Copper Catalyzed [3+2] Annulation of Indoles with 1,1,2,2-Tetrasubstituted Donor-Acceptor Cyclopropanes

D-A cyclopropanes have emerged as versatile synthons for construction of carbocycles and heterocycles via a [3+2] annulation reactions, and have been used in the total synthesis of natural products. Recently, it has been witnessed tremendous progress within the area of transformation of 2-monosubstituted-cyclopropane-1,1-diesters. However, cyclopropane- 1,1-diesters with full substitution at the donor site have not been well explored. C2, C3-fused indolines are widely existed in a plenty of natural products and biologically active compounds, and have been the synthetic targets for decades. Among the various approaches to access these important structural motifs, the cyclopentannulation of indoles with Donor-Acceptor (D-A) cyclopropanes, represents a concise, economical and effective method. Previously, we have developed a highly diastereo- and enantioselective BOX/Cu(II) catalyzed C2, C3-cyclopentannulation of indoles with 2-monosubstituted-cyclopropane-1,1-diesters, a facile access to a series of enantioenriched cyclopenta-fused indoline products. As our further studies, Lewis acid catalyzed [3+2] annulation of indoles with 1,1,2,2-tetrasubstituted D-A cyclopropanes was reported in this paper. This annulation method of C3-substituted indoles with quaternary donor site D-A cyclopropanes yielded C2, C3-fused indolines, bearing three quaternary stereocentres on the newly built cyclopentane ring without the formation of the common Friedel-Crafts byproducts. The ester groups on cyclopropane, ligand, and protection group of indole have great influence on both yield and dr selectivity. Thus, the reaction between indole (1b, -NMe) and cyclopropane 2 (CO2R2=CO2CH2CF3) can give the highest yield and the best dr in the presence of 10 mol% BOX/Cu(SbF6)(2) in DCM, which is prepared in situ. Under the optimal conditions, the [3+2] annulation reacts smoothly with a wide range of substituted indole derivatives and D-A cyclopropanes, giving the desired products in up to 91% yield with up to >20/1 diastereoselectivity. The relative configuration of the products is determined by X-ray crystallographic analysis of the major diastereoisomer of 3b. The general experimental procedure for the [3+2] annulations is shown below: A mixture of CuBr2 (0.02 mmol), AgSbF6 (0.04 mmol), and bisoxazoline (L, 0.024 mmol) in DCM (1 mL) was stirred at room temperature for 3 h under the atmosphere of nitrogen. Then, the mixture was cooled to degrees C for 20 min and the cyclopropane 1 (0.2 mmol) and the indole derivative 2 (0.4 mmol) in 1 mL DCM were added to the mixture of catalyst via a syringe. After the reaction was complete (monitored by TLC), the reaction was filtered through a glass funnel with thin layer (20 mm) of silica gel (100 similar to 200 mesh) and eluted with DCM (approx 100 mL). The filtrate was concentrated under reduced pressure. After the determination of the diatereoselectivity by H-1 NMR, the residue was purified by flash chromatography to afford the product 3.