Mass spectrometric studies of thyroxine and related compounds. Trimethylsilyl derivatives.
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Chemical Phenomena↗
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Effective ring-opening reaction of aziridines with trimethylsilyl compounds: A facile access to beta-amino acids and 1,2-diamine derivatives
Ring-opening reactions of aziridines with trimethylsilyl compounds triggered by tetrabutylammonium fluoride give the corresponding products regioselectively in excellent yield. It provides a facile and efficient procedure for the ring-opening reactions of aziridines and affords a practical access to the synthesis of cyano-, azido-, or chloroamines because of its efficiency and simplicity. The products are easily transformed to vicinal diamines or beta-amino acids.
Journal Article↗
Quinoxalinol derivatives of aliphatic 2-oxocarboxylic acids. Infrared and mass spectra of the O-trimethylsilylated compounds.
In acidic media o-phenylenediamine and 2-oxoacids react to yield quinoxaline derivatives. On derivatization in pyridine with silylating reagents quinoxalinol-O-TMS ethers or O-TMS-ether-TMS-esters are formed exclusively as shown by gas chromatography infrared spectrometry and gas chromatography mass spectrometry. These derivatives have very favourable properties for gas chromatographic detection and quantitation of the parent 2-oxoacids. The mass spectra have characteristic fragments which facilitate easy identification. In addition, 'single ion monitoring' of all aliphatic 2-oxoacids can be performed with only three fragments at m/e 217,232 and 245. In some aspects the mass spectra resemble those of other heterocyclic compounds which contain an O-alkyl sidechain. The fragmentation mechanisms were deduced by low resolution mass spectrometry with and without deuteration, high resolution mass spectrometry and metastable ion evidence. A new type of rearrangement alpha-beta-elimination of ethylene or propylene from the aliphatic sidechain, is proposed for some O-TMS quinoxalinols.
Chemical Phenomena↗
Generation of a silylethylene-titanium alkoxide complex. A versatile reagent for silylethylation and silylethylidenation of unsaturated compounds
(Trimethylsilyl)ethylene-titanium alkoxide complex (1) was generated from trimethyl(vinyl)silane, Ti(O-i-Pr)(4), and i-PrMgCl as a preformed alkene-titanium complex and reacted with several unsaturated compounds such as aldehyde, imine, other vinylsilane, and acetylene to give the corresponding coupling products 4a-f, 6, 8, and 10a-d in a regioselective manner. Both of the two carbon-titanium bonds of the complex 1 reacted successively with esters to afford silylcyclopropanols 11a-j, 13, and 15, some synthetic applications of which were illustrated in the preparation of beta-silyl ketones 16 and cyclopropenes 17. Asymmetric addition of 1 to a chiral acyloxazolidinone 19 gave optically active cyclopropanol (+)-11a of 50% ee.
Journal Article↗
Synthesis and characterization of a series of zinc bis[(alkyl)(trimethylsilyl)amide] compounds.
A series of 21 secondary (alkyl)(trimethylsilyl)amines HNR(TMS) [R = n-propyl (1), i-propyl (2), n-butyl (3), i-butyl (4), s-butyl (5), tert-butyl (6), c-pentyl (7), n-pentyl (8), i-pentyl (9), l-methylbutyl (10), 2-methylbutyl (11), 1-ethylpropyl (12), 1,2-dimethylpropyl (13), tert-pentyl (14), phenyl (15), c-hexyl (16), n-hexyl (17), N,N-dimethyl-3-aminopropyl (18), benzyl (19), n-heptyl (20), 1,1,3,3-tert-butyl (21); TMS = Si(CH3)3] has been prepared and fully characterized by elemental analyses, multinuclear (1H, 13C, 29Si, 14N) NMR, IR, UV/vis, MS, and boiling point. A new method for determination of boiling points of milligram-size samples, based on DSC (differential scanning calorimetry), is described. Each amine has been converted to the corresponding zinc bis(amide) compound Zn[N(TMS)(R)]2 [R = n-propyl (22), i-propyl (23), n-butyl (24), i-butyl (25), s-butyl (26), tert-butyl (27), c-pentyl (28), n-pentyl (29), i-pentyl (30), 1-methylbutyl (31), 2-methylbutyl (32), 1-ethylpropyl (33), 1,2-dimethylpropyl (34), tert-pentyl (35), phenyl (36), c-hexyl (37), n-hexyl (38), N,N-dimethyl-3-aminopropyl (39), benzyl (40), n-heptyl (41), 1,1,3,3-tert-butyl (42); TMS = Si(CH3)3] and subsequently fully characterized by elemental analyses, multinuclear (1H, 13C, 29Si, 14N) NMR, IR, UV/vis, MS, and TGA. The experimental IR has been compared to the computationally calculated one for compound 27. Observed trends in volatility of the compounds are discussed in the context of the dominant intermolecular forces present in the condensed phase.
