30pi aromatic meso-substituted heptaphyrin isomers: syntheses,characterization, and spectroscopic studies

The syntheses of new aromatic 30pi heptaphyrins either through a [5 + 2] or a [4 + 3] acid-catalyzed condensation and oxidative coupling reactions of easily available and air-stable precursors are reported. The methodology followed is not only simple and efficient but also allows synthesis of a range of heptaphyrins with different heteroatoms in the core. The oxidative coupling reactions of modified tripyrranes 11 and tetrapyrranes 12 were found to be dependent on the acid concentration used and as well as the substituents present on the meso position. The change of meso aryl substituents in 11 and 12 to meso mesityl substituents gave a new heptaphyrin 18. The structural characterization has been done with extensive 1H and 2D NMR studies. The heptaphyrins reported here show rich structural diversity when the connections of the heterocyclic rings are altered, and accordingly, one ring and two ring inversions have been observed. By a judicious choice of the precursors it has been possible to control the site of ring inversion either in the bithiophene unit or in the tripyrrane unit. Theoretical calculations performed on three different heptaphyrins, 4, 5, and 17, also reveal that the inverted structures are approximately 35-40 kJ lower in energy relative to the corresponding noninverted structures. Furthermore, one of the heptaphyrins 10c shows the presence of two conformers in solution in the ratio 1:2 and no interconversion between the conformers have been observed in the temperature range of 343-228 K. On protonation, the aromaticity and the ring inversions are retained and the deltadelta values vary in the range 10.07-20.59 ppm. The energies of the Soret maxima and the HOMO-LUMO gap vary linearly with the increase in pi electrons further justifying the aromatic nature of the heptaphyrins.     

Conformational Change from a Twisted Figure‐Eight to an Open‐Extended Structure in Doubly Fused 36π Core‐Modified Octaphyrins Triggered by Protonation: Implication on Photodynamics and Aromaticity

Two examples of core‐modified 36π doubly fused octaphyrins that undergo a conformational change from a twisted figure‐eight to an open‐extended structure induced by protonation are reported. Syntheses of the two octaphyrins (in which Ar=mesityl or tolyl) were achieved by a simple acid‐catalyzed condensation of dipyrrane unit containing an electron‐rich, rigid dithienothiophene (DTT) core with pentafluorobenzaldehyde followed by oxidation with 2,3‐dichloro‐5,6‐dicyano‐1,4‐benzoquinone (DDQ). The single‐crystal X‐ray structure of the octaphyrin (in which Ar=mesityl) shows a figure‐eight twisted conformation of the expanded porphyrin skeleton with two DTT moieties oriented in a staggered conformation with a π‐cloud distance of 3.7 Å. Spectroscopic and quantum mechanical calculations reveal that both octaphyrins conform to a [4n]π nonaromatic electronic structure. Protonation of the pyrrole nitrogen atoms of the octaphyrins results in dramatic structural change, which led to 1) a large redshift and sharpening of absorption bands in electronic absorption spectrum, 2) a large change in chemical shift of pyrrole β‐CH and ? NH protons in the ¹H NMR spectrum, 3) a small increase in singlet lifetimes, and 4) a moderate increase in two‐photon absorption cross‐section values. Furthermore, nucleus‐independent chemical shift (NICS) values calculated at various geometrical positions show positive values and anisotropy‐induced current density (AICD) plots indicate paratropic ring‐currents for the diprotonated form of the octaphyrin (in which Ar=tolyl); the single‐crystal X‐ray structure of the diprotonated form of the octaphyrin shows an extended structure in which one of the pyrrole ring of each dipyrrin subunit undergoes a 180 ° ring‐flip. Four trifluoroacetic acid (TFA) molecules are bound above and below the molecular plane defined by meso‐carbon atoms and are held by N? H ??? O, N? H ??? F, and C? H ??? F intermolecular hydrogen‐bonding interactions. The extended‐open structure upon protonation allows π‐delocalization and the electronic structure conforms to a [4n]π Hückel antiaromatic in the diprotonated state.     

