关键词: 有机胺催化, [4+2]环加成, 对映选择性, 非对映选择性, 非对映选择性反转, 环状烯酮, 1-氮杂二烯

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 β-substituted 2-cyclopentenones or simple 2-cyclohexenone with bis(electrophilic) 1-azadienes, 3-vinyl-1,2-benzoisothiazole-1,1-dioxides, through α'-regioselective Michael addition followed by γ-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 β-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 α',β-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～>19:1 dr, 86%～98% ee) in toluene at 50 ℃, 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 β-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 ℃, providing the exo-selective cycloadducts with good results in terms of diastereo- and enantioselectivity (4:1～>19:1 dr, 83%～95% ee). 6'-OH-9-amino- 9-deoxyepiquinine could afford the exo-cycloadducts with an opposite configuration, but with less satisfactory stereoselectivity (3 examples, 5:1～9:1 dr, 62%～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.

Key words: aminocatalysis, [4+2] cycloaddition, enantioselectivity, diastereoselectivity, diastereodivergence, cyclic enones, 1-azadienes