Formal (4+1)-Cyclization of Ammonium Ylides with Vinylogous para-Quinone Methides

Catalytic Asymmetric Synthesis of Inherently Chiral Eight-Membered O-Heterocycles through Cross-[4+4] Cycloaddition of Quinone Methides

Inherently chiral eight-membered rings embedded in tetraphenylene derivatives and hetero-analogues exhibit unique properties and allow diverse applications. A conceptually viable and straightforward approach to these frameworks is [4+4] cycloaddition, which still remains elusive. Herein, we describe the stereoselective cross-[4+4] cycloaddition of quinone methides (QMs), leading to the formation of oxa-analogues of tetraphenylene with exceptional chemo-, diastereo-, and enantioselectivity. The structures of these novel rigid eight-membered O-heterocycles were explored by single-crystal X-ray diffraction, and their stereochemical stability was elaborated through both density functional theory (DFT) calculations and thermal racemization experiments. The developed methodology exhibited broad substrate scope and the resulting cross-[4+4] cycloadducts could be readily transformed into valuable chiral building blocks. Our findings expand the library of inherently chiral medium-sized rings and also contribute to the advancement of asymmetric cross-[4+4] cycloadditions of quinone methides.

A reusable polymer anchored pyridine mediated formal [4 + 1] annulation reaction for the diastereoselective synthesis of 2,3-dihydrobenzofurans

We have developed a highly stereoselective formal [4 + 1] annulation reaction to construct trans-2,3-dihydrobenzofurans utilising in situ generated supported pyridinium ylide. This approach has excellent substrate versatility and gram-scale synthesis capability. Moreover, the polymer-anchored pyridine has been recovered and reused multiple times. The product has been transformed into valuable molecules.

Tandem 1,6-addition/cyclopropanation/rearrangement reaction of vinylogous para-quinone methides with 3-chlorooxindoles: construction of vicinal quaternary carbon centers

A novel tandem 1,6-addition/cyclopropanation/rearrangement reaction of vinylogous para-quinone methides with 3-chlorooxindoles has been developed, providing dispirooxindole–cyclopentane–cyclohexadienones with vicinal quaternary carbon centers.

Feeding sequence-regulated divergent (4 + 1 + 1’) annulations of α-bromo carbonyls and 1-azadienes via computational calculations-based mechanism elucidation

By investigating the mechanism of our previously reported (4 + 1 + 1) annulations of α-bromo carbonyls and 1-azadienes via density functional theory (DFT) calculations, the formation of a carbon...

Isothiourea-Catalyzed Enantioselective α-Alkylation of Esters via 1,6-Conjugate Addition to para-Quinone Methides

The isothiourea-catalyzed enantioselective 1,6-conjugate addition of para-nitrophenyl esters to 2,6-disubstituted para-quinone methides is reported. para-Nitrophenoxide, generated in situ from initial N-acylation of the isothiourea by the para-nitrophenyl ester, is proposed to facilitate catalyst turnover in this transformation. A range of para-nitrophenyl ester products can be isolated, or derivatized in situ by addition of benzylamine to give amides at up to 99% yield. Although low diastereocontrol is observed, the diastereoisomeric ester products are separable and formed with high enantiocontrol (up to 94:6 er).

Diverting β-Hydride Elimination of a π-Allyl PdII Carbene Complex for the Assembly of Disubstituted Indolines via a Highly Diastereoselective (4 + 1)-Cycloaddition

A Pd⁰-catalyzed formal (4 + 1)-cycloaddition approach to 2,3-disubstituted dihydroindoles is described. The diastereoselective formation of dihydroindoles that is highlighted by a carbene migratory insertion/reductive elimination sequence proceeding via a π-allyl Pd^II-species compliments existing methods of indoline assembly.

CF3-Containing para-Quinone Methides for Organic Synthesis

A new family of CF3-containing para-quinone methides (CF3-QMs) was systematically investigated for its suitability in organic synthesis. Addition of different nucleophiles gives access to target molecules with a benzylic CF3-containing stereogenic center straightforwardly. The electrophilicity parameter E of the prototypical CF3-QM 2,6-di-tert-butyl-4-(2,2,2-trifluoroethylidene)cyclohexa-2,5-dien-1-one was determined to be -11.68 according to the Mayr scale, making it one of the most reactive quinone methides known so far.

Electrophilic Reactivities of Vinyl p-Quinone Methides

The electrophilic reactivity of a series of 8-arylated vinyl p-quinone methides (pVQMs) was determined by analyzing the kinetics of their reactions with carbanions in DMSO at 20 °C according to the linear free energy relationship log k = s_N(N + E). The electrophilicity parameters E for pVQMs were used to successfully predict Michael-additions with structurally diverse C-, N-, S-, and H-nucleophiles.

A thermal decarboxylative Cloke–Wilson rearrangement of dispirocyclopropanes derived from para-quinone methides and bromo-Meldrum's acids: an approach to synthesize spirobutyrolactone para-dienones

An unprecedented approach to synthesize spirobutyrolactone para-dienones from para-quinone methides and bromo-Meldrum's acids has been developed.

(3 + 1) Annulation/Rearrangement Cascade of C,N-Cyclic Azomethine Imines and 3-Chlorooxindoles: Construction of Hexahydroindeno[2,1-c]pyrazole Spirooxindole Frameworks

The Brønsted base promoted (3 + 1) annulation reaction of C,N-cyclic azomethine imines and 3-chlorooxindoles was investigated, finally delivering complex hexahydroindeno[2,1-c] pyrazoles incorporating a spirocyclic oxindole motif after an unexpected rearrangement process. The asymmetric version of this new transformation could be accomplished, but slow racemization of the chiral product was observed at ambient temperature. Experiments and DFT calculations were further conducted to elucidate the reaction process and racemization mechanism.

[4 + 1 + 1] Annulations of α-Bromo Carbonyls and 1-Azadienes toward Fused Benzoazaheterocycles

An unexpected [4 + 1 + 1] annulation between α-bromo carbonyls and 1-azadienes, derived from 2-methylenebenzofuran-3(2 H)-ones or 2-methylenebenzo[ b]thiophene-3(2 H)-ones, has been observed in the presence of DABCO and Cs₂CO₃. These reactions stand in contrast to the common [4 + 1] cyclization reactions of azadienes with the related sulfonium ylides. A range of fused benzofuro[3,2- b]pyridines and benzo[4,5]thieno[3,2- b]pyridines have been efficiently constructed in fair to excellent yields.