Current Organic Chemistry - Volume 7, Issue 9, 2003

Linear solvation energy relationship, LSER, and theoretical linear solvation energy relationship, TLSER, formalisms are applied to the analysis of solvent effects on singlet oxygen reactions. Treatments allow quantitative evaluation of solvent effects and are powerful tools for interpreting the mechanism of the process.Both formalisms are applied to several amino derivatives, where for all types of solvents there is a single pattern, implying a common reaction mechanism involving charge transfer intermediates. The relative contribution of the different descriptors depends upon the compound considered, but a common feature is a significant negative dependence on the α parameter, which measures solvent acidity. Correlation equations obtained from analysis of singlet oxygen reactions with 1,3-dienes also exhibit a common dependence on the ρH parameter, that accounts for the cohesive energy of the solvent, and reflects the negative activation volume associated with concerted or partially concerted reactions. These formalisms can be also used to determine the main reaction center in polyfunctional compounds (e.g. boldine, vincamine, reserpine, and furosemide), to detect changes in the reaction mechanism with solvent properties (furosemide), and to evaluate the relative contribution of tautomers in equilibrium to the total reaction rate (piroxicam). This treatment could be used to analyze singlet oxygen reactivity in microheterogeneous systems and to predict relative rate constants in these systems.

In this review some results are reported that allow the acetal group of α,β-unsaturated carbonyl derivatives to be considered not simply a protective function but also a synthetic tool for transforming aldehydes (and ketones) into a cheap and easily accessible starting material for the construction of various and more complex structures. The reported results can be grouped into three main sections: α,β-unsaturated as masked acyl anions, as precursor of dienophilic moieties, and as reagent in organometallic syntheses. In many reported examples the key reagents are polar organometallic reagents, and in particular, the bimetallic (lithium-potassium) mixed base LICKOR, which is also known as Schlosser's superbase. We are not acquainted with previous reviews on these topics, and in our survey we have considered the literature later than the eighties.

Regio- and stereoselective synthesis of oligosaccharides with unprotected or lightly protected gluco-, manno-, and rhamnose as glycosyl acceptors was achieved via sugar orthoester formation-rearrangement. α-(1→6)- Linked mannose oligosaccharides, β-(1→6)-linked glucose oligosaccahrides, and 3,6-branched mannose and glucose oligosaccharides were conveniently synthesized with quite simplified procedure. Biologically important oligosaccharides such as phytoalexin elicitor glucohexaose, antitumor fragments of lentinan, and some repeating units of biologically important polysaccharides were prepared readily. Recent progress in regioselective synthesis of oligosaccharides with lightly protected glucosamine and galactose as acceptors was also outlined.

This review covers literature on non-stabilized functionalized organolithium compounds since 1997. In this overview, we consider intermediates with different separation between the functional group and the carbonlithium bond (dn-reagents) and having different hybridization at the carbanionic centre. Special emphasis is given to the synthetic applications of these intermediates in order to prepare polyfunctionalized molecules.