Diastereo- and Enantioselective [4+2] Cycloadditions of Cyclic Enones with Cyclic 1-Azadienes

Asymmetric aminocatalysis provides versatile tools for the stereoselective functionalizations of cyclic enone substrates at various sites. Recently, we developed an unusual [5+3] formal cycloaddition reaction of beta-substituted 2-cyclopentenones or simple 2-cyclohexenone with bis(electrophilic) 1-azadienes, 3-vinyl-1,2-benzoisothiazole-1,1-dioxides, through alpha'-regioselective Michael addition followed by gamma-regioselective intramolecular Mannich reaction via cascade cross-conjugated dienamine-endo-dienamine catalysis of a chiral primary amine. However, a completely different reaction pathway was disclosed when other type of cyclic enones, such as beta-substituted 2-cyclohexenones or simple 2-cyclopentenone, were used under the similar aminocatalytic conditions. In this case, the same 1-azadiene partners act as electron-deficient dienophiles, and alpha',beta-regioselective [4+2] cycloadditions occurred via cross-conjugated dienamine catalysis, giving bridged [2.2.2] octane or [2.2.1] heptane architectures with densely adorned functionalities. After systematically screening a number of reaction parameters, such as catalyst, acid additive and solvent, we can successfully realize the diastereodivergence in the above mentioned [4+2] cycloadditions. The endo-selective cycloadducts were efficiently obtained in moderate to excellent stereoselectivity (5 : 1 similar to>19 : 1 dr, 86%similar to 98% ee) in toluene at 50 degrees C, by employing a (R, R)-1,2-diphenylethanediamine derived bifunctional primary amine catalyst with a benzothiadiazine-1,1-dioxide group as hydrogen bonding donor and in combination of benzoic acid. Even exclusive endo-selectivity (>19 : 1 dr) was consistently observed in the reactions of simple 2-cyclopentenone. A combination of 9-amino-9-deoxyepiquinidine and salicylic acid also produced the endo-selective cycloadduct but with slightly lower diastereoselectivity. In contrast, the diastereoselective [4+2] cycloadditions of beta-substituted 2-cyclohexenones and 3-vinyl-1,2-benzoisothiazole-1,1-dioxides could be pleasingly switched by using 6'-OH-9-amino-9-deoxyepiquinidine and benzoic acid in PhCF3 at 50 degrees C, providing the exo-selective cycloadducts with good results in terms of diastereo- and enantioselectivity (4 : 1 similar to>19 : 1 dr, 83%similar to 95% ee). 6'-OH-9-amino-9-deoxyepiquinine could afford the exo-cycloadducts with an opposite configuration, but with less satisfactory stereoselectivity (3 examples, 5 : 1 similar to 9 : 1 dr, 62%similar to 71% ee). Moreover, the analogous 4-styryl-1,2,3-benzoxathiazine-2,2-dioxide could be smoothly utilized as the dienophilic partner, which further enriched the functionalities of the [4+2] cycloadducts.