Organocatalytic Intramolecular [4+2] Cycloaddition between In Situ Generated Vinylidene ortho‐Quinone Methides and Benzofurans

Abstract

Described herein is the enantioselective construction of oxygen‐containing [5‐6‐5] tricyclic heterocycles by an organocatalyzed asymmetric [4+2] cycloaddition of vinylidene ortho‐quinone methides and benzofurans. According to this methodology, a series of oxygen‐containing [5‐6‐5] tricyclic heterocycles with various functional groups were synthesized in excellent enantio‐ and diastereoselectivities (>99 % ee, >20:1 d.r.). Furthermore, the deuterium‐labeling experiments and high‐resolution mass spectroscopy demonstrated that a vinylidene ortho‐quinone methide intermediate was involved and possibly resulted from a prototropic rearrangement of 2‐ethynylphenol. Remarkably, a catalyst loading as low as 0.1 mol %, and a gram‐scale synthesis were achieved for this transformation.

Organocatalytic Atroposelective Intramolecular [4+2] Cycloaddition: Synthesis of Axially Chiral Heterobiaryls

“…Recently,w er eported the first enantioselective intramolecular [4+ +2] cycloaddition of vinylidene ortho-quinone methides (VQMs), derived from 2-ethynylphenol derivatives, with benzofuran. [11] To explore the application of VQMs in enantioselective synthesis,wepresumed that the highly active VQM intermediate might be subjected to ah etero-Diels-Alder cycloaddition in the presence of alkyne to form chiral aryl-naphthopyran skeleton (a new member of heterobiaryl skeletons). Herein, we report our successful development of this transformation to provide an efficient route for the enantioselective synthesis of chiral aryl-naphthopyran skeletons with excellent chemical yield and enantioselectivity.…”

Rhodium‐Catalyzed Cascade Synthesis of Benzofuranylmethylidene‐Benzoxasiloles: Elucidating Reaction Mechanism and Efficient Solid‐State Fluorescence

“…The calculated Gibbs free energy diagram is shown in Figure 3. Once deprotonation of rhodium(I)-bisphenol complex K affords dearomatized allenylrhodiumc omplex L, [26] nucleophilic attack of the carbonyl oxygen to the silicon atom affords rhodium acetylide M,w hich is lower in energy than L (ca. 18 kcal mol À1 ), not throught he expected1 ,2-silicon migration butr ather through an unprecedented 1,5-silicon migration [27] via transition state LM_TS.T he interaction between the basic b-carbon of acetylide and the silicon atom then affords rhodium vinylidene A [28,29] via transition state MA_TS.R hodiumv inylidene A is transformedi nto h 3 -allenyl/propargyl-rhodium complex B through 1,3-carbon (alkyne) migration via transition state AB_TS.O nt he other hand, 1,2-silicon migration withoutd eprotonation would afford rhodiumv inylidene N via transition state KN_TS.S ubsequent deprotonation would also afford the rhodiumv inylidene A mentioned above.H owever,a ctivation energy of the step from L to M via LM_TS (6.5 kcal mol À1 )i sc a.…”

Organocatalytic Atroposelective Intramolecular [4+2] Cycloaddition: Synthesis of Axially Chiral Heterobiaryls

“…[11] To explore the application of VQMs in enantioselective synthesis,wepresumed that the highly active VQM intermediate might be subjected to ah etero-Diels-Alder cycloaddition in the presence of alkyne to form chiral aryl-naphthopyran skeleton (a new member of heterobiaryl skeletons). [11] To explore the application of VQMs in enantioselective synthesis,wepresumed that the highly active VQM intermediate might be subjected to ah etero-Diels-Alder cycloaddition in the presence of alkyne to form chiral aryl-naphthopyran skeleton (a new member of heterobiaryl skeletons).…”