Antiaromaticity of Cycloheptatrienyl Anions: Structure, Acidity, and Magnetic Properties

Investigations of the nature and degree of antiaromaticity of cycloheptatrienyl anion derivatives using both experimental and computational tools are presented. The ground state of cycloheptatrienyl anion in the gas phase is triplet, planar and Baird-aromatic. In DMSO, it assumes a singlet distorted allylic form with a paratropic ring current. The other derivatives in both phases assume either allylic or diallylic conformations depending on the substituent pattern. A combination of experimental and computational methods was used to determine the pKa values of 16 derivatives in DMSO, which ranged from 36 to -10.7. We revealed that the stronger stabilization of the anionic system, which correlates with acidity, does not necessarily imply a lower degree of anti-aromaticity in terms of magnetic properties. Conversely, the substitution pattern first affects the geometry of the ring through the bulkiness of the substituents and their better conjugation with a more distorted system. Consequently, the distortion reduces the cyclic conjugation in the π-system and thereby decreases the paratropic current in a magnetic field, which manifests itself as a decrease in the NICS. The triplet-state geometries and magnetic properties are nearly independent on the substitution pattern, which is typical for simple aromatic systems.

Three-Component Synthesis of Substituted Perhydropyrans from β-Styrylmalonates, Aldehydes, and Alkoxyaluminum Dichlorides

A three-component synthesis of substituted dimethyl dihydro-2H-pyran-3,3(4H)-dicarboxylates in up to 80% yields by the reaction of β-styrylmalonates with aromatic or aliphatic aldehydes in the presence of ROAlCl2 prepared in advance either by exposure of EtAlCl2 with air access or by mixing equimolar amounts of AlCl3 with a primary or secondary alcohol has been developed. If EtAlCl2, itself, is used, dihydro-2H-pyran-3,3(4H)-diesters are not formed at all, while dimerization of styrylmalonates by (4 + 2)-annulation-type to give substituted tetrahydronaphthalenes is the main process. The possibility of using the CH-O-Al fragment of alkoxyaluminum dichlorides in cycloaddition reactions with α-CH-functionalization has been shown for the first time.

Boronyl borinic esters: preparation as B2pin2/ sec BuLi/TFAA adducts, structural insights and reactivity in Pd-catalyzed allylic borylation

Novel boronyl borinic ester I was generated by quenching the B2pin2/ sec BuLi-ate complex with trifluoroacetic acid anhydride (TFAA) via ring-opening in the 1,3,2-dioxaborolane moiety on ate-boron. Detailed NMR studies of the B2pin2/ sec BuLi-ate complex in solution and in solid state allowed us to assume its oligomeric nature in solids with only ate-boron involved in the oligomerization process.  The O-trifluoroacetyl pinacolate residue on borinic ester I initially formed on quenching with TFAA undergoes an unusual intramolecular transesterification with the carbonyl group of trifluoroacetyl forming othroester moiety in a few hours at r.t. to give boronyl borinic ester II. A solution of these reagents I/II was proved to be efficient for borylation of (2-fluoroallyl)pyridinium salts that are highly base sensitive.

Synthesis of the Indole-Based Inhibitors of Bacterial Cystathionine γ-Lyase NL1-NL3

Bacterial cystathionine γ-lyase (bCSE) is the main producer of H₂S in pathogenic bacteria such as Staphylococcus aureus, Pseudomonas aeruginosa, etc. The suppression of bCSE activity considerably enhances the sensitivity of bacteria to antibiotics. Convenient methods for the efficient synthesis of gram quantities of two selective indole-based bCSE inhibitors, namely (2-(6-bromo-1H-indol-1-yl)acetyl)glycine (NL1), 5-((6-bromo-1H-indol-1-yl)methyl)- 2-methylfuran-3-carboxylic acid (NL2), as well as a synthetic method for preparation 3-((6-(7-chlorobenzo[b]thiophen-2-yl)-1H-indol-1-yl)methyl)- 1H-pyrazole-5-carboxylic acid (NL3), have been developed. The syntheses are based on the use of 6-bromoindole as the main building block for all three inhibitors (NL1, NL2, and NL3), and the designed residues are assembled at the nitrogen atom of the 6-bromoindole core or by the substitution of the bromine atom in the case of NL3 using Pd-catalyzed cross-coupling. The developed and refined synthetic methods would be significant for the further biological screening of NL-series bCSE inhibitors and their derivatives.

