Diels-Alder reactions of cyclic ketones

The nickel-iminophosphine-catalysed 4- -2-cycloaddition of enones with allenes formed highly substituted dihydropyrans. The enantioselective amine-catalysed 4-I-2-cycloaddition of allenoates with oxo-dienes produced polysubstituted dihydropyrans in high yields and with high enantioselectivities. Novel enam-ine/metal Lewis acid bifunctional catalysis has been used in the asymmetric inverse-electron-demand hetero-Diels—Alder reactions of cyclic ketones with Q ,j9-unsaturated a-ketoesters. The 4- -2-cycloaddition of acylketenes (80) with 2-unsubstituted and 2-monosubstituted 3-aryl-2//-azirines (81) produced 1 1 (82) or 2 1 (83) adducts, being derivatives of 5-oxa-l-azabicyclo[4.1.0]hept-3-ene or 5,7-dioxa-l-azabicyclo[4.4.1]undeca-3,8-diene. The formation of the monoadducts proceeds via a stepwise non-pericyclic mechanism (Scheme 25). A-heterocyclic carbene-catalysed 4- -2-cycloaddition of ketenes with 1-azadienes yielded optically active 3,4-dihydropyrimidin-2-ones (93% ee) ... 

Employing a bifunctional enamine/metal Lewis acid catalyst enabled Wang and coworkers to perform a highly chemo-and enantioselective inverse-electron-demand hetero-Diels-Alder reaction of cyclic ketones with 8,y-unsaturated a-ketoesters (eq 12).21... 

The difficult translation of SOMO activation to cyclic ketones was achieved with the help of catalyst 6, which was developed earUer for Diels-Alder reactions of cyclic ketones via iminium activation. The difficulty Hes in the fact that aldehydes and ketones exhibit largely different steric and electronic properties with respect to catalyst interactions [16]. Competitive catalyst degradation under the oxidative conditions (i.e., oxidation of the methyl group attached to furan) was circumvented by transforming the methyl into a trifluoromethyl group, or by direct replacement of the furyl ring with less electron-rich heteroaryl systems such as benzofuran (8) or benzothiophene (9), which are less sensitive to oxidation (Scheme 39.5). 

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See also in sourсe #XX -- [ ]

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