Aromatic Substitution Pattern Research Articles

Synthetic Placement of Active Sites in MFI Zeolites for Selective Toluene Methylation to para-Xylene.

Brønsted acidic zeolites are ubiquitous catalysts in fuel and chemical production. Broadening the catalytic diversity of a given zeolite requires strategies to manipulate the acid site placement at framework positions within distinct microporous locations. Here, we combine experiment and theory to elucidate how intermolecular interactions between organic structure-directing agents (OSDAs) and framework Al centers influence the placement of H+ sites in distinct void environments of MFI zeolites and demonstrate the catalytic consequences of active site location on kinetically controlled (403 K) toluene methylation to xylene regioisomers. Kinetic measurements, interpreted using mechanism-derived rate expressions and transition state theory, alongside density functional theory (DFT) calculations show that larger intersection environments similarly stabilize all three xylene isomer transition states without altering well-established aromatic substitution patterns (ortho/para/meta ∼ 60%:30%:10%), while smaller channel environments preferentially destabilize transition states that form bulkier ortho- and meta-isomers, thereby resulting in high intrinsic para-xylene selectivity (∼80%). DFT calculations reveal that the flexibility of nonconventional OSDAs (e.g., 1,4-diazabicyclo[2.2.2]octane) to reorient within MFI intersections and their ability to hydrogen-bond to form protonated complexes favor the placement of Al in smaller channel environments compared to conventional quaternary OSDAs (e.g., tetra-n-propylammonium). These molecular-level insights establish a mechanistic link between OSDA structure, active site placement, and transition state stability in MFI zeolites and provide active site design strategies that are orthogonal to crystallite design approaches harnessing complex reaction-diffusion phenomena to enhance regioisomer selectivity in the industrial production of valuable polymer precursors.

A Photochemical Strategy for the Conversion of Nitroarenes into Rigidified Pyrrolidine Analogues.

Bicyclic amines are important motifs for the preparation of bioactive materials. These species have well-defined exit vectors that enable accurate disposition of substituents toward specific areas of chemical space. Of all possible skeletons, the 2-azabicyclo[3.2.0]heptane framework is virtually absent from MedChem libraries due to a paucity of synthetic methods for its preparation. Here, we report a modular synthetic strategy that utilizes nitroarenes as flat and easy-to-functionalize feedstocks for the assembly of these sp3-rich materials. Mechanistically, this approach exploits two concomitant photochemical processes that sequentially ring-expand the nitroarene into an azepine and then fold it into a rigid bicycle pyrroline by means of singlet nitrene-mediated nitrogen insertion and excited-state-4π electrocyclization. A following hydrogenolysis provides, with full diastereocontrol, the desired bicyclic amine derivatives whereby the aromatic substitution pattern has been translated into the one of the three-dimensional heterocycle. These molecules can be considered rigid pyrrolidine analogues with a well-defined orientation of their substituents. Furthermore, unsupervised clustering of an expansive virtual database of saturated N-heterocycles revealed these derivatives as effective isosteres of rigidified piperidines. Overall, this platform enables the conversion of nitroarene feedstocks into complex sp3-rich heterocycles of potential interest to drug development.

Molecular Reconstruction with Stereochemical Relay: An Investigation into the Rearrangement of Spiro[4.5]decadienone to Benzoxepane.

Herein, we report the unusual skeletal rearrangement of spiro[4.5]decadienone to benzoxepane. In particular, Lewis acid-promoted epoxide-opening ipso-cyclization of aryl epoxides afforded spiro[4.5]decadienone intermediates. Subsequent thermal activation assembled a benzoxepane core via rearomative molecular reorganization. The sequence was high-yielding and highly diastereoselective but sensitive to the aromatic substitution pattern and the epoxide side chain. Mechanistic studies suggested that the rearrangement proceeded via an uncommon intramolecular enolate attack onto the electrophilic O of p-quinone oxonium zwitterion. DFT calculations helped rationalize the product distribution and the origin of diastereoselectivity. Initial investigation into the application of this chemical transformation is also presented.