Cascade Vinylation/8π-Electrocyclization and Cu(II)-Catalyzed Dehydrogenation toward Highly Stable Formally Antiaromatic Cycloheptatrienyl Anions

An approach to the synthesis of seven-membered systems via the chain elongation of nucleophilic propenes and subsequent 8π-electrocyclization is proposed. The cascade reaction yields either cycloheptadienes or bicycloheptenes, and the latter are formed via a 6π-electrocyclization of intermediate cycloheptadienyl anion which was proved to be reversible in a basic medium. The electrocyclic nature of the ring-closing reactions was supported by density functional theory and DLPNO/CCSD(T) calculations. Highly electron-deficient cycloheptatrienes can be obtained from cycloheptadienes or bicycloheptenes via oxidation either introduced into the cascade reaction or performed as a separate reaction, with the overall yield of up to 81%. The oxidation step was performed by means of a rarely encountered Cu(II)-catalyzed dehydrogenation of cycloheptadienes or bicycloheptenes, and so the reaction mechanism was proposed. Stable formally 8π-antiaromatic cycloheptatrienyl-anion containing compounds were obtained, and some correlations between their UV-vis spectra and the structure of the distorted cycloheptatrienyl-anion moiety were clarified. Additionally, a base-induced retro-[2 + 2]-cycloaddition in a bicycloheptene derivative gave cyanotetra(methoxycarbonyl)cyclopentadienyl cesium.

Lewis Acid-Catalyzed Formal (4+2)-Cycloaddition between Cross-Conjugated Azatrienes and Styrylmalonates: The Way to Functionalized Quinolizidine Precursors

Quinolizidine and azaphenalene alkaloids are common in nature and exhibit a pharmaceutical activity, which stirs up increased interest in expanding the range of methods for the synthesis of the corresponding derivatives. In this work, we attempted to adapt our previously presented method for the synthesis of tetrahydropyridines to the preparation of potential precursors for these heterocycles as a separate development of a necessary intermediate stage. To this end, we studied the reactions of β-styrylmalonates with N-protected cross-conjugated azatrienes in the presence of Sn(OTf)₂. Moreover, the regioselectivity of the process involving unsymmetrically substituted azatrienes was estimated. The diene character of vinyltetrahydropyridines was studied in detail with the participation of PTAD. Finally, for the Ts-protected highly functionalized vinyltetrahydropyridines synthesized, a detosylation method to give new desired azadiene structures as precursors of the quinolizidine core was suggested.

Coupling of Styrylmalonates with Furan and Benzofuran Carbaldehydes: Synthesis and Chemistry of Substituted (4-Oxocyclopent-2-enyl)malonates

On the basis of the reaction of β-styrylmalonates with furfural derivatives in the presence of GaCl₃ that occurs with opening of the furan ring, an efficient diastereoselective method for the synthesis of trisubstituted cyclopentenones containing a 1,4-diketone moiety was developed. The regularities of the reaction were studied, and a number of chemical reactions of the resulting substrates were carried out. A mechanism for the formation of substituted (4-oxo-2-arylcyclopent-2-enyl)malonates was suggested.

(2-Fluoroallyl)pyridinium tetrafluoroborates: novel fluorinated electrophiles for Pd-catalyzed allylic substitution

An efficient two-step approach to 2-fluoroallyl amines was developed that involves the synthesis of (2-fluoroallyl)pyridinium tetrafluoroborates from readily available gem-bromofluorocyclopropanes and the application of the former as novel and stable 2-fluoroallyl electrophiles for Pd-catalyzed allylic substitution.

