Chiral Holmium Complex-Catalyzed Diels–Alder Reaction of Silyloxyvinylindoles: Stereoselective Synthesis of Hydrocarbazoles

Abstract

The catalytic and asymmetric cycloaddition between 3-[1-(silyloxy)vinyl]indoles and electron-deficient olefins gave substituted hydrocarbazoles in up to 99% yield and 94% ee. This reaction was catalyzed by a novel chiral holmium(III) complex. Alkylation of the cycloadduct gave a tricyclic compound with four continuous chiral centers, one of which was a quaternary carbon.

Recent developments in enantioselective lanthanide-catalyzed transformations

“…Ever since, other chiral lanthanide catalysts have been applied to promote asymmetric Diels-Alder cycloadditions. As a recent example, Nishida et al reported asymmetric Diels-Alder cycloadditions of 3-[1-(silyloxy)vinyl]indole 51 with N-acyloxazolidinones 44a-k promoted by a novel chiral holmium catalyst in situ generated from Ho(NTf 2 ) 3 and chiral bis-thiourea ligand 52 [32]. The reaction was performed in dichloromethane in the presence of 10 mol% of DBU that was supposed to deprotonate the NH of the ligand.…”

“…Holmium-catalyzed Diels-Alder cycloaddition of a 3-[1-(silyloxy)vinyl]indole with N-acyloxazolidinones, and synthesis of (À)-minovincine [32].…”

Construction of Optically Active Isotwistanes and Aminocyclitols Using Chiral Cyclohexadiene as a Common Intermediate

“…Yb(OTf) 3 (47.1 mg, 76.0 µmol) and (R)-BINUREA (6) 3) (44.1 mg, 76.0 µmol) taken in a test tube with a stirring bar were heated at 120°C under reduced pressure (<0.1 mmHg) for 30 min. After being allowed to cool to room temperature, the test tube was charged with dry argon.…”

“…Both functionalized compounds could be potential synthetic intermediates, and we have previously demonstrated the synthetic utility of this reaction. [4][5][6][7] In the present study, silyloxy-substituted cyclohexadiene 7 derived from 4 was shown to be a key intermediate for two types of synthetically useful chiral building blocks, i.e., the tricyclo[4.3.1.0 3,7 ] decane (isotwistane 8, n=1) and tricyclo [5.3.1.0 3,8 ] undecane (homoisotwistane 8, n=2) skeletons via the intramolecular Diels-Alder reaction, and aminocyclitol 10 via the intermolecular hetero-Diels-Alder reaction with nitrosobenzene (Chart 2).…”

Recent applications of rare-earth metal(III) triflates in cycloaddition and cyclization reactions

“…The reported [4+2]-cycloaddition of silyloxyvinylindoles 4 provides an expedient stereoselective entry into hydrocarbazoles 7 (Scheme 2), as well as hydroindoles and hydrobenzofurans. 29 The obtained chiral cycloadducts could serve as potential synthetic intermediates for Strychnos alkaloids.…”