Journal Article↗
Antimalarial activity of new dihydroartemisinin derivatives. 5. Sugar analogues.
A series of dihydroartemisinin derivatives containing a sugar moiety was prepared in the search for analogues with good water solubility and high antimalarial activity. The preparation of the new compounds was achieved by treatment of dihydroartemisinin (2) with chlorotrimethylsilane in pyridine solution at -10 degrees C to give a nearly quantitative yield of 10-O-(trimethylsilyl)dihydroartemisinin (3), which was then condensed with 1-hydroxypolyacetylated sugars 5 to give dihydroartemisinin derivatives 7a-d. Deacetylation of intermediates 7 gave the desired sugar derivatives 8. The resulting derivatives, tested in vitro against Plasmodium falciparum, were found to be more effective against W-2 than D-6 clones and were not cross-resistant with existing antimalarials. Trimethylsilylated compound 3 is more effective than derivatives 7a-d, which possess activity comparable to or better than that of artemisinin itself. Deacetylated compounds 8a-d were substantially less active than 7 in both cell lines. In P. berghei-infected mice, 7a-c showed 5/5, 2/5, and 3/5 cures, respectively, at 320 mg/kg per day x 3, whereas 7d showed no activity at the same dosage. However, 7d did prolong the life span in 3/5 of the infected mice at 640 mg/kg per day x 3 dose level. Trimethylsilylated compound 3 was also the most effective among the compounds studied, with 5/5 cures at 80 mg/kg per day x 3. The deacetylated sugar derivatives 8a-d showed only slight in vivo antimalarial activity.
Animals↗
Acyclic, ring, and cage As,C compounds from trimethylsilyl ylides and AsCl3
From the reaction of trimethylsilyl ylides with AsCl3 the dichloroarsanyl ylides 2b and 5b are obtained. As shown by X-ray structure determination, their AsCl2 groups deviate systematically from the symmetric orientation. This conformation enables an effective charge transfer from the ylide moiety to one of the As-Cl bonds, which as a consequence is up to 15 pm longer than the other. At the same time the length of the As-C bonds in 2b and 5b indicates a partial double bond. The effects observed here are of the same type as those observed for the corresponding dichlorophosphanyl ylides; they are, however, more pronounced. The 1:1 condensation of the bis(trimethylsilyl) ylide 3 and AsCl3 yields the oligomers (Ph3PCAsCl)2,3,4. The dimer 7b has a diarsetane structure. HCl adds readily to one of its As-C bonds without opening it. The trimer and the tetramer are ionic. The cation of the trimer forms a six-membered ring with a delocalized arsenium/phosphonium charge, the cation of the tetramer forms a barrelane cage with a phosphonio substituent, and the anion is AsCl4- in both cases (10, 13). An arsa-phosphocyanine cation as in 10 is also part of the diphosphonio isoarsindolide tetrachloroarsenate(III) 12. The structures of 7b.HCl, 10, and 13 reflect again an ylide to As-Cl charge transfer. The As-Cl bonds of 13 are by far the longest ones known for a chloroarsine (average 249 pm).
Journal Article↗
A new method for the preparation of silyl enol ethers from carbonyl compounds and (trimethylsilyl)diazomethane in a regiospecific and highly stereoselective manner.
The reaction of lithium (trimethylsilyl)diazomethane with aldehydes and ketones has been investigated, and it has been found that quenching at low temperature with MeOH followed by addition of Rh2(OAc)4 gave silyl enol ethers in high yields. Quenching with other electrophiles (e.g., deuterium, MeI) gave terminal and substituted silyl enol ethers with complete control over regio- and stereochemistry. The mechanism of this novel process has been mapped out through a combination of deuterium labeling, ReactIR, and isolation of reaction intermediates.
Journal Article↗
Structural analysis by mass spectrometry of the major mammalian retinal ganglioside, a sialyl-sialyl-dihexosyl-ceramide.
The major mammalian retinal ganglioside was analysed by mass spectrometry. Three types of derivatives were used, the fully methylated, the methylated and reduced and the methylated, reduced and trimethylsilylated compound. The mass spectra unambiguously showed the ganglioside to be a sialyl-sialyl-dihexosyl-ceramide. This is the first conclusive proof that the two N-acetylneuraminic acids are bound to each other constituting the terminal disaccharide. The major species contained sphingosine and stearic acid.
Animals↗
Studies of the in vitro intestinal metabolism of isoflavones aid in the identification of their urinary metabolites.