GaCl3-Mediated Cascade [2 + 4]-Cycloaddition/[4 + 2]-Annulation of Donor-Acceptor Cyclopropanes with Conjugated Dienes: Strategy for the Construction of Benzobicyclo[3.3.1]nonane Skeleton

Structurally important benzobicyclo[3.3.1]nonane derivatives were synthesized by a gallium trichloride mediated reaction of readily available donor-acceptor cyclopropanes (DACs) with 1,3-dienes as a one-pot cascade ionic [2 + 4]-cycloaddition/Friedel-Crafts-type cyclization process. At the first stage, DACs act as sources of formal gallium 1,2-zwitterionic complexes to give 6-benzylcyclohex-3-ene-1,1-dicarboxylates that are converted under certain conditions in the presence of GaCl₃ to give benzobicyclo[3.3.1]nonanes in 30-74% yields. The process is tolerant of varying substituents at different positions of the main framework. Further, potentially useful modifications of benzobicyclo[3.3.1]nonane derivatives are demonstrated. 2-Cyclopropylbutadiene reacts with DAC at higher temperature more deeply with cleavage of three-membered rings in both cyclopropane substrates, and twofold alkylation of the Ph-substituent at ortho- and meta-positions, that leads to a 1,2,7,8,9,10-hexahydro-6H-7,10a-methanocycloocta[cd]indene skeleton. Cyclopropane-1,1-diesters containing a bulky substituent in the ester group react with isoprene in a different way to give bicyclic lactones containing a 2-oxabicyclo[2.2.2]octan-3-one moiety.

"Cyclopropanation of Cyclopropanes": GaCl3-Mediated Ionic Cyclopropanation of Donor-Acceptor Cyclopropanes with Diazo Esters as a Route to Tetrasubstituted Activated Cyclopropanes

A new ionic cyclopropanation process involving the addition of diazo esters to donor-acceptor cyclopropanes (DAC) activated by GaCl₃ has been developed. The reactions occur via 1,2-zwitterionic gallium complexes with elimination of nitrogen in all cases to give 1,1,2,3-tetrasubstituted cyclopropanes as the main products. Also, a number of related processes with the formation of various polysubstituted cyclopropanes, alkenes, and cyclobutanes, including products of multiple diazo ester addition, have been developed. Obtained by the developed method tetrasubstituted cyclopropanes are activated cyclopropanes such as DAC and can be used for further synthesis in this capacity. Their new reaction with benzaldehyde promoted by TiCl₄ and involving one of the additional functional groups has been demonstrated, which leads to five-membered lactones. The mechanisms of the occurring processes, as well as the structures and stereochemistry of a rich range of products formed, are discussed in detail.

Reactions of Styrylmalonates with Aromatic Aldehydes: Detailed Synthetic and Mechanistic Studies

Reactions of styrylmalonates with aromatic aldehydes in the presence of Lewis and Brønsted acids and their regularities have been studied in detail. Approaches to the synthesis of various polysubstituted 5,6-dihydropyran-2-ones, indenes, aryl-containing dienes and trienes, cyclopentadienes, and polycyclic lactones have been developed with chemo-, regio-, stereo-, and diastereoselectivity control. The mechanisms of these reactions were studied in depth using multinuclear NMR experiments, monitoring the reactions in the NMR tube, and ¹⁸O isotope labeling.

Recent advances in the catalytic cyclopropanation of unsaturated compounds with diazomethane

The main achievements and development trends of the past 10–15 years related to the catalytic cyclopropanation of unsaturated compounds with diazomethane are integrated and analyzed. The attention is focused on the most efficient catalysts based on palladium compounds. Data on the effects of substrate structure and nature of catalyst components on the regio- and stereoselectivity of these reactions are systematized. Characteristic features of safe methods for diazomethane generation are considered, including the use of membrane technologies and continuous-flow and in situ preparation methods, which have prospects for industrial application.

The bibliography includes 281 references.