Study on the Influence of Coal Structure and Oxidation Performance by Endogenous Bacterium

In order to solve the defects of traditional coal spontaneous combustion prevention technology in a closed goaf, a strain of aerobic endogenous bacteria was isolated from coal and used as a blocking raw material. Based on the metabolic and reproductive characteristics of microorganisms, the experimental study on the inhibition of coal spontaneous combustion by microorganisms was carried out. The colonies were isolated and purified by the dilution concentration plate method and the scribing plate method. The growth morphology of microorganisms was analyzed, and the growth curve was determined. The strains were identified by seamless cloning technology for high-throughput sequencing. The surface morphology of coal was analyzed by SEM, the differences of oxidation characteristic temperature points were analyzed by TG–DTG–DSC images, a programmed heating experiment was used to analyze the concentration of the indicator gas CO, and the changes in microscopic groups before and after microbial action were analyzed by FTIR and XPS spectra. Therefore, the inhibition of coal oxidation by endogenous bacteria was verified from macroscopic and microscopic perspectives. The results show that the coal bacteria isolated from the coal is Lysinibacilus sp. After the culture of Lysinibacilus sp., the surface of the coal demonstrated less detritus, and was relatively smooth. In the early stage of low temperature oxidation of coal spontaneous combustion, the characteristic temperature point of coal oxidation and the reaction between coal and O2 could be delayed by Lysinibacilus sp., and the total heat release was reduced in the combustion process. Not only that, Lysinibacilus sp. could also reduce the CO concentration during coal heating. After the coal was decomposed by Lysinibacilus sp., the C=C thick ring skeleton structure had little effect; however, the aromatic substitution pattern changed. This bacterium had an effect on the C-O bond, reducing the percentage of -CH2- and increasing the percentage of -CH3. It might also use the crystalline water in coal for life activities. The carboxyl carbon in coal changed the most, with a decrease of 12.03%, so it might become the carbon source required for microbial growth. The reproductive metabolism of microorganisms also affected the form of nitrogen, and the percentage of pyridine nitrogen in coal was reduced. The ratio of single-bond carbon to double-bond carbon in raw coal was about 3:2, but after this bacterial action, the ratio of the two was about 1:1. The analytical conclusions of XPS and FTIR spectra were consistent, and the results supported each other.

Effect of aromatic substituents on the outcome of base-induced rearrangements of 3-acetyl-2,4-dimethyl-3,4-dihydro-2H-1,2,3-benzothiadiazine 1,1-dioxides

In continuation of our studies on the remarkable rearrangements of 3-acetyl-7,8-dichloro-2,4-dimethyl-3,4-dihydro-2H-1,2,3-benzothiadiazine 1,1-dioxide leading to the corresponding 2,3-dihydro-1,2-benzisothiazole 1,1-dioxide or 3,4-dihydro-2H-1,2-benzothiazine 1,1-dioxide, depending on the amount of the base applied in the reaction, we have now investigated the influence of the aromatic substitution pattern on product distribution. It was found that the decreasing electron withdrawing ability of the substituents promoted the formation of enamide intermediates, potential precursors of both rearranged products. Density functional theory (DFT) has also been employed to understand the impact of the substitution pattern on product distribution.

Reduction of Substituted Benzo-Fused Cyclic Sulfonamides with Mg-MeOH: An Experimental and Computational Study.

A study involving the use of Mg-MeOH for the double reductive cleavage of both N–S and C–S bonds in a series of 11 benzo-fused cyclic sulfonamides is reported. Examples where the sulfonamide nitrogen atom is part of a pyrrolidine ring effectively undergo reduction, as long as a methoxy substituent is not para-positioned in the aromatic ring, relative to the sulfonyl group. In contrast, if the nitrogen atom is contained within an aromatic ring (pyrrole or indole), the presence of a para-methoxy substituent does not prohibit reduction. If deuterated methanol is used, aromatic ortho-deuterium incorporation was observed. To better understand how structure affects reactivity, density functional theory calculations were performed using three functionals. Results using CAM-B3LYP were found to best correlate with experimental observations, and these demonstrate the impact that the different aromatic substitution patterns and types of N-atom have on the lowest unoccupied molecular orbital (LUMO) energies and adiabatic electron affinities.

Reversible, Red-Shifted Photoisomerization in Protonated Azobenzenes.

