Effect of the sarcosine residue on sequence scrambling in peptide b5 ions

The effect of N‐methylation on sequence scrambling in the fragmentation of b₅ ions has been investigated by studying a variety of peptides containing sarcosine (N‐methylglycine). The product ion mass spectra for the b₅ ions derived from Sar‐A‐A‐A‐Y‐A and Sar‐A‐A‐Y‐A‐A show only minor signals for non‐direct sequence ions the major fragmentation reactions occurring from the unrearranged structures. This is in contrast to the b₅ ions where the Sar residue is replaced by Ala and sequence scrambling occurs. The b₅ ion derived from Y‐Sar‐A‐A‐A‐A shows a product ion mass spectrum essentially identical to the spectrum of the b₅ ion derived from Sar‐A‐A‐A‐Y‐A, indicating that in the former case macrocyclization has occurred but the macrocyclic form shows a strong preference to reopen to put the Sar residue in the N‐terminal position. Similar results were obtained in the comparison of b₅ ions derived from A‐Sar‐A‐A‐Y‐A and Sar‐A‐A‐Y‐A‐A. The product ion mass spectra of the MH⁺ ions of Y‐Sar‐A‐A‐A‐A and A‐Sar‐A‐A‐Y‐A show substantial signals for non‐direct sequence ions indicating that fragmentation of the MH⁺ ions channels extensively through the respective b₅ ions and further fragmentation of these species. Copyright © 2014 John Wiley & Sons, Ltd.     

Peptide sequence scrambling through cyclization of b5 ions

The CID mass spectra of the MH(+) ions and the b(5) ions derived therefrom have been determined for the hexapeptides YAAAAA, AYAAAA, AAYAAA, AAAYAA, and AAAAYA. The CID mass spectra for the b(5) ions derived from the five isomers are essentially identical and show abundant ion signals for nonsequence b ions. This result is consistent with cyclization of the b(5) ions to a cyclic pentapeptide before fragmentation; this cyclic peptide can open at various positions, leading to losses of amino acid residues that are not characteristic of the original amino acid sequence. These nonsequence b ions are also observed in the fragmentation of the MH(+) ions and increase substantially in importance with increasing collision energy. A comparison of the fragmentation of AAAYAA and Ac-AAAYAA indicates that N-acetylation eliminates the cyclization of b(5) ions and, thus, eliminates the nonsequence ions in the CID mass spectra of both b(5) and MH(+) ions.     

Richter cyclization and co-cyclization reactions of triazene-masked diazonium ions

The conventional Richter cyclization involves diazotization of 2-alkynylanilines with HX (aq) (X = Br or Cl) and NaNO₂, followed by spontaneous ring closure to give a mixture of 4-halocinnoline and 4-cinnolinone products. The different products result from competing attack of X⁻ and H₂O, respectively, upon an intermediate 2-alkynylphenyl diazonium ion during the cyclization step. In order to improve the chemoselectivity of this reaction, we have utilized triazenes as masked diazonium ions. These can be unmasked using MeSO₃H in anhydrous solvents and the resultant 2-alkynylphenyl diazonium ion cyclized chemoselectively by the incorporation of a specifically added nucleophile. This process has been extended to tethered nucleophiles, leading to a Richter induced co-cyclization process to give ring-fused cinnolines.     

The formation of medium-sized rings by the intramolecular cyclization of arylnitrenium ions

The synthesis of an aminodibenzo[a,c]octadiene and an aminodibenzo[a,c]octatriene, guided by molecular mechanics calculations, was effected by the intramolecular cyclization of the corresponding open chain nitrenium ions. Also reported is the predicted failure to synthesize another aminodibenzo[a,c]octadiene using this method.     

Synthesis of spirocyclic azacycles from the cyclization of furan tethered N-acyliminium ions

The protic and Lewis acid promoted cyclization reactions of tethered furan-4,5-dihydroxypiperid-2-ones, furan-4,5-diacetoxypiperid-2-ones and furan-3,4-diacetoxypyrrolid-2-ones, via their corresponding N-acyliminium ion intermediates, have been studied. In the case of the furan-4,5-dihydroxypiperid-2-one 2a and its diacetate derivative 2b, macrocyclic products were formed from an initial intermolecular reaction between 2a or 2b, via the nucleophilic C5 furan carbon, and their corresponding N-acyliminium ion intermediates. When the furan C5 position of 2b was blocked by substitution with bromine then TFA or Sc(OTf)₃ catalysed cyclization reactions gave a spirotricyclic product (a 5-6-6-tricycle) in a highly diastereoselective manner. Cyclization of the analogous C5-Br-furan-pyrrolidone 29 with TFA resulted in a related spirotricyclic (a 5-6-5 tricycle) product. Attempts to prepare an analogous azepine system, a 5-7-5 tricycle, were not successful. Cyclization reactions of the C5-PhS-furan- or C5-phenylsulfonyl-pyrrolidone analogues of 29 with TFA were also not successful.     

Regio- and enantioselective catalytic cyclization of pyrroles onto N-acyliminium ions

The regio- and enantioselective cyclization of pyrroles onto N-acyliminium ions generated in situ from hydroxylactams is reported. Modest to excellent ee's and yields are obtained in these novel Pictet-Spengler-type reactions with a chiral thiourea-pyrrole catalyst. Useful synthetic transformations of the versatile pyrroloindolizidinone and pyrroloquinolizidinone products are presented.     