26pi aromatic core-modified hexaphyrins: syntheses, characterization, and structural diversities

Synthesis and characterization of several 26pi core-modified hexaphyrins are reported. The synthetic methodology involved a well-known acid-catalyzed MacDonald-type condensation reaction of the required tripyrrane with electron deficient pentafluorobenzaldehyde. The nature of the product and yield depends on the nature of the acid catalyst and its concentration. Dioxahexaphyrin 9 was isolated only when 0.5 equiv of TFA was used as a catalyst, while dithiahexaphyrin 10 and diselenahexaphyrin 11 were formed with TFA, PTSA, and even in the absence of catalyst. The detailed 1H and 2-D COSY as well as HSQC experiments reveal the solution structure as well as the conformational mobility of hexaphyrins. In the tetracationic state, 10 and 11 exhibit a four heterocyclic ring inverted structure, while only two completely inverted heterocyclic rings were observed for 9. The other four heterocyclic rings are only partially inverted in 9. All the hexaphyrins reported here show aromatic character inferred from large Deltadelta values (difference in chemical shift between the most shielded and the most deshielded protons). Electronic absorption spectral studies also support the conformational changes observed upon protonation.     

Polyfuryl(Aryl)alkanes and their derivatives. 6. Stereochemistry of intramolecular rotations about the C-C+ bonds in aryl(Alkyl)difurylcarbonium ions

It was demonstrated by dynamic ¹H NMR spectroscopy that for difurylmethyl and gem-difurylethyl cations conformational changes are realized through rotation of one of the furan rings, whereas in the case of aryldifurylmethyl cations conformational transformations take place as the simultaneous rotation of two aromatic rings; the correlated rotation of the two furan rings proceeds with smaller energy expenditures.For Communication 5 see [1].Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 4, pp. 450–455, April, 1993.     

Protonated free-base corroles: acidity, electrochemistry, and spectroelectrochemistry of [(Cor)H4]+, [(Cor)H5]2+, and [(Cor)H6]3+

Protonated meso-substituted free-base macrocycles of the form [(Cor)H4]+, [(Cor)H5]2+, and [(Cor)H6]3+ where Cor is the trianion of a given corrole, were chemically generated from neutral (Cor)H3 in benzonitrile by addition of trifluoroacetic acid (TFA) and characterized as to their relative acidity, electrochemistry, and spectroelectrochemistry. Three types of protonated free-base corroles with different electron-donating or electron-withdrawing substituents at the meso positions of the macrocycle were investigated. One is protonated exclusively at the central nitrogens of the corrole forming [(Cor)H4]+ from (Cor)H3, while the second and third types of corroles undergo protonation at one or two meso pyridyl substituents prior to protonation of the central nitrogens and give as the final products [(Cor)H5]2+ and [(Cor)H6]3+, respectively. Altogether the relative deprotonation constants (pKa) for 10 different corroles were determined in benzonitrile and analyzed with respect to the molecular structure and/or type of substituents on the three meso positions of the macrocycle. Mechanisms for oxidation and reduction of the protonated corroles are proposed in light of the electrochemical and spectroelectrochemical data.     

“To Twist or Not to Twist”: Figure‐of‐Eight and Planar Structures of Octaphyrins

Amongst the various porphyrinoids, octaphyrin has attracted significant attention owing to its diverse syntheses, conformations, and metal‐ligation properties. Octaphyrin is a higher homologue of porphyrin and is formed by linking together heterocycles such as pyrrole, furan, thiophene, and selenophene through α‐α or α‐meso carbon bonds. The planar conformation is mainly achieved through inversion of the heterocyclic units from the center of macrocycle; avoiding meso ‐bridges; introducing a para ‐quinodimethane bridge; employing a neo‐confusion approach; protonation; and by generating dianionic species. In this Focus Review, recent synthetic advancements in the field of octaphyrins are summarized. The twisted conformation of the octaphyrin binds to two metal ions in a tetracoordinate geometry. The diphosphorus complex of octaphyrin represents the first example of a stable expanded isophlorin.     

Figure-eight aromatic core-modified octaphyrins with six meso links: syntheses and structural characterization

The synthesis and structural characterization of the first examples of aromatic core-modified figure-eight octaphyrins with six meso links and their formation with and without acid catalysts are highlighted.     