Azobenzenes are among the best-studied molecular photoswitches and play a key role in the search for red-shifted photoresponsive materials for extended applications. Currently, most approaches deal with aromatic substitution patterns to achieve visible light application, on occasion paired with protonation to yield red-shifted absorption of the azonium species. Appropriate substitution patterns are essential to stabilize the latter approach, as conventional acids are known to induce a fast Z- to E-conversion. Here, we show that steady-state protonation of the azo-bridge instead is possible in simple azobenzenes when the pKa of the acid is low enough, yielding both the Z- and E-azonium as supported by UV–vis- and 1H NMR spectroscopy as well as density functional theory calculations. Moreover, the steady-state protonation of para-methoxyazobenzene, specifically, yields photoisomerizable azonium ions in which the direction of switching is essentially reversed, that is, visible light produces the out-of-equilibrium Z-azonium. Although the current conditions render the visible light photoswitch unsuitable for in vivo and material application, the demonstrated understanding of simple azobenzenes paves the way for a great range of further work on this already widely studied photoswitch.

Practical Synthesis of Variously Substituted 2,3,4-Benzothiadiazepine 2,2-Dioxides

AbstractThe significant pharmacological activity of 2,3-benzodiazepine-4-ones led to an increased interest in this compound family. However, the literature of the closely related 2,3,4-benzothiadiazepine 2,2-dioxides is rather scarce. Earlier we elaborated a synthesis of 5-aryl congeners variously substituted at the aromatic ring. In the present study, a new synthetic route was investigated via highly versatile intermediates, to extend the reaction to a wider aromatic substitution pattern and to the preparation of 5-alkyl and 5-H derivatives. The essence of this approach is, starting from benzaldehyde acetals, acetophenone and benzophenone ketals, to synthesize ortho-formyl- and ortho-acyl-arylmethanesulfonyl chlorides, which are suitable precursors of the target compounds. The synthesis was implemented as follows: ortho-lithiation of the corresponding acetals and ketals and subsequent treatment with paraformaldehyde, or alternatively in two steps, with DMF or ethyl formate, followed by reduction with NaBH4, led to the corresponding hydroxymethyl derivatives. Reaction of these latter with methanesulfonyl chloride gave mesylates that were used to S-alkylate thiourea to afford S-alkylisothiouronium salts. The final steps of the synthesis were carried out in a telescoped reaction. Treatment of the S-alkylisothiouronium salts with N-chlorosuccinimide resulted in the corresponding arylmethanesulfonyl chlorides. Subsequent reaction with hydrazine hydrate followed by an acidic treatment gave 2,3,4-benzothiadiazepine 2,2-dioxides.

Third Time is a Charm – Protonating Tricarboxybenzenes

AbstractTriprotonation of the three constitution isomers of tricarboxybenzene was accomplished. Furthermore, the preparation of selected mono‐ and dications showed the sequence of protonation steps. The cations were mostly isolated as [SbF6] and [AsF6] species, which were characterized by Raman and NMR spectroscopy as well as X‐ray structure determination. To further elucidate the experimental results, quantum chemical calculations are employed, especially in regard to charge distribution with NPA charges and aromaticity by NICS(0) values. Thermal decomposition of the compounds was investigated in order to explore the possibility of acyl cation formation. The influence of the aromatic system, substitution pattern and anomeric effect concerning the properties of the respective compounds was thus explained.

Surface Tuning of Wood via Covalent Modification of Its Lignocellulosic Biopolymers with Substituted Benzoates-A Study on Reactivity, Efficiency, and Durability.

Chemical modification of wood applying benzotriazolyl-activated carboxylic acids has proven to be a versatile method for the durable functionalization of its lignocellulosic biopolymers. Through this process, the material properties of wood can be influenced and specifically optimized. To check the scope and limitations of this modification method, various benzamide derivatives with electron-withdrawing (EWG) or electron-donating (EDG) functional groups in different positions of the aromatic ring were synthesized and applied for covalent modification of Scots pine (Pinus sylvestris L.) sapwood in this study. The bonded amounts of substances (up to 2.20 mmol) were compared with the reactivity constants of the Hammett equation, revealing a significant correlation between the modification efficiency and the theoretical reactivity constants of the corresponding aromatic substitution pattern. The successful covalent attachment of the respective substituted benzamides was proven by attenuated total reflection infrared (ATR-IR) spectroscopy, while the stability of the newly formed ester bond was proven in a standardized leaching test.

