Current Organic Chemistry - Volume 9, Issue 2, 2005

The photooxygenation of heterocycles represents a versatile and widely accepted tool for introducing oxygenated functions in a mild, simple and selective way. The review evidences the synthetic potential of the photooxygenation with particular attention to the reaction of Type II involving singlet oxygen in the first electronically excited state (1Δg), which has been applied to the most studied heterocycles as furans, thiophenes, pyrroles, oxazoles, imidazoles, indoles, nitrogen-containing six-membered systems. The singlet oxygenation of these systems occurs mainly via [4+2] cycloaddition leading to unstable endoperoxides which, in addition to the classical transformations of peroxides (reduction, hydrolysis, deoxygenation, generally performed at low temperature), afford characteristic rearranged products depending on the heteroatom, substitution pattern and experimental conditions. 1,2-Oxygen addition can sometimes compete with the Diels-Alder-type reaction, especially for pyrroles, imidazoles and indoles. The attention has been also focused to the oxygenation of some biomolecules as histidine, triptophan and guanine which play a significant role in biological processes as photodynamic effects or in the photoinduced deactivation of nucleic acids.

New developments in the chemistry of quinoline derivatives are reviewed. Two general synthetic routes based on the utilization of mono-substituted or ortho-substituted anilines are discussed. Their major methods and modifications are analyzed.

Due to the potent biological activity exhibited by various indole derivatives, there is a continuous demand for novel synthetic procedures in this area. In 1989, the reaction of vinyl magnesium halides with ortho-substituted nitroarenes was discovered to lead to indoles. In the 1990s, it has attracted much attention, as it employs simple and readily available starting materials. This reaction is now frequently reported as the “Bartoli reaction” or the “Bartoli indole synthesis” and has rapidly become the shortest and most flexible route to 7-substituted indoles, as classical indole syntheses generally fail in their preparation. The flexibility of Bartoli reaction is great as it can be extended to heteroaromatic nitro derivatives and can be run on solid support. The necessity of an ortho-substituent on the aromatic ring is the limit of the Bartoli indole synthesis, because o,o'- unsubstituted nitroarenes follow a completely different pathway when reacting with vinyl Grignard reagents. Bromine, however, should be a transient group, which can enforce the sigmatropic rearrangement, as requested by the mechanism, and is easily removed. A combination of the two methodologies can give a significant reduction of steps required for the preparation of many complex 7-unsubstituted indoles, whose functions are tolerant to the reaction conditions, but not to classical indole syntheses. This review will focus both the use of the Bartoli indole synthesis as key step in many preparation of complex indoles and the improvements of the reaction.

This review provides a comprehensive survey of the “one pot glycosylation” (OPG) strategy for the chemical synthesis of oligosaccharides, covering literatures from the first example reported by Kahne and Raghavan in 1993 through May 2003. The essence of the OPG is to distinguish the reactivity difference of a pair of the glycosylation donors or acceptors so as to carry out two glycosylation steps sequentially without purification of the first-step coupling product. Accordingly, the literature reports are grouped based on the major stereoelectronic factors causing the reactivity differences, those include the “armed-disarmed effect”, “orthogonality of leaving groups”, “distinguishable acceptors”, and “the hybrid”. “The hybrid” OPG procedure takes advantage of a combination of the reactivity disparity of a set of the armed-disarmed donors, orthogonal leaving groups, as well as acceptors so as to proceed three or more steps of glycosylation sequentially in one pot. Relevant conception and exploitation of the reactivity differences of the donors and acceptors in the synthesis of oligosaccharides, which finally evolve the OPG or advance parallelly, are briefly described at the beginning.

Organic solvents are extensively used in organic synthesis and for this reason they are a matter of much concern due to characteristics such as: high flammability, volatility, hazardness, and toxicity. Thus the search for environmentally benign substitutes for organic solvents has recently gained more attention in view of the increasing importance of Green Chemistry. In this review, recent developments in the fields of supercritical fluids, ionic liquids, low melting polymers (especially PEG), perfluorinated solvents and water in many types of organic reactions will be disclosed.