Soy isoflavones have recently gained considerable interest due to their possible health benefits. However, detailed studies on the metabolism of isoflavones are lacking. The aims of the investigation presented here were (1) to study the in vitro intestinal metabolism of isoflavones and their hydroxylated analogues 3'-OH-daidzein, 6-OH-daidzein, 8-OH-daidzein, and 3'-OH-genistein and (2) to characterize the structures of some earlier identified urinary metabolites of soy isoflavones, for which no authentic reference compounds have been available. Isoflavone standards (1-2 mg) were fermented with human fecal flora (16.7%) for 24 h. Metabolites formed during the fermentation were tentatively identified by interpretation of the mass spectra of trimethylsilylated compounds obtained by GC-MS. Compounds having hydroxyl groups at 5-position (i.e., genistein and 3'-OH-genistein) were completely converted to metabolites that could not be detected by the methods used in this study. The metabolism of daidzein and its hydroxylated analogues, 3'-OH-daidzein, 6-OH-daidzein, and 8-OH-daidzein, occurred to a much lesser extent. Minor amounts of reduced metabolites (i.e., isoflavanones and alpha-methyldeoxybenzoins) of these compounds were tentatively identified in fermentation extracts. The retention times and the mass spectra of reduced isoflavone metabolites, obtained from in vitro fermentations of pure compounds, were utilized to identify unknown urinary metabolites of soy isoflavones. Four novel isoflavone metabolites were identified in human urine collected after soy supplementation: 3' '-OH-O-desmethylangolensin, 3',4',7-trihydroxyisoflavanone, 4',7,8-trihydroxyisoflavanone, and 4',6,7-trihydroxyisoflavanone.
Feces↗
Retinobenzoic acids. 5. Retinoidal activities of compounds having a trimethylsilyl or trimethylgermyl group(s) in human promyelocytic leukemia cells HL-60.
The retinoidal activities of trimethylsilyl or trimethylgermyl-containing retinobenzoic acids are discussed on the basis of differentiation-inducing activity on human promyelocytic leukemia cells HL-60. Compounds with a trimethylsilyl or trimethylgermyl group at the meta position of the generic formula 2 have more potent activities than the corresponding retinobenzoic acids with a m-tert-butyl group. Compounds having two m-trimethylsilyl or -trimethylgermyl groups also have strong activities, and (E)-4-[3-[3,5-bis(trimethylsilyl)phenyl]-3-oxo-1-propenyl]benzoic acid (22, Ch55S) and (E)-4-[3-[3,5-bis(trimethylgermyl)phenyl]-3-oxo-1- propenyl]benzoic acid (35, Ch55G) are more active than retinoic acid by 1 order of magnitude. However, in the para-substituted chalcone derivatives, the replacement of a tert-butyl group (49, Ch40) with a trimethylsilyl (27, Ch40S) or a trimethylgermyl (30, Ch40G) group caused the disappearance of the activity.
Antineoplastic Agents↗
Glycosidation reactions of silyl ethers with conformationally inverted donors derived from glucuronic acid: stereoselective synthesis of glycosides and 2-deoxyglycosides.
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Glucuronic Acid↗
A close look at short C-CH3...potassium contacts: synthetic and theoretical investigations of [M2Co2(mu3-OtBu)2(mu2-OtBu)4(thf)n] (M = Na, K, Rb, thf = tetrahydrofuran).
Agostic interactions of the type Si-CH3M+ (M = alkali metal) are frequently mentioned in discussions of solid-state structures of trimethylsilyl compounds and the purpose of this work was to elucidate if they also exist in the related tert-butyl species by using density functional theory. The compounds [M2Co2(mu3-OtBu)2(mu2-OtBu)4(thf)n] (M = Na, n = 2; M = K, n = 0; M = Rb, n = 1) have been synthesised and their crystal structures determined. Close contacts of methyl groups with K atoms are observed in the solid-state structure of [K2Co2(mu3-OtBu)2(mu2-OtBu)4], and calculations of the rotational barrier of a tert-butoxy group about the axis through the C-O bond were performed. It was shown that apparent short C-CH3K distances are in this case a consequence of the packing in the extended solid-state structure.
Journal Article↗
Studies in C-terminal sequencing: new reagents for the synthesis of peptidylthiohydantoins.
