Mannich base-connected syntheses mediated by ortho-quinone methides

Ortho-quinone methide driven synthesis of kynurenic acid lactams

Lactam formation of different KYNA amides and Mannich bases mediated by ortho-quinone methide has been investigated. The efficiency of the two routes of the cyclization process was revealed and the influence of diverse amide side chains was explored. In this regard compounds bearing a tertiary amine function in the amide side chain resulted in the formation of the lactam product, while the formation of dimer derivatives was observed in the case of other KYNA amides. Furthermore, derivatives bearing different substituents on the KYNA B ring were synthesized and their effects on the ring-closure reaction were investigated.

The Versatile and Strategic O-Carbamate Directed Metalation Group in the Synthesis of Aromatic Molecules: An Update

The aryl O-carbamate (ArOAm) group is among the strongest of the directed metalation groups (DMGs) in directed ortho metalation (DoM) chemistry, especially in the form Ar-OCONEt2. Since the last comprehensive review of metalation chemistry involving ArOAms (published more than 30 years ago), the field has expanded significantly. For example, it now encompasses new substrates, solvent systems, and metalating agents, while conditions have been developed enabling metalation of ArOAm to be conducted in a green and sustainable manner. The ArOAm group has also proven to be effective in the anionic ortho-Fries (AoF) rearrangement, Directed remote metalation (DreM), iterative DoM sequences, and DoM-halogen dance (HalD) synthetic strategies and has been transformed into a diverse range of functionalities and coupled with various groups through a range of cross-coupling (CC) strategies. Of ultimate value, the ArOAm group has demonstrated utility in the synthesis of a diverse range of bioactive and polycyclic aromatic compounds for various applications.

Alkoxyalkylation of Electron-Rich Aromatic Compounds

Alkoxyalkylation and hydroxyalkylation methods utilizing oxo-compound derivatives such as aldehydes, acetals or acetylenes and various alcohols or water are widely used tools in preparative organic chemistry to synthesize bioactive compounds, biosensors, supramolecular compounds and petrochemicals. The syntheses of such molecules of broad relevance are facilitated by acid, base or heterogenous catalysis. However, degradation of the N-analogous Mannich bases are reported to yield alkoxyalkyl derivatives via the retro-Mannich reaction. The mutual derivative of all mentioned species are quinone methides, which are reported to form under both alkoxy- and aminoalkylative conditions and via the degradation of the Mannich-products. The aim of this review is to summarize the alkoxyalkylation (most commonly alkoxymethylation) of electron-rich arenes sorted by the methods of alkoxyalkylation (direct or via retro-Mannich reaction) and the substrate arenes, such as phenolic and derived carbocycles, heterocycles and the widely examined indole derivatives.

Synthesis of Indole-Coupled KYNA Derivatives via C–N Bond Cleavage of Mannich Bases

KYNAs, a compound with endogenous neuroprotective functions and an indole that is a building block of many biologically active compounds, such as a variety of neurotransmitters, are reacted in a transformation building upon Mannich bases. The reaction yields triarylmethane derivatives containing two biologically potent skeletons, and it may contribute to the synthesis of new, specialised neuroprotective compounds. The synthesis has been investigated via two procedures and the results were compared to those of previous studies. A possible alternative reaction route through acid catalysis has been established.

Regioselective synthesis, physicochemical properties and anticancer activity of 2-aminomethylated estrone derivatives

