B. Madhav

Aqueous-Phase Synthesis of α-Hydroxyphosphonates Catalyzed by β-Cyclodextrin

Abstract α-Hydroxyphosphonates were synthesized from aromatic/heteroaromatic aldehydes with triethyl phosphite in the presence of β-cyclodextrin in an aqueous medium. The β-cyclodextrin can be recovered and reused without loss of catalytic activity. GRAPHICAL ABSTRACT

Recent Advances in Biomimetic Synthesis Involving Cyclodextrins

Modern bioorganic chemistry is interested in the mimicking of enzymes in their capability to bind substrates selectively and catalyze chemical reactions since biochemical selectivity will be superior to chemical selectivity in various aspects. Laboratory organic chemistry differs from that used in living systems to perform biochemical reactions. In general, organic chemists allow small reactive reagents to attack a free substrate randomly in a solution. The selectivity that is achieved is a result of selective reactivity of a particular region of the substrate or steric crowding or blocking certain approach directions. In contrast, biochemical reactions involving enzymes bind and then orient the reactants. Biochemical selectivity usually reflects such orientation, rather than the intrinsic reactivity of the substrate molecule. For instance, it is common to observe the selective oxidation of an unreactive region of a substrate molecule in an enzymatic reaction while much more reactive segments are left untouched. Enzymatic processes frequently achieve higher levels of selectivity which are not attainable by simple chemical means. Most enzyme catalyzed reactions are stereoselective, or in the choice of substrates, selective either in the type of chemical reactions performed and selective in the region of the molecule to be attacked. However, regioselectivity and stereoselectivity, in particular the formation of pure product enantiomers from achiral precursors, are aspects of enzymatic chemistry which are to be admired and imitated by synthetic chemists. Biochemical selectivity is the result of the geometry of enzyme-substrate complexes, in which only certain substrates can fit in the enzyme and only certain points in the substrates are then in a position to be attacked. Geometric control was attained by using the reagentsubstrate complexes in which a relatively rigid reagent would direct the attack into a particular region of the substrate and this is called “biomimetic control”. The term “biomimetic” has since come into wider use, generally referring to any aspect in which a chemical process imitates a biochemical reaction. Certain supramolecular hosts, with their cavities have the potential to perform novel chemical transformations, mimicking the biochemical selectivity exhibited by enzymes. Binding of substrates to these supramolecular hosts involving intermolecular interactions of non covalent nature such as hydrogen bonding, van der Waals forces, etc. results in host guest complexation akin to biological receptors and substrates. The formation of such inclusion complexes involves molecular recognition capability of the supramolecular hosts. In fact molecular recognition involves both binding and selection of the substrate by the