Catalytic Asymmetric Synthesis of Chiral Dihydrobenzofurans through a Formal [4+1] Annulation Reaction of Sulfur Ylides and In Situ Generated ortho‐Quinone Methides

Abstract

The first example of a catalytic asymmetric formal [4+1] annulation reaction between sulfur ylides and in situ generated ortho‐quinone methides (o‐QMs) is reported in this work. A C2‐symmetric chiral urea was identified to be the best H‐bonding catalyst, affording a wide range of chiral 2,3‐dihydrobenzofurans in high yields and moderate enantioselectivities [70–98 % yields, up to 89:11 e.r. (enantiomeric ratio)].

Recent Advances in Enantioselective Photochemical Reactions of Stabilized Diazo Compounds

“…A variety of chiral quinolinones 86 were produced with high reaction efficiency and stereocontrol in this transformation (Scheme 22). This study features the substantial practicality of the visible-light-driven photoconversion of α-diazoketones and the unique reactivity of the resulting ketenes in Pd-catalyzed asymmetric cycloadditions [64,65]. As depicted in the proposed reaction pathway for the above process, Pd-containing 1,4-dipole 87 was generated in situ by decarboxylative process of vinyl benzoxazinone 83a with a chiral Pd(0) catalyst; meanwhile, reactive ketene intermediate 88 was formed via the visible-light-induced Wolff rearrangement of α-diazoketone 84 .…”

Organocatalytic Asymmetric Synthesis of Tetrahydrofuran and 1,2‐Dihydrobenzofuran Scaffolds

“…Yang and Xiao, in 2016, released the first catalytic asymmetric formal [4+1] annulation reaction of in situ generated ortho ‐quinone methides ( o ‐QMs) 150 and sulfur ylides 147 for the synthesis of optically active chiral 2,3‐dihydrobenzofurans 149 . Using C2‐symmetric chiral urea catalyst 148 forms multiple‐H‐bondings with sulfur ylides to form complex 147a , which binds to in situ generated o ‐QMs in the presence of CsF and 18‐crown‐6, which then undergoes intermolecular [4+1] annulation reaction to provide desired 2,3‐dihydrobenzofurans 149 in good yields and ee values.…”

Transition‐Metal Free Catalytic Synthesis of Trifluoromethyl Indolines by [4+1] Cycloaddition of Trifluoromethyl Benzoxazinones with Sulfur Ylides

“…The key for this catalytic transformation is the dual nucleophilic attack of sulfur ylides in a reaction that proceeds through an azaortho-quinone methide intermediate. [32][33][34][35][36] While the reaction mechanism is entirely different from that of Pd-catalysis, the outcome is almost the same as Pdcatalysis. This transition-metal free protocol was extended to the synthesis of non-fluorinated indolines using methyl, vinyl-or ethynyl-substituted indolines, while the CF 3 -analog was more suitable for this transformation than the others due to the electronwithdrawing property of the CF 3 group.…”

“…A wide variety of combinations of CF 3 ‐benzoxazinones and sulfur ylides were tolerated under those conditions to yield desired CF 3 ‐indolines in moderate to good yields. The key for this catalytic transformation is the dual nucleophilic attack of sulfur ylides in a reaction that proceeds through an aza‐ ortho ‐quinone methide intermediate [32–36] . While the reaction mechanism is entirely different from that of Pd‐catalysis, the outcome is almost the same as Pd‐catalysis.…”