Intermolecular Hetero and Diels-Alder Reactions

Intermolecular Diels-Alder or hetero Diels-Alder reactions have been greatly improved by using microwave technology - again with higher reaction rates and improved yields [3j]. Remarkable improvements in rate acceleration and selectivity enhancement for a variety of intermolecular Diels-Alder reactions have also been accomplished in the past two decades by application of catalysts such as Lewis acids. Recently, many such examples have been reported under microwave conditions in polar solvents or ionic liquids as energy-transfer medium. These reactions have also been developed in open vessels by adsorption of the reactants on mineral solid supports or using neat reactants. 

Another technique used for intermolecular Diels-Alder (DA) reaction with microwave irradiation takes advantage of the intrinsic properties of the solvent [3jj. In this context, an interesting example is the microwave-induced Diels- 

The intermolecular Diels-Alder or hetero Diels-Alder reaction has also been performed under solvent-free microwave conditions in open vessels (Section 11.3). 

Intermolecular and intramolecular [3+2] cycloaddition reactions are among the most efficient and tvidely used procedures for synthesis of five-membered heterocycles. The reactive partners in these reactions are 1,3-dipoles and dipolarophiles such as alkenes and alkynes. 1,3-Dipoles vary in stability some can be isolated and stored, others are relatively stable, but they are usually employed immediately. Others are so unstable that they have to be generated and reacted in situ. There are tv ro general classes of dipole, often referred to as sp (Fig. 11.1) and sp -hybridized dipoles (Fig. 11.2). 

In the sections below the more representative 1,3-dipolar cycloadditions performed under microwave conditions will be reviewed according to the nature of the dipole. 

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