Synthesis of Substituted β-Styrylmalonates by Sequential Isomerization of 2-Arylcyclopropane-1,1-dicarboxylates and (2-Arylethylidene)malonates

A method has been developed for the synthesis of substituted β-styrylmalonates by conversion of 2-arylcyclopropane-1,1-dicarboxylates (ACDCs) in the presence of gallium trichloride into the corresponding­ 1,2-zwitterionic intermediates or (2-arylethyl­idene)malonates, followed by treatment with pyridine at room temperature leading to an isomerization of the emerging double bond. This method allows one to expand these reactions to include ACDCs with acceptor substituents at the aromatic ring.

Donor-Acceptor Bicyclopropyls as 1,6-Zwitterionic Intermediates: Synthesis and Reactions with 4-Phenyl-1,2,4-triazoline-3,5-dione and Terminal Acetylenes

The bicyclopropyl system activated by incorporation of donor and acceptor groups in the presence of Lewis acids was used as a synthetic equivalent of 1,6-zwitterions. Opening of both cyclopropane rings in 2'-aryl-1,1'-bicyclopropyl-2,2-dicarboxylates (D-A bicyclopropyl, ABCDs) in the presence of GaI₃ + Bu₄N⁺GaI₄^- results in 5-iodo-5-arylpent-2-enylmalonates as products of HI formal 1,6-addition to the bicyclopropyl system. The use of GaCl₃ or GaBr₃ as a Lewis acid and terminal aryl or alkyl acetylenes as 1,6-zwitterion interceptors allows the alkyl substituent to be grown to give the corresponding acyclic 7-chloro(bromo)-hepta-2,6-dienylmalonates. The reaction of ABCDs with 4-phenyl-1,2,4-triazoline-3,5-dione (PTAD) catalyzed by Yb(OTf)₃ also results in the opening of both cyclopropane rings. The reaction products are tetrahydropyridazine derivatives - (7,9-dioxo-1,6,8-triazabicyclo[4.3.0]non-3-en-2-ylmethyl)malonates - containing one more PTAD moiety in the malonyl group.

Cascade Cleavage of Three-Membered Rings in the Reaction of D-A Cyclopropanes with 4,5-Diazaspiro[2.4]hept-4-enes: A Route to Highly Functionalized Pyrazolines

A new cascade process for reactions of donor-acceptor cyclopropanes (DACs) with spiro[cyclopropanepyrazolines] in the presence of EtAlCl₂ or Ga halides is reported. The action of a Lewis acid results in DAC activation and addition of the carbocationic intermediate to the azocyclopropane system of the pyrazoline with opening of the second three-membered ring and addition of a halide anion from the Lewis acid. A specific feature of this process is that one activated cyclopropane ring activates another one, and depending on the component ratio, the process can involve two DAC molecules and one pyrazoline molecule or one DAC molecule and two pyrazoline molecules. The process is tolerant to various functional groups and occurs with a wide range of substrates to give polyfunctionalized structures based on a 2-pyrazoline moiety.

Branching tryptamines as a tool to tune their antiproliferative activity

The influence of a series of tryptamine derivatives on the viability of normal (HEK293) and tumor (HepG2, Jurkat and SH-SY5Y) cells has been evaluated. All tryptamines tested were three different substitution types: C- and N-branching, and indole benzylation. All the derivations enhance the activity of compounds separately, although the effects of different substitutions were not additive. Thus, combinations of C- and N-branchings as well as C-branching and indole benzylation gave little or no increase in activity.

Synthesis of Branched Tryptamines via the Domino Cloke-Stevens/Grandberg Rearrangement

The rearrangement of cyclopropylketone arylhydrazones generated in situ from arylhydrazine hydrochlorides and ketones leads to formation of tryptamine derivatives. The use of (2-arylcyclopropyl)ethanones in the reactions with model 4-bromophenylhydrazine hydrochloride gives branched tryptamines with aryl groups in the α-position to the amino group, while (2-methylcyclopropyl)ethanone gives a mixture of α- and β-substituted products in a ratio of 1:3. The method was found effective in the synthesis of enantiomerically pure tryptamine. Thus, (R,R)-(2-phenylcyclopropyl)ethanone gives the (S)-α-phenyltryptamine derivative with an enantiomeric excess over 99%.