Ylide Hydrolysis in Tandem Reactions: A Highly Z/E-Selective Access to 3-Alkylidene Dihydrobenzofurans and Related Analogues

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.

Multicomponent benzannulation of allylic P-ylides with isocyanates or aldehydes for construction of anilines and biaryls

The reactivity of allylic phosphorus ylides generated in situ from alkoxycarbonylmethylenephosphoranes and propiolates is investigated toward isocyanates and aromatic aldehydes, which leads to one-pot multicomponent benzannulations for efficient synthesis of polysubstituted anilines and biaryls, respectively. The mechanism may involve a tandem [2+2] cycloaddition/fragmentation/Wittig/cyclization/elimination/aromatization sequence.

Synthesis of Aryldiphenylphosphine Oxides by Quaternization of Tertiary Diphenylphosphines with Aryl Bromides Followed by the Wittig Reaction

A two-step method for the synthesis of aryldiphenylphosphine oxides from tertiary diphenylphosphines and aryl bromides is developed. The first step is the quaternization of methyldiphenylphosphine or benzyldiphenylphosphine with aryl bromides. This quaternization can be nickel-catalyzed (metal-free in some cases), and tolerate of a variety of functional groups, furnishing quaternary phosphonium salts in 48-90% yields. The second step is Wittig reactions of these quaternary phosphonium salts with furan-2-carbaldehyde or p-chlorobenzaldehyde to provide aryldiphenylphosphine oxides in 27-90% yields. The use of the Wittig reaction for the synthesis of tertiary phosphine oxides is in contrast to its traditional use for the synthesis of olefins, leaving tertiary phosphine oxide as a byproduct. This quaternization-Wittig method can be applied to synthesize aryldiphenylphosphine oxides that are difficult to access by the alkaline hydrolysis of aryltriphenylphosphonium salts, especially those bearing an electron-deficient aryl group. The ligand-coupling mechanism for the alkaline hydrolysis of (p-acylphenyl)triphenylphosphonim salts is also discussed.

A three-component reaction of phosphorus ylides with isocyanates: facile synthesis of 2-amino-3-carboxylate-4-quinolones

A three-component reaction between one molecule of phosphorus ylides (P-ylides) and two molecules of isocyanates for the rapid assembly of 2-amino-3-carboxylate-4-quinolones is described. The mechanism may involve the addition of a P-ylide to an isocyanate followed by 1,3-H shift to form a carbamoyl stabilized P-ylide. The intermediate then reacts with another aryl isocyanate via Wittig/ketenimine-ketene rearrangement/6π-electrocyclization/1,3-H shift to finally afford the 4-quinolones.

Domino Reaction of Pyrrolidinium Ylides: Michael Addition/[1,2]-Stevens Rearrangement

A novel domino reaction featuring a Michael addition/[1,2]-Stevens rearrangement reaction of pyrrolidinium ylides with electrophilic alkenes is described. Ylides generated under mild conditions from 2-aryl- N-cyanomethyl- N-methylpyrrolidinium salts entered the Michael addition, followed by a [1,3]-hydrogen shift and finally the [1,2]-Stevens rearrangement to give 3-aryl-2-cyano-2-(2-EWG-ethyl)-1-methylpiperidines.

Collective Syntheses of 2-(3-Methylbenzofuran-2-yl)phenol-Derived Natural Products by a Cascade [3,3]-Sigmatropic Rearrangement/Aromatization Strategy

A cascade [3,3]-sigmatropic rearrangement/aromatization strategy to the synthesis of 2-(3-methylbenzofuran-2-yl)phenol derivatives was developed and applied to the collective syntheses of seven 2-arylbenzofuran-containing natural products, namely glycybenzofuran, glycyuralin E, lespedezol A₁, puerariafuran, 7,2',4'-trihydroxy-3-benzofurancarboxylic acid, coumestrol, and 4'-O-methylcoumestrol. Among them, the total syntheses of glycybenzofuran, glycyuralin E, puerariafuran, 7,2',4'-trihydroxy-3-benzofurancarboxylic acid, and 4'-O-methylcoumestrol were reported for the first time. The practicality of this novel strategy in preparation of the key intermediates was demonstrated by performing the reaction on gram scale and by synthesizing a series of natural products with 2-(3-methylbenzofuran-2-yl)phenol scaffolds in a common strategy.

Stereoselective Synthesis of (E)-β-Aryloxyl and Alkyloxyl Acrylates Through DABCO-Catalysed Michael Additions of Phenols and Alcohols to Ethyl 2,3-Butadienoate

The development of DABCO-catalysed Michael addition of phenols and alcohols to ethyl 2,3-butadienoate provides an efficient synthetic pathway to ( E)- β-aryloxyl and alkyloxyl acrylates in i-PrOH or under solvent-free conditions. The major advantages of the present method are wide substrate scope, mild reaction conditions, high stereoselectivity, and good reaction yields.

Palladium-Catalyzed Synthesis of 2,3-Disubstituted Benzofurans: An Approach Towards the Synthesis of Deuterium Labeled Compounds

Palladium-catalyzed oxidative annulations between phenols and alkenylcarboxylic acids produced a library of benzofuran compounds. Depending on the nature of the substitution of the phenol precursor, either 2,3-dialkylbenzofurans or 2-alkyl-3-methylene-2,3-dihydrobenzofurans can be synthesized with excellent regioselectivity. Reactions between conjugated 5-phenylpenta-2,4-dienoic acids and phenol gave 3-alkylidenedihydrobenzofuran alkaloid motifs while biologically active 7-arylbenzofuran derivatives were prepared by starting from 2-phenylphenols. More interestingly, selective incorporation of deuterium from D2O has been discovered, which offers an attractive one-step method to access deuterated compounds.

Gilheany D. First ever observation of the intermediate of phosphonium salt and ylide hydrolysis: P-hydroxytetraorganophosphorane

P-Hydroxytetraorganophosphorane, the putative intermediate in phosphonium salt & ylide hydrolysis reactions, has been observed and characterised for the first time, thus establishing unequivocally its existence & involvement in these reactions.