in organic synthesis.

The syntheses of a variety of organic compounds, with a particular emphasis on nitrogen-containing substrates, with the use of simple electrochemical methods are illustrated. Electrochemical methods allow for the formation of useful and / or novel organic compounds under mild conditions, and involving electrons transfer without the use of any oxidizing or reducing reagents.

para-Regioselective electrophilic aromatic substitution reactions, for example nitration, alkylation, acylation, sulfonylation and halogenation, can be achieved by carrying out reactions over solid catalysts such as zeolites, under modest conditions via shape-selectivity. By contrast, organolithium reagents can play an important role when ortho-products are desirable. ortho-Lithiation has been applied for the synthesis of more complex substituted heterocycles that are difficult to prepare by other means.

2H-Azirines are the smallest unsaturated heterocyclic compounds containing nitrogen. Despite their strain many stable azirines are known. Due to the high reactivity 2H-azirines have been explored extensively for various synthetic purposes. The ring system occurs naturally in the antibiotics azirinomycin, isolated from Streptomyces auras cultures. Moreover, both enantiomers of (R)-(+)- and (S)-(-)-dysidazirine and (S)-(+)- antazirine were isolated from the marine sponge Dysidea fragilis. The chemistry of 2H-azirines is dominated by ring strain and processes in which the strain is relieved are very favoured. 2H-Azirines undergo reactions in which they can function either as a nucleophile or as an electrophile. They can participate in cycloadditions as 2π-components and undergo ring cleavage on photochemical excitation to give nitrile ylides which can act as 4π-component in dipolar cycloadditions. Thermal ring cleavage originates vinyl nitrenes which allows for ring expansion reactions. X-Ray crystallographic structure determinations reported to highly substituted 2H-azirines indicate an unusually long C2-N bond explaining the preferential C2-N cleavage in thermal reactions of 2H-azirines. The photolysis of azirines bearing a conjugative substituent (e.g. phenyl, vinyl) at C-2 leads to nitrile ylides with conjugative groups which can participate in 1,5-dipolar electrocyclic ring closure. The study of the reactivity of 2-halo-2H-azirines is of particular interest since this system bears an extra functional group. In contrast with other 2H-azirine derivatives the reactivity of 2-halo-2H-azirines has been less explored. However, the development of a general route to these heterocycles raised the interest on their use in organic synthesis. This review will focus on the chemistry of 2-halo-2H-azirines (1) taking into account the known chemistry of the non-halogenated-2H-azirines.

The biological potential of RNA along with the variety of possible applications has led to an increased demand for large-scale production of RNA molecules. Recent developments in the field allow for the synthesis of long RNA molecules (up to 100 nts) of excellent quality. This review focuses on current strategies (i.e. 2'-O-TBDMS, 2'-O-TOM and 2'-O-ACE chemistry) for chemical RNA synthesis.