Functionalized Tris(anilido)triazacyclononanes as Hexadentate Ligands for the Encapsulation of U(III), U(IV) and La(III) Cations

Tripodal multidentate ligands have become increasingly popular in f-element chemistry for stabilizing unusual bonding motifs and supporting small molecule activation processes. The steric and electronic effects of ligand donor atom substituents have proved crucial in both of these applications. In this study we functionalized the previously reported tris-anilide ligand {tacn(SiMe2NPh)3} (tacn = 1,3,7-triazacyclononane) to incorporate substituted aromatic rings, with the aim of modifying f-element complex solubility and ligand steric effects. We report the synthesis of two proligands, {tacn(SiMe2NHAr)3} (Ar = C6H3Me2-3,5 or C6H4Me-4), and their respective group 1 transfer agents—{tacn(SiMe2NKAr)3}, M(III) complexes [M{tacn(SiMe2NAr)3}] for M = La and U, and U(IV) complexes [M{tacn(SiMe2NAr)3}(Cl)]. These compounds were characterized by multinuclear NMR and FTIR spectroscopy and elemental analysis. The paramagnetic uranium complexes were also characterized by solid state magnetic measurements and UV/Vis/NIR spectroscopy. U(III) complexes were additionally studied by EPR spectroscopy. The solid state structures of all f-block complexes were authenticated by single-crystal X-ray diffraction (XRD), together with a minor byproduct [U{tacn(SiMe2NC6H4Me-4)3}(I)]. Comparisons of the characterization data of our f-element complexes with similar literature examples containing the {tacn(SiMe2NPh)3} ligand set showed minor changes in physicochemical properties resulting from the different aromatic ring substitution patterns we investigated.

Imidazolium based gemini amphiphiles derived from L-valine. Structural elements and surfactant properties

Imidazolium amphiphiles often display excellent surfactants properties, while amide functionalities and chirality can implement self-assembly. The introduction of these functionalities in imidazolium gemini amphiphiles is expected to allow tuning their aggregation, achieving a higher emulsifying capacity of interest for practical applications. l-valine-derived gemini surfactants have been prepared and their aggregation investigated using NMR, UV-CD, fluorescence, LS or microscopy. The imidazolium gemini amphiphiles synthetized, presented excellent surfactant properties. The aromatic substitution pattern defines additional geometric constrains facilitating a fine tune of their properties, displaying the para-substituted compound best properties. This can be associated to facilitating the required conformational organization of polar and apolar fragments, while reducing coulombic repulsions between imizadolium rings. Two critical aggregation concentrations (CACs) were observed by fluorescence in aqueous media, with the formation of well-defined vesicles in the second step. Dynamic equilibria between aggregates of different sizes seem to be present. These gemini surfactants revealed of interest as emulsifiers for oil/water systems and for controlled fragrance delivery using (R)-limonene as a benchmark volatile compound.

GC–MS analysis of N-(bromodimethoxybenzyl)-2-, 3-, and 4-methoxyphenethylamines: Inverse analogues of the psychoactive 25B-NBOMe drug

The substituted phenethylamine analogues in this study are derivatives of N-(2-methoxybenzyl)-4-bromo-2,5-dimethoxyphenethylamine (25B-NBOMe) having the reverse substitution pattern for the two aromatic rings. These compounds contain only a single methoxy substituent in the phenethylamine part, and a bromodimethoxy substitution pattern in the benzyl portion. The regioisomers were prepared with variations in the substitution pattern in both the bromodimethoxy substitution pattern of the benzyl part as well as the methoxy substitution pattern in the phenethylamine part. The nine regioisomeric compounds were prepared from the regioisomeric 2-, 3-, and 4- methoxyphenethylamines via N-reductive alkylation with the three regioisomeric 2,4,5-substituted bromodimethoxybenzaldehydes.The EI-MS spectra of these regioisomers gave two major bromine containing ions at m/z 229/231 (base peak) and 258/260 and two non-brominated fragments at m/z 91 and 121. The relative abundance of the m/z 91 was higher for the 2-methoxyphenethylamine substituted isomers and the relative abundance of the m/z 121 fragment was higher in the 4-methoxy substituted isomers. The gas chromatographic separation for the regioisomeric methoxyphenethylamines of each bromodimethoxybenzyl aromatic ring substitution pattern showed the 2-methoxyphenethylamine substituted isomer eluted first followed by the 3-substituted isomer and the 4-methoxyphenethylamine substituted isomer was the last to elute and the three bromodimethoxybenzyl regioisomers for the 2-methoxyphenethylamine subseries were resolved as the trifluoroacetamides. The EI-MS for the TFA derivatives of the 2-bromo-4,5-dimethoxybenzyl substituted isomer gave unique fragment ions resulting from displacement of bromine from the molecular ion. This fragmentation pathway was confirmed using the model compounds N-(2-, 3- and 4-bromobenzyl)phenethylamine trifluoroacetamides.

