Intrinsic Gas-Phase Electrophilic Reactivity of Cyclic N-Alkyl- and N-Acyliminium Ions

Catalytic Asymmetric Additions of Enol Silanes to in situ Generated Cyclic, Aliphatic N-Acyliminium Ions

Strong and confined imidodiphosphorimidate (IDPi) catalysts enable highly enantioselective substitutions of cyclic, aliphatic hemiaminal ethers with enol silanes. 2‐Substituted pyrrolidines, piperidines, and azepanes are obtained with high enantioselectivities, and the method displays a broad tolerance of various enol silane nucleophiles. Several natural products can be accessed using this methodology. Mechanistic studies support the intermediacy of non‐stabilized, cyclic N‐(exo‐acyl)iminium ions, paired with the confined chiral counteranion. Computational studies suggest transition states that explain the observed enantioselectivity.

One-Step Synthesis of the 1-Azaspiro[5.5]undecane Skeleton Characteristic of Histrionicotoxin Alkaloids from Linear Substrates via Hg(OTf)2 -Catalyzed Cycloisomerization

Histrionicotoxin (HTX) alkaloids isolated from the poison arrow frogs possess a unique structure characterized by a 1-azaspiro[5.5]undecane skeleton common to the HTX family. The unique molecular architecture of HTXs and the interest as potential target drugs have prompted synthetic chemists to promote the total synthesis so far. However, all of the synthetic strategies to access the 1-azaspiro[5.5]undecane framework of HTXs take a multistep approach from linear starting materials due to stepwise construction of either six-membered carbo- or azacycle. Herein, we report the direct one-step construction of the 1-azaspiro[5.5]undecane skeleton from linear amino ynone substrates bearing an N-methoxycarbonyl group utilizing our mercuric triflate (Hg(OTf)₂ )-catalyzed cycloisomerization reaction. The utility of this novel methodology was demonstrated by the total and formal syntheses of HTX-235A and HTX-283A, respectively, from the azaspirocycle.

Addition of carbon nucleophiles to hemiaminals promoted by a Lewis acidic polyoxotungstate

Organic soluble TBA₅K[α₁Hf(H₂O)₄P₂W₁₇O₆₁] (POM/Hf) catalyzes the addition of pronucleophiles (silylenolethers, diketones, ketoesters, allylsilane) to unactivated hemiaminals.

Exploring the intrinsic polar [4+2(+)] cycloaddition reactivity of gaseous carbosulfonium and carboxonium ions

Gas-phase reactions of model carbosulfonium ions (CH(3)-S(+)=CH(2;) CH(3)CH(2)-S(+)=CH(2) and Ph-S(+)=CH(2)) and an O-analogue carboxonium ion (CH(3)-O(+)=CH(2)) with acyclic (isoprene, 1,3-butadiene, methyl vinyl ketone) and cyclic (1,3-cyclohexadiene, thiophene, furan) conjugated dienes were systematically investigated by pentaquadrupole mass spectrometry. As corroborated by B3LYP/6-311 G(d,p) calculations, the carbosulfonium ions first react at large extents with the dienes forming adducts via simple addition. The nascent adducts, depending on their stability and internal energy, react further via two competitive channels: (1) in reactions with acyclic dienes via cyclization that yields formally [4+2(+)] cycloadducts, or (2) in reactions with the cyclic dienes via dissociation by HSR loss that yields methylenation (net CH(+) transfer) products. In great contrast to its S-analogues, CH(3)-O(+)=CH(2) (as well as C(2)H(5)-O(+)=CH(2) and Ph-O(+)=CH(2) in reactions with isoprene) forms little or no adduct and proton transfer is the dominant reaction channel. Isomerization to more acidic protonated aldehydes in the course of reaction seems to be the most plausible cause of the contrasting reactivity of carboxonium ions. The CH(2)=CH-O(+)=CH(2) ion forms an abundant [4+2(+)] cycloadduct with isoprene, but similar to the behavior of such α,β-unsaturated carboxonium ions in solution, seems to occur across the C=C bond.

