Arylnitroso dienophiles

Many examples of the use of catalytic monoclonal antibodies for a variety of organic transformations and especially for Diels-Alder reactions have been described in the last years since its discovery by Lerner and Schultz [563], Recently, a hetero Diels-Alder reaction of an arylnitroso dienophile 9-4 and ( )-piperylene 9-3 to give the two regioisomeric cycloadducts 9-5 and 9-6 in the presence of a catalytic antibody has been published by Pandit and his group [564]. The most successful hapten used was the bridged compound 9-7 (Fig. 9-2). 

Also, the use of ionic liquids such as [BMlm]SbF6, [BMImjPFG, and [BMImjOTf dramatically increased the rate and selectivity of normal Diels-Alder reactions [30]. Arylnitroso dienophiles areseenin equilibrium with their dimeric counterparts, which in turn form stable bidentate complexes with Sc(OTf)3 and react with cyclohexadiene to give the corresponding Diels-Alder adduct the same rates as that in the normal thermal processes [31]. 

Arylnitroso compounds of various types react with both electron-rich and electron-deficient dienes under mild conditions (usually 0-l(X)°C) to produce dihydrooxazines. As mentioned in the preceding section, Kresze s group has looked in considerable detail at orientational preferences and kinetics of [4 + 2] cycloadditions with various phenylnitroso compounds and substituted 1,3-dienes. Table 3-1 contains some of the results of cycloadditions of aromatic nitroso dienophiles with representative unsymmetrical 1- and 2-substituted dienes." Scheme 3-1 shows the subtle regiochemical trends that can be observed in a series of these reactions. These results were rationalized based on consideration of putative dipolar and nonpolar transition states and their relative stabilization by aromatic electron-donating and -withdrawing substituent groups. 

The normal demand hetero Diels-Alder cycloaddition between nitroso reagents 171 and dienes 172 leads to the formation of 3,6-dihydro-1,2-oxazines 173 via the concomitant stereoselective formations of a C—N and a C O bonds (Scheme 41.36). Several nitroso dienophiles like the a-chloronitroso, arylnitroso, or acylnitroso compounds are known to react efficiently in cycloadditions. Chiral nonracemic chloronitroso and acylnitroso compounds have been developed to achieve diastereoselective nitroso Diels-Alder reactions, and recently Yamamoto and co-workers reported on the first enantioselective-catalyzed cycloaddition with a pyridylnitroso dienophile. ° ... 

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