In previous studies aimed at the sequencing of peptides and proteins from the carboxy terminus, we have derivatized the C-terminus to a thiohydantoin using acetic anhydride and trimethylsilylisothiocyanate (TMS-ITC) and subsequently hydrolyzed it to form a shortened peptide capable of further degradation and an amino acid thiohydantoin which can be identified by reverse-phase HPLC. Current limitations to this chemistry include an inability to derivatize proline and low yields with asparagine and aspartic acid residues (Bailey et al., 1992). In an attempt to solve some of these problems, we have investigated the use of reagents other than acetic anhydride for the activation of the C-terminal carboxylic acid. These include 2-fluoro-1-methylpyridinium tosylate, 2-chloro-1-methylpyridinium iodide, and acetyl chloride. Addition of TMS-ITC to peptides activated by the 2-halo-pyridinium salts formed the expected peptidylthiohydantoin, but in addition formed a peptide chemically modified at the C-terminus which was blocked to C-terminal sequence analysis. This derivative was not obtained when either acetic anhydride or acetyl chloride was used for activation. Formation of this derivative was found to require the presence of an isothiocyanate reagent in addition to the halo-pyridinium salt. Sodium thiocyanate, TMS-ITC, and a new reagent for thiohydantoin synthesis, tributyltinisothiocyanate (TBSn-ITC), were all found to be capable of forming this analogue. Structural elucidation of the C-terminally modified amino acid revealed it to be a 2-imino-pyridinium analogue. Formation of this C-terminally blocked peptide could be minimized by the use of the 2-chloro-pyridinium reagent, rather than the 2-fluoro reagent, and by performing the reaction at a temperature of 50 degrees C or lower. The 2-halo-pyridinium reagents offer potential advantages over the use of acetic anhydride for activation of the C-terminal carboxylic acid. These include: milder reaction conditions, faster reaction times, and the ability to sequence through C-terminal aspartic acid. The TBSn-ITC reagent was found to be comparable to TMS-ITC for formation of peptidylthiohydantoins.
Acetic Anhydrides↗
Comparison of electron and chemical ionization mass spectrometry of sialic acids.
Sialic acids were analyzed as per-O-trimethylsilylated compounds by gas-liquid chromatography/mass spectrometry either on electron or chemical ionization by isobutane. Electron ionization mass spectra of these derivatives are very similar to those of the corresponding methyl esters described earlier whereas chemical ionization mass spectra are characterized in the high mass range by loss of the C-2 and the C-4 substituents from the M + 1 ion. Together with other fragment ions the seven different sialic acids analyzed could be clearly identified.
Esters↗
Synthesis of the alpha-Neu5Ac-(2----6)-Gal structure. Facile 6-O-sialylation following stannylene activation of an unprotected D-galactopyranoside.
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Carbohydrate Sequence↗
Synthesis of [18F]2-deoxy-2-fluoro-D-glucose from highly reactive [18F]tetraethylammonium fluoride prepared by hydrolysis of [18F]fluorotrimethylsilane.
18F-Labeled fluorotrimethylsilane was prepared by nucleophilic substitution of chlorotrimethylsilane with reactor produced [18F]fluoride. Hydrolysis of fluorotrimethylsilane by aqueous tetraethylammonium hydroxide followed by removal of water with a mechanical pump gave a powerful source of no carrier added nucleophilic 18F. Reaction of this purified 18F preparation with 4,6-benzylidene-1-beta-O-methyl D-mannopyranoside-2.3-cyclic sulfate was complete in 2 min at 80 degrees C and gave two labeled products with similar retention times on reverse phase HPLC. Allowing for decay and handling losses during deprotection, the maximum yield of [18F]2-deoxy-2-fluoro-D-glucose from no-carrier added tetraethylammonium fluoride was 50%. Incorporation of 18F into organic products was 30% complete in 10 min at room temperature. An identical time-course was observed for reaction of 3-O-triflyl-1,2-5,6-diisopropylidene-D-allofuranose, the starting material for 3-deoxy-3-fluoro-D-glucose. Reaction of tetraethylammonium fluoride with chlorotrimethylsilane was more rapid and much more tolerant of water than the fluorosugar reactions. Chlorotrimethylsilane can be used to recover unreacted 18F from reaction mixtures.
Chemical Phenomena↗
Stability of cannabinoids in dried samples of cannabis dating from around 1896-1905.
Cannabinoids from three samples of cannabis obtained from the Pitt-Rivers Museum, Oxford, and dating from the turn of the century were examined by gas chromatography and mass spectometry for the presence of cannabinoids. Although the samples were from different geographical locations, the profiles of constituent cannabinoids were similar. In common with other aged material, most of the cannabinoid content was present as cannabinol (CBN), the main chemical degradation product of the major psychoactive constituent, delta-9-tetrahydrocannabinol (delta-9-THC). However, a substantial concentration of CBN acid-A was also present; this compound is unstable to heat and readily undergoes decarboxylation to CBN. Methyl and propyl homologues of CBN, together with delta-9-THC and its naturally occurring acid-A were also found at low concentrations in all samples. Intermediates in the formation of CBN from delta-9-THC, previously identified in aged solutions of the drug, were absent or present in only trace concentrations. However, oxidation products involving hydroxylation at the benzylic positions, C-11 and C-1', not seen in solution, were identified in substantial abundance. The results suggest that decomposition of cannabis samples may proceed more slowly than originally thought.
Aluminum↗