1,6-Conjugate-Addition-Induced [2 + 1] Annulation of para-Quinone Methides and Pyrazolones: Synthesis of Bis-Spiro Compounds with Contiguous Quaternary Spiro-Centers

A one-pot synthesis of 2,3-dihydrobenzofurans, benzofuran-2(3H)-ones, and indoles via a [4 + 1] annulation reaction of ortho-substituted para-quinone methides and bromonitromethane

Under mild and metal-free reaction conditions, a facile one-pot synthesis of oxygen- and nitrogen-containing benzoheterocycles has been achieved through [4 + 1] annulation of para-quinone methides, followed by an oxidation/elimination sequence, efficiently synthesizing functionalized 2,3-dihydrobenzofuran, benzofuran-2(3H)-one, and indole derivatives in moderate to good yields (up to 72%). A novel functional group tolerant oxidative Nef reaction utilizes DDQ without a base.

BF3 ⋅Et2 O-Catalyzed Selective C-4 Alkylation of Isoquinolin-1(2H)-ones Employing p-Quinone Methides

The direct C-4 alkylation of isoquinolin-1(2H)-one moiety is a challenging transformation in organic synthesis. Here we present a practical and efficient synthesis of C-4 alkylated isoquinolin-1(2H)-ones through conjugate addition of isoquinolin-1(2H)-ones to p-quinone methides for the first time. The process is facilitated by Lewis acid catalysis and this operationally straightforward, mild, metal-free and one-pot transformation provides a wide range of C-4 alkylated isoquinolin-1(2H)-ones at ambient temperature in good to excellent yields.

Metal-Free Approach for Oxa-spirocyclohexadienones through [3 + 2]/[4 + 2] ipso-Cyclization of para-Quinone Methides with Halo Alcohols

A facile one-pot metal-free, base-mediated formal [3 + 2] and [4 + 2] dearomative ipso-cycloaddition of para-quinone methides (p-QMs) with halo alcohols has been designed for the efficient construction of 2-oxa-spirocyclohexadienones in excellent yield under mild reaction conditions. The commercial availability of the bases, reagents, and convenient reaction procedure makes it an attractive method for ipso-cyclization.

Sydnone: Synthesis, Reactivity and Biological Activities

Sydnones are among the most well-known mesoionic compounds. Since their synthesis in 1935 by Earl and Mecknay, numerous researches have shown that the chemical behavior, physical and biological properties of sydnones make them the most useful compounds in organic chemistry. Sydnones undergo thermal 1,3-dipolar cycloaddition reaction with dipolarophiles (alkynes or alkenes) to give exclusively derivatives containing a pyrazole moiety exhibiting numerous applications, such as pharmaceuticals and agrochemicals. However, the sydnone cycloaddition reaction with alkynes requires harsh conditions, like high temperatures and long reaction times, giving poor regioselectivity to the resulting products. To overcome these constraints, new reactions named CuSAC (Copper- Catalyzed Sydnone-Alkyne Cycloaddition) and SPSAC (Strain-Promoted Sydnone- Alkyne Cycloaddition) have been developed, leading to pyrazoles with interesting constant kinetics.

Metal-Free Reductive Coupling of para-Quinone Methides with 4-Cyanopyridines Enabled by Pyridine-Boryl Radicals: Access to Pyridylated Diarylmethanes with Anti-Cancer Activity

Reported herein is a streamlined protocol to produce pyridylated diarylmethanes through pyridine-boryl radical induced reductive coupling between para-quinone methides (p-QMs) and 4-cyanopyridines using bis(pinacolato)diboron (B₂ pin₂ ) as a templated reagent. The metal-free process is characterized by an operationally simple approach, excellent chemoselectivity (1,2- vs. 1,6-selectivity), and a broad substrate scope with good functional group compatibility. The mechanistic studies provided important insights into the reductive cross-coupling process between diarylmethyl radical and pyridine-boryl radical. Moreover, part of the obtained pyridylated diarylmethane products were screened against a panel of cancer cell lines, and 3 v was confirmed to significantly inhibit the proliferation of head and neck squamous cell carcinoma (HNSCC) cells. This method offers a platform for the preparation of new lead compounds with antitumor activity.

Selective sulfonylation and isonitrilation of para-quinone methides employing TosMIC as a source of sulfonyl group or isonitrile group

Chemoselective sulfonylation and isonitrilation reactions for the divergent synthesis of valuable diarylmethyl sulfones and isonitrile diarylmethanes starting from easy-to-synthesize para-quinone methides (p-QMs) and commercially abundant p-toluenesulfonylmethyl isocyanide (TosMIC) by using respectively zinc iodide and 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) as catalysts were developed. The distinguishing feature of this method is that TosMIC plays a dual role from the same substrates in the reaction: as a sulfonyl source or as an isonitrile source. The synthetic utility of this protocol was also demonstrated in the synthesis of difluoroalkylated diarylmethane 5 and diarylmethane ketone derivatives 6 and 7, which are important core structures in natural products and medicines.

