**1.1 Evolution of water as a solvent**

Although Diels and Alder themselves used water as reaction medium in the cycloaddition of furan with maleic anhydride in 1931, but it was Breslow's observations (in 1980) confirming the acceleration of some Diels-Alder reaction when carried out in water with respect to other organic solvents that played a pivotal role in the development of organic synthesis in aqueous media [4, 5]. Prior to this, in 1948, Woodward and Baer also employed aqueous maleic acid as dienophile, and in 1973 the beneficial effect of aqueous medium on the reaction was also successfully investigated by Koning and Carlson. In fact, Breslow's kinetic work was the first that quantitatively showed the valuable effects of water on the reactivity and selectivity of an organic reaction. Herein, the reaction of butanone with cyclopentadiene in water was enhanced by 740 times as compared to that carried out in isooctane [6–8]. Though, there are numerous advantages of using an aqueous reaction media. Among them, the possibility of recovery and reuse of the aqueous medium containing all the soluble species dissolved in it (catalysts and reactants), and the possibility of controlling the pH is most prominent.