Inhibition of (+)-aristolochene synthase with iminium salts resembling eudesmane cation

Trigonal iminium halides of (4aS,7S)-1,4a-dimethyl- and (4aS,7S)-4a-methyl-7-(prop-1-en-2-yl)-2,3,4,4a,5,6,7,8-octahydroquinolinium ions, aimed to mimic transition states associated with the aristolochene synthase-catalyzed cyclization of (-)-germacrene A to eudesmane cation, were evaluated under standard kinetic steady-state conditions. In the presence of inorganic diphosphate, these analogues were shown to competitively inhibit the enzyme, suggesting a stabilizing role for the diphosphate leaving group in this apparently endothermic transformation.

Dimerization of ionized 4-(methyl mercapto)-phenol during ESI, APCI and APPI mass spectrometry

A novel ion/molecule reaction was observed to occur under electrospray ionization (ESI), atmospheric pressure chemical ionization (APCI), and atmospheric pressure photo ionization (APPI) conditions, leading to dimerization of ionized 4-(methyl mercapto)-phenol followed by fast H(*) loss. The reaction is particularly favored during ESI, which suggests that this ion/molecule reaction can occur both in the solution inside the ESI-charged droplets and in the gas-phase environment of most other atmospheric pressure ionization techniques. The dimerization reaction is inherent to the electrolytic process during ESI, whereas it is more by ion/molecule chemistry in nature during APCI and APPI. From the tandem mass spectrometry (MS/MS) data, accurate mass measurements, hydrogen/deuterium (H/D) exchange experiments and density functional theory (DFT) calculations, two methyl sulfonium ions appear to be the most likely products of this electrophilic aromatic substitution reaction. The possible occurrence of this unexpected reaction complicates mass spectral data interpretation and can be misleading in terms of structural assignment as reported herein for 4-(methyl mercapto)-phenol.

Cyclization reactions of acylium and thioacylium ions with isocyanates and isothiocyanates: gas phase synthesis of 3,4-dihydro-2,4-dioxo-2H-1,3,5-oxadiazinium ions

Gas-phase reactions of several acylium and thioacylium ions, that is H2C=N-C+=O, H2C=N-C+=S, O=C=N-C+=O, S=C=N-C+=O, H3C-C+=O, and (CH3)2N-C+=O, with both a model isocyanate and isothiocyanate, that is, C2H5-N=C=O and C2H5-N=C=S, were investigated using tandem-in-space pentaquadrupole mass spectrometry. In these reactions, the formation of mono- and double-addition products is observed concurrently with proton transfer products. The double-addition products are far more favored in reactions with ethyl isocyanate, whereas the reactions with ethyl isothiocyanate form, preferentially, either the mono-addition product or proton transfer products, or both. Retro-addition dominates the low-energy collision-induced dissociation of the mono- and double-addition products with reformation of the corresponding reactant ions. Ab initio calculations at Becke3LYP//6-311 + G(d,p) level indicate that cyclization is favored for the double-addition products and that products equivalent to those synthesized in solution, that is, of 3,4-dihydro-2,4-dioxo-2H-1,3,5-oxadiazinium ions and sulfur analogs, are formed.

Validated method for determination of bromopride in human plasma by liquid chromatography--electrospray tandem mass spectrometry: application to the bioequivalence study