5,10,15-Triaryl-21,23-dioxacorrole and its isomer with a protruding furan ring

An oxa-analogue of 5,10,15-triarylcorrole, i.e., 5,10,15-triaryl-21,23-dioxacorrole (21,23-O(2)Cor)H, where two pyrrole rings are replaced by furan moieties, has been produced by condensation of 2,5-bis(arylhydroxymethyl)furan, 2-phenylhydroxymethylfuran, and pyrrole. 2-Phenylhydroxymethylfuran serves as the suitable synthone to introduce a furan ring with the ability create a direct pyrrole-furan alpha-alpha bond. The replacement of 2-phenylhydroxymethylfuran by 3-phenylhydroxymethylfuran led to a nonaromatic isomer of (21,23-O(2)Cor)H, i.e., (iso-21,23-O(2)Cor)H, which accommodates two furan rings. The protruding furan is built into the macrocycle via beta and beta' carbon atoms with the oxygen atom pointing outward. Crystal structures of the [(21,23-O(2)Cor)H(2)](2)[ZnCl(4)] and [(iso-21,23-O(2)Cor)H(2)]Cl have been studied by X-ray crystallography. The complex ZnCl(4)(2-) anion is located in a clam-shell-like cavity formed by two 21,23-dioxacorrole cations of the [(21,23-O(2)Cor)H(2)](2)[ZnCl(4)] unit. The 21,23-dioxacorrole cation is only slightly distorted from planarity. In [(iso-21,23-O(2)Cor)H(2)]Cl, the macrocycle is strongly puckered as the internal ring is contracted by two carbon atoms when compared to regular porphyrin. The chloride anion is located over the center of the macrocycle and is involved in two intra (N)H...Cl and two intermolecular (C)H...Cl interactions to be classified as a tetrafurcate system. The (21,23-O(2)Cor)H molecule preserves aromaticity of the parental corrole with characteristic downfield positions of furan and pyrrole resonances in (1)H NMR accompanied the NH resonance at the upfield position (-2.53 ppm). The temperature-dependent features detected in (1)H NMR spectra of (21,23-O(2)Cor)H are consistent with the existence of a tautomeric equilibrium which involves two tautomers alternatively protonated on N(22) or N(24) nitrogen atoms. The density functional theory (DFT) has been applied to model the molecular and electronic structure of two tautomers of 21,23-dioxacorrole [22-N, 24-NH], [22-NH, 24-N]. The total energies calculated using the B3LYP/6-31G**//B3LYP/6-31G* approach demonstrate a very small energy difference (1.4 kcal/mol) between tautomers suggesting their simultaneous presence in equilibrium. Insertion of nickel(II) into (21,23-O(2)Cor)H yields five-coordinate (21,23-O(2)Cor)Ni(II)Cl--the first high-spin nickel(II) in a corrole-like macrocyclic environment.     

O-confused oxaporphyrin--an intermediate in formation of 3-substituted 2-oxa-21-carbaporphyrins

[Reaction: see text]. A condensation of 2,4-bis(phenylhydroxymethyl)furan with pyrrole and p-toluylaldehyde in the presence of ethanol formed 5,20-diphenyl-10,15-di(p-tolyl)-2-oxa-3-ethoxy-3-hydro-21-carbaporphyrin [(H,EtO)OCPH]H2. The new carbaporphyrinoid has 1H NMR features of an aromatic molecule, including the upfield shift of the inner H(21) atom (-5.48 ppm). Addition of acid removes the ethoxy substituent and converts [(H,EtO)OCPH]H2 into the dication of "true" O-confused oxaporphyrin {[(H)OCPH]H3}2+ via an exocyclic C(3)-O bond cleavage followed by an elimination of the ethoxy group as determined by 1H NMR. Addition of ethanol, water, or pyrrole converts {[(H)OCPH]H3]2+ into [(H,EtO)OCPH]H2, [(H,OH)OCPH]H2, or pyrrole appended O-confused porphyrin [(H,pyrrole)OCPH]H2, respectively. The reaction of [(H,OEt)OCPH]H2 with silver(I) acetate yields a stable Ag(III) complex [(H,OEt)OCP]AgIII substituted at the C(3) position by the ethoxy group and hydrogen. Coordination through the nitrogen donors is reflected by the presence of 107/109Ag scalar splitting seen for the selected -H pyrrolic signals. Addition of TFA to [(H,OEt)OCP]AgIII produces a weakly aromatic O-confused porphyrin complex {[(H)OCP]AgIII}+. In the course of this reversible process the tetrahedral-trigonal rearrangements originate at the C(3) atom but extend its consequences on the whole structure. Insertion of silver into the hydroxy analogue of [(H,OEt)OCPH]H2, i.e., [(H,OH)OCPH]H2, was accompanied by ligand oxidation, yielding carbaporpholactone which contains the lactone functionality instead of the regular furan moiety embedded in the carbaporphyrin ligand of [(O)OCP]AgIII. The structure was determined by X-ray crystallography. The macrocycle is only slightly distorted from planarity, and silver(III) is located in the NNNC plane.     