The unique estrogen receptor (ER)-independent antiproliferative and apoptotic activity of 2-methoxyestradiol (2ME2) is well known, however, its use has been limited because of its poor oral bioavailability. In this study, novel 2-aminomethylated estrone (E) and estradiol (E2) derivatives structurally related to 2ME2 were synthesized, and their physicochemical properties as well as their in vitro cytotoxic effects were investigated in the hope of finding more selective antiproliferative agents with improved pharmacokinetic profile. The target compounds were synthesized from 2-dimethylaminomethylated E obtained regioselectively by a three-component Mannich reaction. Quaternization with methyl iodide followed by reacting the ammonium salt with various dialkyl and alicyclic secondary amines afforded the desired products in good yields. The reactions proceeded via a 1,4-nucleophilic addition of the applied secondary amines to the ortho-quinone methide (o-QM) intermediates, generated in situ from the salt by base-promoted β-elimination. The compound library has been enlarged with structurally similar E2 analogues obtained by stereoselective reduction and with some 17β-benzylamino derivatives prepared by reductive amination. The potential values of the novel E and E2 derivatives were characterised by means of three different approaches. At the first step compounds were virtually screened using physicochemical parameters. Physicochemical characterization was completed by kinetic solubility and in vitro intestinal-specific permeability measurement. Antiproliferative effects were additionally determined on a panel of malignant and non-cancerous cell lines. The evaluation of the pharmacological profile of the novel E and E2 derivatives was completed with the calculation of lipophilic efficacy (LiPE).

Multicomponent Reactions (MCRs) with o-Quinone Methides

This article presents a comprehensive overview of multicomponent reactions (MCRs) that proceed via ortho-quinone methide intermediates (o-QM) generated in the reaction medium. Examples of applications involving these highly reactive intermediates in organic synthesis and biological processes (e. g., biosynthetic pathways, prodrug cleavage and electrophilic capture of biological nucleophiles) are also described. QMs are often generated by eliminative processes of phenol derivatives or by photochemical reactions, including reversible generation in photochromic substances. This class of compounds can undergo various reaction types, including nucleophilic attack at the methide carbon, with subsequent rearomatization, and react with electron-rich dienophiles in inverse-electron demand hetero-Diels-Alder reactions. Its versatile reactivity has been explored in the context of cascade reactions for the construction of several classes of substances, including complex natural products.

Hetero-Diels-Alder Reactions of Quinone Methides: The Origin of the α-Regioselectivity of 3-Methylene-1,2,4-naphthotriones

The regioselective formation of α- and β-lapachone via hetero-Diels-Alder reactions was investigated by experimental and computational approaches. The experimentally observed α-selectivity was explored in detail, revealing that the lower energy barrier for the formation of α-lapachone is a result of lower Pauli repulsion throughout the reaction path, when compared to the β-isomer. By comparing equivalent points on both α- and β-lapachone potential energy surfaces (PES), according to the activation strain model (ASM) and energy decomposition analysis (EDA), we were able to demonstrate that the Pauli repulsion term increases more significantly when going from reactants to TS_β than to TS_α, resulting in lower interaction energy in the early stages of the reaction path and in a later transition state for β-lapachone. Moreover, we confirmed that regio- and diastereoselectivity trends previously reported for other quinone methide intermediates are also observed for 3-methylene-1,2,4-naphthotriones, such as small endo/exo diastereoselectivity, as well as pronounced ortho/meta regioselectivity for reactions performed with dienophile containing electron-releasing groups. The results presented here provide a deeper understanding of the reactivity of quinone methide derivatives, aiding the future rational design of the reaction condition, structural modification of possible quinone methide intermediates, and the development of more selective synthetic routes for quinone derivatives.

Synthesis and Conformational Analysis of Naphthoxazine-Fused Phenanthrene Derivatives

The synthesis of new phenanthr[9,10-e][1,3]oxazines was achieved by the direct coupling of 9-phenanthrol with cyclic imines in the modified aza-Friedel-Crafts reaction followed by the ring closure of the resulting bifunctional aminophenanthrols with formaldehyde. Aminophenanthrol-type Mannich bases were synthesised and transformed to phenanthr[9,10-e][1,3]oxazines via [4 + 2] cycloaddition. Detailed NMR structural analyses of the new polyheterocycles as well as conformational studies including Density Functional Theory (DFT) modelling were performed. The relative stability of ortho-quinone methides (o-QMs) was calculated, the geometries obtained were compared with the experimentally determined NMR structures, and thereby, the regioselectivity of the reactions has been assigned.

