Room-Temperature Cascade Electrophilic Addition/Cyclization/Oxidation Reactions: Divergent Selective Synthesis of Brominated 2H-Chromenes, 2H-Chromen-2-ols and 2H-Chromen-2-ones

The room temperature metal-free cascade electrophilic addition/cyclization/oxidation reactions of (3-phenoxyprop-1-yn-1-yl)benzenes to divergently synthesize various brominated benzopyran derivatives (3-bromo-2H-chromenes, 3-bromo-2H-chromen-2-ols and 3-bromo coumarins) by tuning the amount of Br2 and H2O have been developed. The method exhibited high selectivity, mild reaction conditions, broad substrate scope, high efficiency, and the applicability for derivatization of the brominated products. The importance of the strategies provides a great advantage for selective synthesis of brominated benzopyran derivatives.

Y(OTf)3-catalyzed phosphorylation of 2H-chromene hemiacetals with P(O)-H compounds to 2-phosphorylated 2H-chromenes

A facile synthesis of 2-phosphorylated 2H-chromenes has been accomplished herein via a Y(OTf)₃-catalyzed dehydrative coupling of 2H-chromene hemiacetals with P(O)-H compounds. This protocol features low catalyst loading, mild reaction conditions, broad substrate scope and easy elaboration of the products.

Design of a Functional Chromene-Type Kobayashi Precursor: Gram-Scale Total Synthesis of Natural Xanthones by Highly Regioselective Aryne Annulation

The 2,2-dimethyl-2H-chromene motif is widely found in many bioactive molecules, and is a privileged structure in the pharmaceutical arena. We have developed a concise and regioselective approach to chromenes and chromanes through an aryne-based synthetic strategy. A practical, gram-scale synthetic route to a chromene-type aryne precursor was explored. Subsequently, cyclization under mild conditions afforded tetracyclic xanthone skeletons with excellent regioselectivity. Our approach provides a concise strategy for the gram-scale synthesis of chromene-type xanthones such as 6-deoxyisojacareubin, cylindroxanthone D, staudtiixanthone D, brasilixanthone A and cudracuspixanthone O.

Synthesis of 2H-Chromenes via Unexpected [4 + 2] Annulation of Alkynyl Thioethers with o-Hydroxybenzyl Alcohols

A novel Brønsted acid-catalyzed reaction of alkynyl thioethers with o-hydroxybenzyl alcohols via an unexpected formal [4 + 2] annulation has been developed. This metal-free protocol leads to the facile and practical synthesis of valuable polysubstituted 2H-chromenes in mostly good to excellent yields under mild reaction conditions and features a wide substrate scope and excellent functional group tolerance.

Copper-Catalyzed Asymmetric Hydroboration of 2 H-Chromenes Using a Chiral Diphosphine Ligand

A highly regioselective asymmetric hydroboration of 2 H-chromenes catalyzed by the complex of CuCl and diphosphine ligand ( S, R)-DuanPhos has been realized under mild conditions to produce 3-boryl chromans, achieving good yields and excellent enantioselectivities up to 96% ee. This work provides an efficient approach to the synthesis of chiral 3-boryl chromans and derivatives.

TfOH-promoted transformation of TMS-ethers of diarylsubstituted CF3-allyl alcohols with arenes into CF3-indanes

Novel synthesis of 1,3-diaryl-1-trifluoromethyl indanes, having predominantly a trans-configuration of aryl groups, was developed on the basis of the reaction of TMS-ethers of 2,4-diaryl-1,1,1-trifluorobut-3-en-2-ols with arenes in superacid TfOH.

Boronic acid/Brønsted acid co-catalyst systems for the synthesis of 2H-chromenes from phenols and α,β-unsaturated carbonyls

Protocols for the synthesis of substituted 2H-chromenes from α,β-unsaturated carbonyls and phenols are described. Optimal combinations of arylboronic acids and Brønsted acids have been identified, such that both can be employed in catalytic quantities to accelerate these condensations. The method has been used to synthesize a variety of substituted 2H-chromenes, as well as photochromic naphthopyrans. The use of pentafluorophenylboronic acid and diphenylphosphinic acid enabled an expansion of the electrophile scope to include α,β-unsaturated ketones. Hall's 'phase-switching' of boronic acids has been exploited to achieve the separation of the two co-catalysts from unpurified reaction mixtures by a simple liquid-liquid extraction.

A cascade Claisen rearrangement/o-quinone methide formation/electrocyclization approach to 2H-chromenes

A new approach to 8-substituted 2H-chromenes is developed, featuring a novel cascade aromatic Claisen rearrangement/o-quinone methide formation/6π-electrocyclization.

Synthesis of Allylic Amide Functionalized 2H-Chromenes and Coumarins Using a One-Pot Overman Rearrangement and Gold(I)-Catalyzed Hydroarylation

A four-step synthesis of allylic trichloroacetimidates bearing a 2-proparyloxyaryl group has been developed from readily available 2-hydroxybenzaldehydes, and these have been used for the preparation of allylic amide derived 2H-chromenes using an Overman rearrangement and a 6-endo-dig hydroarylation. High yields of the 2H-chromenes were achieved using a stepwise approach involving an Overman rearrangement under thermal conditions followed by a hydroarylation reaction with a gold(I) triflimide catalyst. An alternative method where both reactions were performed as a one-pot process was also developed and instead used a gold(I) chloride catalyst activated by silver(I) hexafluoroantimonate for the cycloisomerization step. The allylic amide derived 2H-chromenes were converted to the corresponding coumarin analogues by a pyridinium dichromate (PDC)-mediated chemoselective allylic oxidation.