(18-Crown-6)-2,3,11,12-tetracarboxylic acid as a chiral NMR solvating agent for determining the enantiomeric purity and absolute configuration of β-amino acids

(18-Crown-6)-2,3,11,12-tetracarboxylic acid is an excellent chiral NMR solvating agent for cyclic β-amino acids and acyclic derivatives with aliphatic, aromatic, and heterocyclic aromatic moieties. The β-amino acids are mixed with the crown ether in methanol-d₄ in either their neutral or protonated form. Substantial enantiomeric discrimination typically occurs for the resonances of the α-methylene and β-methine hydrogen atoms. Resonances of the substituent group of the β-amino acid often exhibit enantiomeric discrimination. The enantiomeric discrimination of the α-methylene and β-methine resonances of specific groups of compounds shows consistent patterns that correlate with the absolute configuration.     

Synthesis and comparative study of the heterocyclic rings on liquid crystalline properties of 2,5-aryl-1,3,4-oxa(thia)diazole derivatives containing furan and thiophene units

A series of heterocyclic liquid crystalline compounds containing 1,3,4-oxadiazole/thiadiazole, furan and thiophene units were synthesised and characterised by means of electrospray ionisation-mass spectrometry (ESI-MS), high-resolution mass spectroscopy (HRMS), ¹H nuclear magnetic resonance (NMR) and ¹³C NMR. The thermal behaviours were investigated by differential scanning calorimetry (DSC) and polarised optical microscopy (POM). The effect of the 1,3,4-oxadiazole, 1,3,4-thiadiazole, furan, thiophene and benzene rings on the liquid crystalline properties was discussed briefly in context with the geometrical and electronic factors. The results showed that the tendency to form mesophases follows the sequence: 1,4-disustituted benzene >2,5-disubstituted thiophene >2,5-disustituted furan and 1,3,4-thiadiazole >1,3,4-oxadiazole.     

Synthesis and special characterization through X-ray analysis of 1,8-dioxooctahydroxanthenes

Pyran moiety containing heterocyclic compounds have the capability for binding to either side to cyclohex-2-enone rings and to form xanthene derivatives (1,8-dioxooctahydroxanthenes also known as xanthenodiones). These xanthene derivatives display a wide range of biological activities and find applications in laser technologies and photodynamic therapy. This paper describes the preparation, X-ray structural analysis, and theoretical investigation of a series of 1,8-dioxooctahydroxanthenes. The compounds were synthesized via Knoevenagel condensation between different aldehydes and β-diketones. The reactions were performed free of solvents and the 1,8-dioxooctahydroxanthenes were obtained in good yields (70−92%). All the compounds were fully characterized by NMR and IR spectroscopy as well as mass spectrometry. Among the synthesized compounds, seven had their crystal structures elucidated for the first time. In all the new crystal structures, the three fused rings did form an almost completely planar xanthenodione core, except for the side rings that adopt half-chair conformation with both carbons at the flaps oriented toward to the aromatic substituent, or with one of the two carbons pointing opposite to the substituent. Another conformational difference among the new compounds investigated by X-ray diffraction resides in the rotation around the bond axis connecting the xanthenodione core to its aromatic substituent. It was found that different bent levels resulted from weak intermolecular contact patterns. In addition, theoretical calculations for single molecule and dimmers have provided insights into the balance between intramolecular and intermolecular forces driving both conformational features.     

Core‐Modified Analogues of Expanded Porphyrins Containing Rigid β,β′‐Linked o‐Phenylene Bridges

Structural modifications that lead to the creation of π‐extended aromatic macrocyles involving a heterocyclic ring other than pyrrole and rigid β‐β′ linkages have not been well studied up to date. The rigidity caused by the conformational restriction would change the spectroscopic properties of the system as compared with those of the normal congeners. With these considerations, we have synthesized and fully characterized π‐extended, core modified expanded porphyrins bearing rigid bipyrrole units. Core‐modified naphthorubyrins were synthesized by the Lewis acid‐catalyzed condensation of naphthobipyrrole with thiophene/furan diols, whereas naphthosapphyrins were obtained by reacting 2,9‐diformyl‐naphthobipyrrole with 16‐thia/oxatripyrranes under mild reaction conditions. The core‐modified analogues of both naphthorubyrin and naphthosapphyrin displayed the aromatic character. The dithiarubyrin analogues showed a lack of conformational change as expected and displayed well‐resolved ¹H NMR resonances at room temperature. On the other hand, the oxasapphyrin analogue adopts a furan‐inverted geometry, and the ring inversion is independent of the protonation state. The oxanaphthosapphyrin also exhibited a weak fluorescence emission at 613 nm.     

