关键词: 不对称催化, 烯丙基苄基化, 炔酰胺环化, 串联反应, 金, 铱

Asymmetric allylic substitution reactions involving benzyl nucleophilic reagents can rapidly construct chiral molecules containing benzyl fragments, which has attracted widespread attention from organic chemists. Formal asymmetric allylic benzylation reactions have been achieved by utilizing methylene oxazole as an equivalent of benzyl nucleophile. However, the development of highly efficient asymmetric allylic benzylation reactions remains a great challenge mainly due to the poor stability and synthetic difficulty of methylene oxazole. In this work, we have developed gold- and iridium- catalyzed alkynylamide cyclization/asymmetric allylic benzylation cascade reactions. In the presence of gold-carbene complex (Au1) and the combination of [Ir(cod)Cl]₂ and (S_a)-Carreira ligand, a wide range of enantioenriched oxazole derivatives, bearing a benzylic stereogenic center, were obtained in 49%~87% yields with 98%~>99% ee. A general procedure is described as the following: To a dried Schlenk tube were added [Ir(cod)Cl]₂ (5.4 mg, 0.008 mmol, 4 mol%), (S_a)-L1 (16.2 mg, 0.032 mmol, 16 mol%) and 1,2-dichloroethane (1 mL) under argon atmosphere. The mixture was stirred at room temperature for 15 minutes to give a chiral iridium complex solution. Under an argon atmosphere, alkynylamide (0.2 mmol, 1.0 equiv.), allyl alcohol (0.4 mmol, 2.0 equiv.), Au1 (0.02 mmol, 12.4 mg, 10 mol%), Fe(OTf)₂ (0.2 mmol, 70.6 mg, 1.0 equiv.) and 3 Å molecular sieves (80.0 mg) were added to another dry Schlenk tube, and then the above-prepared iridium catalyst was added. The reaction mixture was stirred at 40 ℃ until the starting materials were consumed (monitored by thin layer chromatography, TLC). The mixture was quenched with water (5 mL), and extracted with dichloromethane (5 mL×3). The combined organic layers were dried over anhydrous Na₂SO₄, filtered, and then concentrated in vacuo to afford the crude product. The residue was purified by column chromatography (V(petroleum ether)/V(ethyl acetate)=15/1 or 10/1) to afford product 3.

Key words: asymmetric catalysis, allylic benzylation, alkynylamide cyclization, cascade reaction, gold, iridium