Highly efficient Synthesis of per -substituted amino-cyclodextrins under Microwave Irradiation in a closed Cavity
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Highly efficient Synthesis of per-substituted amino-cyclodextrins under Microwave Irradiation in a closed Cavity Giancarlo Cravotto*1, Katia Martina1, Marina Caporaso1, Georgios Heropoulos2 and László Jicsinszky3 1 Dipartimento di Scienza e Tecnologia del Farmaco, Università di Torino, Via P. Giuria 9, 10125 Torino, Italy. 2 Institute of Organic and Pharmaceutical Chemistry The National Hellenic Research Foundation, 11635 Athens, Greece. 3 Cyclolab R&D Laboratory, Illatos út 7, H-1097 Budapest, Hungary. *Author for correspondence: E-mail: [email protected]; FAX +39.011.6707687 ABSTRACT Synthetic chemists are paying ever more attention to enabling technologies as a means to opening the paths towards the double goal of achieving high efficiency and meeting green criteria. Non-conventional techniques that create unique environments which promote selective modification have been investigated by many as part of the search for more highly efficient synthetic derivatization of cyclodextrins (CD). A few optimized microwave-assisted protocols have so far been developed for the preparation of selective per-alkylated aminocyclodextrin. In this work, a series of β-CD derivatives, whose primary hydroxyls were all replaced by amino groups, has efficiently been synthesized from per-(6-iodo-6-deoxy)-ȕ-CD via nucleophilic substitution with amines under microwave irradiation in closed vessel (N2 pressure). The reduction of per-(6-azido-6-deoxy)-ȕ-CD to per-(6-amino-6-deoxy)-ȕ-CD via catalytic hydrogenation has also been successfully carried out under dielectric heating. INTRODUCTION After more than 20 years of investigation, the use of dielectric heating to promote chemical reactions is now well established as a reliable technique which can be applied to a range of scales, from milliliters to kilograms [1]. The availability of professional microwave (MW) reactors (batch and flow), which provide control of all the main parameters, over a wide range of temperatures and pressures, often makes the critical conversions of even poorly reactive substrates feasible by cutting down on reaction time [2]. A comparison of processes performed under classic conductive heating and under MW irradiation is not a trivial task due to difficulties in understanding the true MW process temperature. The nature of the MW effect has been studied from the points of view of both reaction medium and mechanism. The effect should be stronger in apolar solvents and solvent-free reactions, with polar transition states and late transition states [3]. It is clear that the irradiation of MW absorbing materials (catalysts, carriers and reaction medium) can cause rapid volumetric heating and modify surface properties. This distinguishes the action of MW from traditional thermal treatments which are used in the preparation of catalysts and in reactions. The unique environments obtained under MW irradiation make this an useful enabling technique for the synthesis of tailor-made macromolecules, of which selective modified cyclodextrins (CDs) are a striking exampl
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