Eco-friendly approaches

Most of these publications describe important accelerations of a wide range of organic reactions especially when performed under solvent-free conditions. The combination of solvent-free reaction conditions and microwave irradiation leads to large reductions in reaction times, enhancement of yield, and, sometimes [3, 4] in selectivity with several advantages of an eco-friendly approach, termed green chemistry . 

Several eco-friendly approaches to vinyl-/i-lactams (219-221) bearing a vinyl substituent at various positions on the ring (Scheme 9.71) have recently been described by Manhas [122]. Vinyl-/i-lactams are efficient synthons for a variety of compounds of biomedical interest, e. g. isocephalosporins, carbapenem intermediates and pyrrolidine alkaloids. MORE chemistry techniques allow highly accelerated syntheses using limited amounts of solvent and with efficient stereocontrol, thus achieving high atom economy . 

Manilas, M.S., Banik, B.K., Mathur, A., Vincent, J.E. and Bose, A.K., Vinyl-(3-lactams as efficient synthons. Eco-friendly approaches via microwave assisted reactions, Tetrahedron, 2000, 56, 5587. 

Sivakumar, V. and Rao, P.G. 2001. Application of power ultrasound in leather processing An eco-friendly approach. Journal of Cleaner Production, 9 25-33. 

Ranu, B.C., Guchhait, S.K., Ghosh, K., and Patra, A. 2000. Construction of bicyclo-[2.2.2] octanone systems by microwave-assisted solid phase Michael addition followed by AI2O3-mediated intramolecular aldolisation. An eco-friendly approach. Green Chemistry, 1 5-6. 

The development of simple, convenient, and an eco-friendly approach for the S3mthesis of these biologically imporiant compoimds is still in demand. For example, the very useful and green concept of a "click" reaction is a facile, selective, high-peld reaction imder mild water-tolerant conditions with little or no by-products [8]. Cascade armulation reactions lead also to the formation of polycydic fused six- and seven-membered heterocycles with indole and indolizine core [9]. 

Ecological considerations and investigations of fire hazards such as CO and smoke production target the inert filler characteristics of nanocomposites. The rather physical mechanisms proposed for nanocomposites are advantageous for such considerations. Nanocomposites appear to be a promising eco-friendly approach to fire retardancy in polymers. 

The approach precludes the usage of volatile organic solvents, is relatively much faster, efficient, and eco-friendly. Significant rate enhancements are reported in the 1,3-dipolar cycloaddition reactions including the use of covalently grafted dipolaro-philes on the ionic liquids [189]. 

Quiroga and co-authors [101] also reported eco-friendly solvent-free approach to the synthesis of fused benzo[fjpyrazolo[3,4-b]quinolines 72 by three-component reaction of 5-aminopyrazoles, aldehydes, and p-tetralone accomplished by fusion procedure (Scheme 32). However, this method was found inapplicable for the similar reaction of a-tetralone - multicomponent procedure allowed obtaining only bispyrazolopyridines 74 instead of benzo[h]pyrazolo[3,4-b]quinolines 73. According to these experimental results, the latter were generated via preliminary synthesis of arylidentetralones 75. 

Partially hydrogenated quinoline cores are also present in some important bioactive compounds. For example, the 4-aza-analogs of Podophyllotoxin, a plant lignan that inhibits microtubule assembly, revealed to be more potent and less toxic anticancer agents. In 2006, Ji s group reported a green multicomponent approach to a new series of these derivatives, consisting of the reaction of either tetronic acid or 1,3-indanedione with various aldehydes and substituted anilines in water under microwave irradiation conditions (Scheme 26) [107]. For this efficient and eco-friendly transformation, the authors proposed a mechanism quite similar to the one that was postulated for the synthesis of tetrahydroquinolines in the precedent section. 

Tetrahydropyrans hydroxylated at the 4-position have good synthetic value [113]. Although many synthetic methods have been reported [17-23,114,115], the search for potential alternate approaches and the development of eco-friendly and high-yielding reactions resulted in the development of a method that poses less problems for the environment. Synthesis of tetrahydropyranol derivatives can be achieved through the Prins-type cyclization reaction of homoallylic alcohols with aldehydes using bismuth triflate as catalyst in [bmim]PF6 solvent system [108] (Fig. 22). 

The examples of this section summarise the recent application of the solvent-drop grinding approach to solid-state cocrystal preparation. The approach has been shown in certain instances to provide for either acceleration of CO crystallisation kinetics or selection of a particular polymorph via solid-state grinding. The approach is attractive, as it appears to incorporate some of the beneficial aspects of solvent participation while maintaining an essentially green, eco-friendly process. 

This can be achieved either in a true multi-component mode, where all the substrates and reagents are mixed together initially and subsequently processed, or in a one-pot multi-step procedure, where one or several ofthe reaction components are added to the reaction at a later stage. Both approaches eliminate the need for work-up or purification between the different reaction steps making the reaction both more eco-friendly and faster. 

Pal, P., Dutta, S. and Bhattacharya, P. (2002). Multi-enzyme immobUization in eco-friendly emulsion liquid membrane reactor—A new approach to membrane formulation. Sep. Purif. Technol., 27, 145-54. 

These studies demonstrate the convenience of biologically assisted synthesis for the development of electrode materials. However, this eco-friendly synthesis approach is still dependent on our ability to control the physiological conditions of bacteria and to accelerate their reaction kinetics. These are the kinds of challenges to which we will devote our future activities in collaboration with our... 

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