A simple, sensitive and specific liquid chromatography-tandem mass spectrometry method for the quantification of bromopride I in human plasma is presented. Sample preparation consisted of the addition of procainamide II as the internal standard, liquid-liquid extraction in alkaline conditions using hexane-ethyl acetate (1 : 1, v/v) as the extracting solvent, followed by centrifugation, evaporation of the solvent and sample reconstitution in acetonitrile. Both I and II (internal standard, IS) were analyzed using a C18 column and the mobile-phase acetonitrile-water (formic acid 0.1%). The eluted compounds were monitored using electrospray tandem mass spectrometry. The analyses were carried out by multiple reaction monitoring (MRM) using the parent-to-daughter combinations of m/z 344.20 > 271.00 and m/z 236.30 > 163.10. The areas of peaks from analyte and IS were used for quantification of I. The achieved limit of quantification was 1.0 ng/ml and the assay exhibited a linear dynamic range of 1-100.0 ng/ml and gave a correlation coefficient (r) of 0.995 or better. Validation results on linearity, specificity, accuracy, precision and stability, as well as application to the analysis of samples taken up to 24 h after oral administration of 10 mg of I in healthy volunteers demonstrated the applicability to bioequivalence studies.

Solvent-Dependent Chemoselectivities in Additions of β-Carbonyl Imines to Allyltrimethylsilane with CTAN

The oxidative coupling of beta-carbonyl imines and allyltrimethylsilane with CTAN were investigated in CH(3)CN and CH(2)Cl(2). In CH(3)CN allylation products were obtained predominantly, while in CH(2)Cl(2), dihydropyrrole products were obtained exclusively. Solvent-assisted nucleophilic cleavage of the intermediate beta-silyl cation is proposed to play a role in the solvent-dependent chemoselectivity.

Gas-phase polar [4+ + 2] cycloaddition with ethyl vinyl ether: a structurally diagnostic ion-molecule reaction for 2-azabutadienyl cations

The intrinsic reactivity of eight gaseous, mass-selected 2-azabutadienyl cations toward polar [4(+) + 2] cycloaddition with ethyl vinyl ether has been investigated by pentaquadrupole mass spectrometric experiments. Cycloaddition occurs readily for all the ions and, with the only exception of those from the N-acyl 2-azabutadienyl cations (N-acyliminium ions), the cycloadducts are found to dissociate readily upon collision activation (CID) both by retro-Diels-Alder reaction and by a characteristic loss of an ethanol (46u) neutral molecule. Ethanol loss from the intact polar [4(+) + 2] cycloadduct functions therefore as a structurally diagnostic test: 72 u neutral gain followed by 46 u neutral loss, i.e., as a combined ion-molecule reaction plus CID 'signature' for N-H, N-alkyl and N-aryl 2-azabutadienyl cations. The two N-acyliminium ions tested are exceptional as they form intact cycloadducts with ethyl vinyl ether which dissociate exclusively by the retro-Diels-Alder pathway.

Probing the mechanism of the Petasis olefination reaction by atmospheric pressure chemical ionization mass and tandem mass spectrometry

Atmospheric pressure chemical ionization mass (APCI-MS) and tandem mass spectrometry (APCI-MS/MS) is used to probe the mechanism of the Petasis olefination reaction. Oxatitanacycle intermediates 4 were transferred from solution to the gas phase, detected as 4H+ by APCI-MS with characteristic Ti-isotopic patterns, and structurally characterized by APCI-MS/MS. Detection of 4H+, which upon collision activation dissociates to both 3H+ and Cp(2)TiOH+, fully supports the Hughes mechanism as depicted above. [reaction: see text]

Reactions of gaseous acylium ions with 1,3-dienes: further evidence for polar [4 + 2+] Diels-Alder cycloaddition