Cyclic C–H⋯O motifs as general supramolecular synthon in aromatic sulfonester trimers

The synthesis, analytical characterisation and four X-ray structures of aromatic sulfonester trimers featuring a meta-substitution are reported. The title compounds feature either resorcinol or a 1,3-benzenedisulfonyl moiety as a central unit; one of the trimers exists in two polymorphic forms. Depending on the Caryl-SO2-O-Caryl torsion angles two main conformations have been detected: U- and W-shape. Only little influenced by the molecular conformation the prevailing non-covalent interaction in the packing of all four title structures are C–H⋯O contacts. Thereby, aromatic H atoms in ortho position to oxysulfonyl groups serve as hydrogen bonding donor and SO units as hydrogen bonding acceptors. In all cases the structures contain cyclic supramolecular synthons featurinR22(10)g either graph-R22(12)set descriptor for the resorcinol trimers and for benzenedisulfonic acid derivatives respectively. One of the two supramolecular synthons is also present in most published sulfontriester X-ray structures uninfluenced from the aromatic substitution pattern.

Structure fragmentation studies of ring-substituted N-trifluoroacetyl-N-benzylphenethylamines related to the NBOMe drugs.

The halogenated derivatives of N-(2-methoxy)benzyl-2,5-dimethoxyphenethylamine (25-NBOMe) such as the 4-bromo analogue (25B-NBOMe) represent a new class of hallucinogenic or psychedelic drugs. The purpose of this study was to determine the role of the electron-donating groups (halogen and dimethoxy) in the pathway of decomposition for the distonic molecular radical cation in the electron ionization mass spectrometry (EI-MS) process of the trifluoroacetamide (TFA) derivatives. The systematic removal of substituents from the 4-halogenated 2,5-dimethoxyphenethylamine portion of the N-dimethoxybenzyl NBOMe analogues allowed an evaluation of structural effects on the formation of major fragment ions in the EI-MS of the TFA derivatives. All six regioisomeric dimethoxybenzyl-substituted analogues (2,3-, 2,4-, 2,5-, 2,6-, 3,4- and 3,5-dimethoxy) for the four series of phenethyl aromatic ring substitution patterns were prepared, derivatized and analyzed via gas chromatography coupled with EI-MS. The analogues yield two unique radical cation fragments from the decomposition of the common distonic molecular radical cation. The substituted phenylethene radical cation (m/z 164) is the base peak or second most abundant ion in all six TFA-2,5-dimethoxyphenethylamine isomers. The dimethoxybenzyltrifloroacetamide radical cation (m/z 263) is the base peak or second most abundant ion in the 2- and 3-monomethoxyphenethylamine isomers. However, the 2- and 3-methoxyphenylethene radical cation (m/z 134) is among the five most abundant ions for each of these twelve isomers. Only one isomer in the phenethylamine series yields the corresponding unsubstituted phenylethene radical cation at m/z 104. The decomposition of the hydrogen-rearranged distonic molecular radical cation favors formation of the dimethoxybenzyltrifloroacetamide (m/z 263) species for the less electron-rich phenethyl aromatic rings. The addition of electron-donating groups to the aromatic ring of the phenethyl group as in the NBOMe-type molecules shifts the decomposition of the common distonic molecular radical cation to favor the formation of the electron-rich substituted phenylethene radical cation.

Subtle variations in the structure of crosslinked epoxy networks and the impact upon mechanical and thermal properties

ABSTRACTPresented here is an investigation of the structure–property relationships of crosslinked networks using three bi‐functional glycidyl ether aromatic epoxy resins, two bi‐aryl and one tri‐aryl, cured with bi‐ and tri‐aryl amines. Subtle changes to the monomer chemistry including changing aromatic substitution patterns from meta to para, methylene to isopropyl and isopropyl to ether were explored. Changing an epoxy resin backbone from methylene to isopropyl enhances backbone rigidity thus increasing glass transition temperature (Tg), yield strength, and strain despite reducing modulus. Changing meta‐substitution to para increases Tg and yield strain while leaving strength unaffected and reducing modulus. Changing isopropyl linkages to ether reduces modulus, strength, Tg, and yield strain reflecting increased molecular flexibility. Using three instead of two aromatic rings increases the molecular weight between crosslinks thereby decreasing Tg and yield strain while increasing modulus and strength. Despite the complexities of multiple systems for varying epoxy resins and amine hardeners, the effect upon network properties is explained in terms of short‐ and long‐range molecular and segmental mobility, crosslink density, and equilibrium packing density. © 2020 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2020, 137, 48874.