Electrochemical Radical δ-H Sulfonylation Reaction for the Synthesis of 4-((Aryl,Arylsul fonyl)methylene)-2,5-Cyclohexadiene Derivatives

A novel and efficient electrochemical radical δ-H sulfonylation reaction of para-quinone methides (p-QMs) and sodium sulfinates has been achieved under common laboratory conditions. In this strategy, a new C(sp²)-S bond was constructed for the synthesis of 4-((aryl,arylsulfonyl)methylene)-2,5-cyclohexadiene derivatives with a broad substrate scope, good functional group tolerance, and mild conditions. Further studies showed that the reaction had an excellent regional selectivity.

Evaluation of the cellular protection by novel spiropyrazole compounds in dopaminergic cell death

The loss of neurons is strongly correlated with aging and aging-associated disorders. In this study, cell viability assays and mitochondrial function were performed to evaluate the effect of new spiro-pyrazole derivatives, prepared from aldehydes and 3-amino-1-phenyl-2-pyrazolin-5-one, on neuroprotection in an in vitro model of dopaminergic cell death induced by 1-methyl-4-phenylpyridinium (MPP⁺). The percentages of neuroprotection by derivatives were found between 21.26% and 52.67% at selected concentrations (10-50 μM) with compound 4d exerting the best neuroprotective effect. The results show that the studied spiropyrazolones perform important roles in dopaminergic neuroprotection and can be used for potential new therapies in the treatment of neurodegenerative disorders including Parkinson's disease.

A three-component, general and practical route for diastereoselective synthesis of aza-spirocyclic pyrazolones via a decarboxylative annulation process

An efficient, general, and practical route for highly diastereoselective synthesis of aza-spirocyclic pyrazolones from easily available α-amino acids, aldehydes, and alkylidene pyrazolones by means of a decarboxylative annulation process is reported.

Tandem isonitrile insertion/azacyclopropylidene-annulated cyclohexenone–tropone rearrangement of p-QMs and TosMIC: de novo synthesis of pyrrolotropones with anti-cancer activity

TosMIC causes ring expansion of p-QMs: highly substituted pyrrolotropones are rapidly assembled in a domino process mediated by DBU. The utility of the method was highlighted by follow-up transformation and biological activity identification.

Microwave-Assisted Copper Catalysis of α-Difluorinated gem-Diol toward Difluoroalkyl Radical for Hydrodifluoroalkylation of para-Quinone Methides

Reported herein is a unified strategy to generate difluoroalkyl radicals from readily prepared α-difluorinated gem-diols by single electron oxidation. Under microwave irradiation, a catalytic amount of oxidant Cu(OAc)₂ succeeds in the formation of transient difluoroalkyl radicals in situ, for the first time. The reaction features a simple protocol, short reaction time, scalability, and high yield. The synthetic utility of this new methodology was also explored for the synthesis of difluoroalkylated spiro-cyclohexadienones, which is an important core structure in natural products and pharmaceuticals.

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.

Diastereoselective synthesis of spiro-cyclopropanyl-cyclohexadienones via direct sulfide-catalyzed [2 + 1] annulation of para-quinone methides with bromides

A sulfide-catalyzed [2 + 1] annulation of para-quinone methides with diverse bromides was disclosed to access appealing spiro-cyclopropanyl-cyclohexadienones with high diastereoselectivity.

V. K, Hari K, Kaliyamoorthy A. Base-promoted 1,6-conjugate addition of alkylazaarenes to para-quinone methides

1,1,2-Triarylethanes embedded with an azaarene unit were prepared in a single step at ambient temperature via the sodium hexamethyldisilazide mediated 1,6-conjugate addition of unactivated alkylazaarenes on para-quinone methides (p-QMs).

Recent developments in 1,6-addition reactions of para-quinone methides (p-QMs)

In this review, we provide a comprehensive overview of recent progress in this rapidly growing field by summarizing the 1,6-conjugate addition and annulation reactions of p-QMs with consideration of their mechanisms and applications.

1,6-Conjugate addition initiated formal [4+2] annulation of p-quinone methides with sulfonyl allenols: a unique access to spiro[5.5]undeca-1,4-dien-3-one scaffolds

An expedient one-pot synthesis of carbocyclic spiro[5.5]undeca-1,4-dien-3-ones via 1,6-conjugate addition initiated formal [4+2] annulation sequences by employing p-quinone methides and sulfonyl allenols.