A novel reaction of acylium and thioacylium ions, polar [4 + 2(+)] Diels-Alder cycloaddition with 1,3-dienes and O-heterodienes, has been systematically investigated in the gas phase (Eberlin MN, Cooks RG. J. Am. Chem. Soc. 1993; 115: 9226). This polar cycloaddition, yet without precedent in solution, likely forms cyclic 2,5-dihydropyrylium ions. Here we report the reactions of gaseous acylium ions [(CH(3))(2)N-C(+)=O, Ph-C(+)=O, (CH(3))(2)N-C(+)=S, CH(3)-C(+)=O, CH(3)CH(2)-C(+)=O, and CH(2)=CH-C(+)=O] with several 1-oxy-substituted 1,3-dienes of the general formula RO-CH=CH-C(R(1))=CH(2), which were performed to collect further evidence for cycloaddition. In reactions with 1-methoxy and 1-(trimethylsilyloxy)-1,3-butadiene, adducts are formed to a great extent, but upon collision activation they mainly undergo structurally unspecific retro-addition dissociation. In reactions with Danishefsky's diene (trans-1-methoxy-3-(trimethylsilyloxy)-1,3-butadiene), adducts are also formed to great extents, but retro-addition is no longer their major dissociation; the ions dissociate instead mainly to a common fragment, the methoxyacryl cation of m/z 85. This fragment ion is most likely formed with the intermediacy of the acyclic adduct, which isomerizes prior to dissociation by a trimethylsilyl cation shift. Theoretical calculations predict that meta cycloadducts bearing 1-methoxy and 1-trimethylsilyloxy substituents are unstable, undergoing barrierless ring opening induced by the charge-stabilizing effect of the 1-oxy substituents. In contrast, for the reactions with 1-acetoxy-1,3-butadiene, both the experimental results and theoretical calculations point to the formation of intrinsically stable cycloadducts, but the intact cycloadducts are either not observed or observed in low abundances. Both the isomeric ortho and meta cycloadducts are likely formed, but the nascent ions dissociate to great extents owing to excess internal energy. The ortho cycloadducts dissociate by ketene loss; the meta cycloadducts undergo intramolecular proton transfer to the acetoxy group followed by dissociation by acetic acid loss to yield aromatic pyrylium ions. Either or both of these dissociations, ketene and/or acetic acid loss, dominate over the otherwise favored retro-Diels-Alder alternative. The pyrylium ion products therefore constitute compelling evidence for polar [4 + 2(+)] cycloaddition since their formation can only be rationalized with the intermediacy of cyclic adducts.

Novel gas-phase ion-molecule aromatic nucleophilic substitution in beta-carbolines

We report a novel gas-phase ion-molecule aromatic-nucleophilic substitution reaction between beta-carbolines and water vapour, that accounts for the observation of ions with higher masses than the precursor ion in the MS/MS spectra.

Mannich-type reactions in the gas-phase: the addition of enol silanes to cyclic N-acyliminium ions

The intrinsic gas-phase reactivity of cyclic N-acyliminium ions in Mannich-type reactions with the parent enol silane, vinyloxytrimethylsilane, has been investigated by double- and triple-stage pentaquadrupole mass spectrometric experiments. Remarkably distinct reactivities are observed for cyclic N-acyliminium ions bearing either endocyclic or exocyclic carbonyl groups. NH-Acyliminium ions with endocyclic carbonyl groups locked in s-trans forms participate in a novel tandem N-acyliminium ion reaction: the nascent adduct formed by simple addition is unstable and rearranges by intramolecular trimethylsilyl cation shift to the ring nitrogen, and an acetaldehyde enol molecule is eliminated. An NSi(CH(3))(3)-acyliminium ion is formed, and this intermediate ion reacts with a second molecule of vinyloxytrimethylsilane by simple addition to form a stable acyclic adduct. N-Acyl and N,N-diacyliminium ions with endocyclic carbonyl groups, for which the s-cis conformation is favored, react distinctively by mono polar [4(+) + 2] cycloaddition yielding stable, ressonance-stabilized cycloadducts. Product ions were isolated via mass-selection and structurally characterized by triple-stage mass spectrometric experiments. B3LYP/6-311G(d,p) calculations corroborate the proposed reaction mechanisms.

Electrochemical oxidation of N-p-toluenesulfinamides

[reaction: see text] Contrasting and interesting electrochemical behavior is observed in anodic oxidation of N-substituted p-toluenesulfinamides under controlled current conditions. For sulfinamides derived from secondary alkylamines and primary arylamines, the N-sulfinyl group is removed and the corresponding amines are formed; for sulfinamides derived from primary alkylamines, sulfur oxidation yields the corresponding sulfonamides in good yields.