Conformational study of anticholinesterase carbamates in the furylbenzene series by 1H and 13C NMR spectroscopy

The structural analysis of carbamates derived from 2-(α-furyl)benzaldoximes and 2-(α-furyl)benzyl alcohols was carried out by ¹H and ¹³C NMR spectroscopy. The conjugative and steric effects of alkyl substituents introduced on the benzene rings were found to modify the relative orientation of the aromatic and furan rings. The existence of a close relationship between the stereochemistry of the studied compounds and their anticholinesterase activity has been proposed.     

Ab initio theoretical study of non-covalent adsorption of aromatic molecules on boron nitride nanotubes

We have studied non-covalent functionalization of boron nitride nanotubes (BNNTs) with benzene molecule and with seven other different heterocyclic aromatic rings (furan, thiophene, pyrrole, pyridine, pyrazine, pyrimidine, and pyridazine, respectively). A hybrid density functional theory (DFT) method with the inclusion of dispersion correction is employed. The structural and electronic properties of the functionalized BNNTs are obtained. The DFT calculation shows that upon adsorption to the BNNT, the center of aromatic rings tend to locate on top of the nitrogen site. The trend of adsorption energy for the aromatic rings on the BNNTs shows marked dependence on different intermolecular interactions, including the dispersion interaction (area of the delocalized π bond), the dipole-dipole interaction (polarization), and the electrostatic repulsion (lone pair electrons). The DFT calculation also shows that non-covalent functionalization of BNNTs with aromatic rings can give rise to new impurity states within the band gap of pristine BNNTs, suggesting possible carrier doping of BNNTs via selective adsorption of aromatic rings.     

NMR studies on derivatives of heteroaromatic compounds: exceptionally small carbon-carbon couplings, 1J(CC), in 2-lithiothiophene, 2-lithio-N-methylpyrrole and 2-lithiofuran

Spin-spin carbon-carbon coupling constants across one bond and carbon proton coupling constants across one, two and three bonds have been measured for a large series of derivatives of five-membered heterocyclic compounds. This included 2-methyl and 2-lithio derivatives of furan, N-methyl pyrrole and thiophene and a series of 2-R-substituted thiophenes where R = O-t-Bu, Cl, Br, I, Si(CH3)3, MgBr and MgTh. For the long-range C-H couplings their signs have been determined in several compounds by the use of modern 2D NMR techniques, and in all the cases they have been found to be positive. A good linear dependence upon electronegativity of the substituent has been observed for 1J(CC), 2J(C2H3), and 3J(C2H4). Very small 1J(CC) couplings of ca. 30 Hz only have been found in all three lithio compounds; they belong to the smallest couplings of this type reported up to now. They are accompanied by very large and positive two-bond couplings, 2J(C2H3) of ca. +20 Hz, which in turn are the largest couplings of this type reported so far. In both cases the changes observed are interpreted in terms of the changes in the Fermi contact contribution.     

Nitrile ylide dimerization: investigation of the carbene reactivity of nitrile ylides

A series of novel hexaaryl diazatrienes 5 ("nitrile ylide dimers") were synthesized directly from the corresponding diaryl ketimines 12 and dichlorotoluenes 13 in a facile one-pot synthesis. The carbene character of the nitrile ylides was investigated by varying the substituents on the aromatic ring adjacent to the carbene center. The isolation of the corresponding carbene dimers as stable crystalline materials with absorption maxima (lambda(max)) from 363 to 422 nm was shown to be promoted by the absence of strongly electron-withdrawing substituents. The crystal structures indicate that the E-isomers were isolated when phenyl, 3-methylphenyl, and 3-chlorophenyl substituents are present at the carbene carbon; the Z-isomer was isolated when the more sterically hindered 2,4,6-trimethylphenyl substituent (Mes) is present. The (1)H NMR spectra of the E-isomers demonstrate the nonequivalence of the aromatic rings, in which two of the aromatic rings of the imine moiety are pseudoaxial and the remaining aromatic rings are pseudoequatorial. The reactions proceed via the intermediate nitrile ylides 1 generated by the base-promoted 1,1-elimination of HCl from the intermediate chloroimine 14. The nitrile ylide was also generated by 1,3-elimination of HCl from the imidoyl chloride 18, confirming common pathways via the nitrile ylide as the dimer products obtained from these different routes were identical. The strongly electron-withdrawing 4-nitrophenyl substituent promotes the linear carbanion character of the 1,3-dipole and no dimer is formed.     