Visual discrimination of aromatic acid substitution patterns by supramolecular nanocooperativity

Physicochemical and, in particular, visual recognition of positional isomers, due to their similar appearance and properties, is an extremely challenging task. Here we present an easy-to-prepare assay for the naked-eye differentiation of all possible isomers of phthalic acids. The desired optical response is attained through specific non-covalent interactions between the acids and a cationic macrocyclic host. These interactions are then translated to and amplified by gold nanoparticles which subsequently aggregate to various extents producing a color palette.

Atomic-level engineering and imaging of polypeptoid crystal lattices

Rational design of supramolecular nanomaterials fundamentally depends upon an atomic-level understanding of their structure and how it responds to chemical modifications. Here we studied a series of crystalline diblock copolypeptoids by a combination of sequence-controlled synthesis, cryogenic transmission electron microscopy, and molecular dynamics simulation. This family of amphiphilic polypeptoids formed free-floating 2-dimensional monolayer nanosheets, in which individual polymer chains and their relative orientations could be directly observed. Furthermore, bromine atom side-chain substituents in nanosheets were directly visualized by cryogenic transmission electron microscopy, revealing atomic details in position space inaccessible by conventional scattering techniques. While the polypeptoid backbone conformation was conserved across the set of molecules, the nanosheets exhibited different lattice packing geometries dependent on the aromatic side chain para substitutions. Peptoids are inherently achiral, yet we showed that sequences containing an asymmetric aromatic substitution pattern pack with alternating rows adopting opposite backbone chiralities. These atomic-level insights into peptoid nanosheet crystal structure provide guidance for the future design of bioinspired nanomaterials with more precisely controlled structures and properties.

Colchicine-like β-acetamidoketones as inhibitors of microtubule polymerization: Design, synthesis and biological evaluation of in vitro anticancer activity.

Objective(s):In this study a series of novel colchicine-like β-acetamidoketones was designed and synthesized as potential tubulin inhibitorsMaterials and Methods:The cytotoxicity of the novel synthesized β-acetamidoketones was assessed against two cancerous cell lines including MCF-7 (human breast cancer cells) and A549 (adenocarcinomic human alveolar basal epithelial cells) employing the MTT test. Tubulin polymerization test was done by using a commercial kit (tubulin polymerization assay kit).Results:In general, the cytotoxicity activities were highly dependent on the aromatic substitution pattern of phenyl ring at β position of β-acetamidoketones. Based upon, compound 4f possessing the same structural elements of colchicine and chalcone 1, revealed the most cytotoxicity more than the other β-acetamidoketone against the cancerous cell lines and showed moderate antitubulin effect. The tubulin inhibitory effect of 4f, colchicine and chalcone 1 were consistent with their antiproliferative activities. Molecular docking studies of 4f, into the colchicine-binding site of tubulin exhibited possible mode of interaction between this compound and tubulin.Conclusion:The structure activity relationship (SAR) data attained showed that the presence of trimethoxy phenyl attached to carbonyl group of β-acetamidoketones and a methoxy group at para position of the other ring are essential for cytotoxic activity. In general, the cytotoxicity activities were highly dependent on the aromatic substitution pattern of phenyl ring at β position of β-acetamidoketones.

"Methylene Bridge" to 5-HT3 Receptor Antagonists: Conformationally Constrained Phenylguanidines.

Arylguanidines, depending upon their aromatic substitution pattern, display varying actions at 5-HT3 receptors (e.g., partial agonist, agonist, superagonist). Here, we demonstrate that conformational constraint of these agents as dihydroquinazolines (such as A6CDQ; 1) results in their conversion to 5-HT3 receptor antagonists. We examined the structure-activity relationships of 1. Replacement/removal of any of the guanidinium nitrogen atoms of 1 resulted in decreased affinity. All three nitrogen atoms of 1 are necessary for optimal binding affinity at 5-HT3 receptors. Introduction of substituents as small as an N2-methyl group abolishes affinity. The results are consistent with homology modeling/docking studies and binding data from site-directed mutagenesis studies. Introducing a "methylene bridge" to the arylguanidine structure, regardless of its functional activity, results in a 5-HT3 receptor antagonist.