Organic transformations that benefit from being performed on water under the conditions defined by Sharpless and co-workers was studied. The Diels-Alder cycloaddition is a powerful synthetic transformation that has been used prolifically by organic chemists, and the effect of aqueous solvents on this large family of reactions. Rideout and Breslow reported in 1990 that both rate enhancement and excellent selectivity could be achieved for certain Diels-Alder reactions when they were performed in dilute aqueous solutions. Sharpless and co-workers in 2005 demonstrated the on-water effect on the Diels-Alder reaction of trans,trans-2,4- hexadienyl acetate 10 and N-propylmaleimide 11. The 1,3-dipolar cycloaddition reactions of phthalazinium ylides on water, reported by Butler and co-workers, are noteworthy because they involve at least one solid reactant. Freeing water from the stereotypes will propel it to a prominent place in the arsenal of tools of synthetic organic chemists.
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