Synthesis and transformation of 6-aminomethyl derivatives of 7-hydroxy-2'-fluoroisoflavones

Mannich aminomethylation of 8-methyl-7-hydroxy-2'-fluoroisoflavones applying bis-(N,N-dimethylamino)methane afforded C-6 substituted N,N-dimethylaminomethyl derivatives. Subsequent acetylation of these compounds in acetic anhydride in the presence of potassium acetate provided access to the corresponding acetoxymethyl derivatives that were converted to hydroxymethyl- and alkoxymethyl-substituted 7-hydroxyisoflavonoids. Addition of 3-(N,N-dimethylamino)-5,5-dimethyl-2-cyclohexen-1-one or 1,3-dimethyl-5-aminopyrazole with thermally generated ortho-quinone methides led to hetero-Diels–Alder or Michael adducts.

Chemoselective acid-catalyzed [4 + 2]-cycloaddition reactions of ortho-quinone methides and styrenes/stilbenes/cinnamates

Chemoselective [4 + 2]-cycloaddition reactions between o-QMs and different olefins—styrenes, stilbenes, and cinnamates—yielded distinct cycloadducts in moderate to good yields.

Adipic Acid as a Biodegradable Solid Acid Catalyst for One-Pot, Three Component Synthesis of 1-Amidoalkyl-2-naphthols

Background:

1-Amidoalkyl-2-naphthols are an attractive group of organic compounds that can be converted to oxazine heterocycles and aminoalkyl naphthols. The derivatives of 1- amidoalkyl-2-naphthols have significant biological activities and act as drug candidates.

Methods:

1-Amidoalkyl-2-naphthols were synthesized via the three-component condensation reaction of 2-naphthol, acetamide/benzamide and various aldehydes in the presence of 10 mol% of adipic acid as an organocatalyst under solvent-free conditions at 120°C.

Results:

A simple, efficient, and operative method for the synthesis of 1-amidoalkyl-2-naphtholes in the presence of adipic acid as the biodegradable catalyst is introduced. Easy operation, acceptable reaction times, eco-friendly, availability of starting materials, simple separation of products, and high yields of products are the significant results of this method.

Conclusion:

In this study, 1-amidoalkyl-2-naphthols were synthesized using commercially available reactants in excellent yields and relatively shorter times. In this process, microwave or ultrasonic waves were not used to provide energy for the reaction.

Ortho-Quinone Methide Driven Synthesis of New O,N- or N,N-Heterocycles

To synthesize functionalized Mannich bases that can serve two different types of ortho-quinone methide (o-QM) intermediates, 2-naphthol and 6-hydroxyquinoline were reacted with salicylic aldehyde in the presence of morpholine. The Mannich bases that can form o-QM and aza-o-QM were also synthesized by mixing 2-naphthol, 2-nitrobenzaldehyde, and morpholine followed by reduction of the nitro group. The highly functionalized aminonaphthol derivatives were then tested in [4+2] cycloaddition with different cyclic imines. The reaction proved to be both regio- and diastereoselective. In all cases, only one reaction product was obtained. Detailed structural analyses of the new polyheterocycles as well as conformational studies including DFT modelling were performed. The relative stability of o-QMs/aza-o-QM were also calculated, and the regioselectivity of the reactions could be explained only when the cycloaddition started from aminodiol 4. It was summarized that starting from diaminonaphthol 25, the regioselectivity of the reaction is driven by the higher nucleophilicity of the amino group compared with the hydroxy group. 12H-benzo[a]xanthen-12-one (11), formed via o-QM formation, was isolated as a side product. The proton NMR spectrum of 11 proved to be very unique from NMR point of view. The reason for the extreme low-field position of proton H-1 could be accounted for by theoretical calculation of structure and spatial magnetic properties of the compound in combination of ring current effects of the aromatic moieties and steric compression within the heavily hindered H(1)-C(1)-C(12b)-C(12a)-C(12)=O structural fragment.

A visible light photoredox catalyzed carbon radical-mediated generation of ortho-quinone methides for 2,3-dihydrobenzofuran synthesis

A visible light photoredox-catalyzed carbon radical-mediated strategy for in situ formation of ortho-quinone methides from 2-vinyl phenols towards 2,3-dihydrobenzofuran synthesis is described.