Hetero Diels-Alder Reactions in Aqueous Solution

SCHEME 13. A Lewis acid catalysed hetero-Diels-Alder reaction in aqueous solution... 

Clearly, an important feature will be the selectivity of these reactions. In this respect, the control of endo- and exo-selectivity using different Lewis acids, the induced diastereoselectivity with chiral heterobutadienes as well as chiral heterodienophiles and finally the use of chiral Lewis acids for the enantioselec-tive synthesis will be discussed. In recent time some attention has been paid to hetero Diels-Alder reactions in aqueous solutions and in the presence of inor-... 

Hetero Diels-Alder Reactions in Aqueous Solution... 

Wijnen, f.W., Zavarise, S., Engberts, J.B.F.N. and Charton, M., Substituent effects on an inverse electron demand hetero Diels Alder reaction in aqueous solution and organic solvents cycloaddition of substituted styrenes to di(2 pyridyl) 1,2,4,5 tetrazine, /. Org. Chem., 1996,61, 2001-2005. 

Reaction between nitrosobenzene (55) and cyclopentadiene (56a) gives an unstable cycloadduct (54a) however, in a highly aqueous medium the adduct is stabilized by hydrogen bonding and the hetero retro-Diels-Alder reaction is retarded, thereby enabling a study of the equilibrium dynamics with both reactants and products present in solution.31 Comparison with the corresponding reaction of cyclohexa-1,3-diene has been made in an attempt to separate the effects of the aqueous medium on the rate constants for the forward and reverse reaction. 

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