22pi smaragdyrin molecular conjugates with aromatic phenylacetylenes and ferrocenes: Syntheses, electrochemical, and photonic properties

Syntheses, spectroscopic, electrochemical, and third-order nonlinear optical susceptibilities of a series of 22pi smaragdyrins and their corresponding Rh(I) derivatives bearing phenylacetylene substituents and ferrocene-containing substituents are reported. The synthetic strategy involved a [3 + 2] acid-catalyzed oxidative coupling reaction of the appropriate dipyrromethane and oxatripyrrane. The desired meso substituents, such as phenylacetylenylphenyl and the ferrocenes, were incorporated to the dipyrromethane unit prior to the oxidative coupling reaction. The optical absorption, emission characteristics, and the quantum yield of the smaragdyrin conjugates depends on the nature of the substituent, nature of linker group, and the spacer length. Theoretical studies at the DFT level suggest high delocalization of electrons confined to only four of the five available heterocyclic rings for the free bases. However, upon Rh(I) metalation, the pi-electron delocalization is extended to all the heterocyclic rings. The two-photon absorption cross section (TPA) values sigma(2) measured through the open aperture Z-scan method, increases linearly with enhanced pi-electron delocalization for the smaragdyrins containing phenylacetylene substituents. The meta branching of substituents decreases sigma(2) values. Introduction of Rh(I) to the smaragdyrin cavity enhances the sigma(2) values by about 3-10 orders of magnitude, attributed to the increased aromatic character upon Rh(I) insertion. The calculated molecular electrostatic potential (MESP) and harmonic oscillator model of aromaticity (HOMA) for the free bases and the Rh(I) derivatives justifies such a conclusion. A linear correlation observed for the second oxidation potential of Rh(I) derivatives and corresponding free bases also support the increased aromaticity upon Rh(I) insertion. The electrochemical data for ferrocene-containing smaragdyrins reveal easier ring oxidation by about 50-130 mV and harder ferrocene oxidation by 40-180 mV suggesting electron-donating nature of the ferrocene upon linking with the smaragdyrin system. The TPA cross section value of 88782 GM observed for 5g represents one of the highest values known for a metalloexpanded porphyrin derivative.     

Structural assignment of monothiocarbonohydrazones by 1H NMR spectroscopy

Analysis of the ¹H NMR spectra of several monothiocarbonohydrazones, some of them synthesized for the first time, shows that they exist as two structural isomers. Whereas, in general, the derivatives of aromatic aldehydes conform to a linear structure, the aliphatic carbonyl derivatives conform to heterocyclic or linear structures, depending on the size of the substituent groups. This dual behaviour is explained in terms of extended conjugation and steric hindrance.     

Electrospray ionization ion-trap time-of-flight tandem mass spectrometry of two furofurans: sesamin and gmelinol

High-resolution electrospray ionization multistage tandem mass spectrometry (MS(1-7)) in positive ion mode was used to determine the accurate masses and the fragmentation pathways of two furofurans, sesamin and gmelinol. The protonated molecules [M+H]+ were absent in the conventional mass spectra of both compounds, but two characteristic ions, [M+H-H(2)O]+ and [M+H-H(2)]+, were always observed. Successive losses of CH(2)O and CO are the common characteristic fragmentations. Based on the exact masses acquired from 21 different tandem mass spectra, two or three fragmentation pathways for each compound are proposed. The consecutive losses of two H(2)O molecules and one H(2) molecule readily take place from the furan rings for both sesamin and gmelinol, resulting in the absence of the protonated molecules in the single-stage experiments. HCHO loss is observed at least three times in the tandem mass spectra, mainly from methylenedioxy groups (OCH(2)O) for sesamin but only from tetrahydrofuran rings for gmelinol. Moreover, CO loss is found at least three times in the tandem mass spectra of both sesamin and gmelinol from the 3,4-methylenedioxyphenyl (ArOCH(2)O) moieties for sesamin and from both the dimethoxyphenyl and the tetrahydrofuran ring moieties for gmelinol. In addition, the disubstituted benzyl cation ArCH(2)+ at m/z 135 for sesamin and at m/z 151 for gmelinol was found in the MS(3) spectra of both sesamin and gmelinol, which is very helpful in the identification of the compositions of 3,4-disubstituted groups on the benzene rings of the furofurans.