Radical cyclization of allyl α-bromocarboxylates into λ-butyrolactones. Effect of the ester structure on the cyclization

Allyl dibromoacetate, allyl α-bromopropionate, and allyl phenylbromoacetate undergo cyclization in benzene in the presence of increased concentration of Fe(CO)₅—DMF as the initiating system to give the corresponding bromo-substituted butyrolactones. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 12, pp. 2210–2212, December, 1997.     

Cyclization of peptide b9 ions

The product ion mass spectra obtained by CID of the b₉ ions derived by loss of neutral alanine from the MH⁺ ion of the peptides Tyr(Ala)₉, (Ala)₄Tyr(Ala)₅, and (Ala)₈TyrAla are essentially identical, indicative of full cyclization reaction to a common intermediate before fragmentation. This leads to abundant nondirect sequence ions in the product ion mass spectra of the b₉ ions. The product ion mass spectra of the b₈ ions from the first two peptides also are essentially identical. The fragmentation of the MH⁺ ions also leads to low intensity nondirect sequence ions in the product ion mass spectra. N-terminal acetylation blocks the cyclization and eliminates nondirect sequence fragment ions in the product ion mass spectra.     

Effect of catalysts on the Fischer cyclization of cyclohexanone 4-pyridylhydrazone

The peculiarities of the Fischer cyclization of cyclohexanone 4-pyridylhydrazone and the formation of 5,6,7,8-tetrahydro--carboline, 4-aminopyridine, 4-ethylaminopyridine, 4-pyridone, and 2-(4-pyridyl)-3,3a,4,5,6,7-hexahydroindazole-3-spirocyclohexane under the conditions of this reaction are examined. In the case of I, as also for the isomeric 2-pyridylhydrazones, catalysts can be arranged in the following order with respect to their ability to increase the yield of the normal indolyzation product: cuprous chloride sulfosalicyclic acid < polyphosphoric acid < p-toluenesulfonic acid < zinc chloride. It was established that the indolyzation of I proceeds also without acid catalysts under the influence of sodium ethoxide.Translated from KMmiya Geterotsiklicheskikh Soedinenii, No. 4, pp. 508–513, April, 1972.     

Effect of ligand structure on the asymmetric cyclization of achiral olefinic organolithiums

The ability of a large and chemically diverse set of 30 chiral ligands to effect asymmetric cyclization of 2-(N,N-diallylamino)phenyllithium (1), derived from N,N-diallyl-2-bromoaniline (2) by low-temperature lithium-bromine exchange, has been investigated in an attempt to elucidate the structural motifs required to provide high enantiofacial selectivity in the ring closure. Although none of the ligands examined in this study afforded 1-allyl-3-methylindoline in significantly higher ee than previously observed for the cyclization of 1 in the presence of the benchmark ligand (-)-sparteine, several ligands, structurally unrelated to sparteine and available in either enantiomeric form, were found to match the utility of (-)-sparteine in this chemistry.     

Promotion of cyclization of linear pentapeptides and heptapeptide by different univalent metal ions

Univalent metal ions such as Na+, K+ and Cs+ can enhance not only the cyclization yields of some linear pentapeptides and heptapeptide but also their cyclization rates while some bivalent and trivalent metal ions such as Mg2+, Ca2+, Zn2+, Fe2+, Ni2+ and Cr3+ elevate neither the cyclization yields nor the cyclization rates and some of them prevent the cyclization.     

Synthesis of 2,3-disubstituted benzo[b]selenophenes via electrophilic cyclization

2,3-Disubstituted benzo[b]selenophenes have been prepared by the electrophilic cyclization of various 1-(1-alkynyl)-2-(methylseleno)arenes by Br2, NBS, I2, ICl, PhSeCl, PhSeBr, and Hg(OAc)2. This method tolerates a wide variety of functional groups, including alcohol, ester, nitrile, nitro, and silyl groups, and proceeds under exceptionally mild reaction conditions.     

Generation and cyclization of nitrilium ions from amides. Asymmetric synthesis of fused azabicyclics

Treatment of 2° amides with PPSE affords nitrilium ions which may be cyclized onto appropriate terminators. The conditions are compatible with esters in the molecule, and further elaboration to pyrrolizidine and indolizidine rings is possible via asymmetric reduction.     

Effect of metal ions on monolayer collapses

A Langmuir monolayer of stearic acid on pure water and in the presence of certain divalent metal ions such as Cd and Pb at pH approximately 6.5 of the subphase water collapses at constant area, while for other divalent ions such as Mg, Co, Zn, and Mn at the same subphase pH the monolayer collapses nearly at constant pressure. Films of stearic acid with Cd, Pb, Mn, and Co in the subphase (at pH approximately 6.5) have been transferred onto hydrophilic Si(001) using a horizontal deposition technique, just after and long after collapse. Electron density profiles obtained from X-ray reflectivity analysis show that a three-molecular-layer structure starts to form just after constant area collapse, where in the lowest molecular layer, in contact with the substrate, molecules are in asymmetric configuration, i.e., both hydrocarbon tails are on the same side of the metal-bearing headgroup that touches the substrate, while the molecules above the first layer are in symmetric conformation of the tails with respect to the headgroups. Further along collapse, when the surface pressure starts to rise again with a decrease in area, more layers with molecules in the symmetric configuration are added, but the coverage is poor. On the other hand, only bimolecular layers form after constant pressure collapse, with the lower and upper layers having molecules in asymmetric and symmetric configurations, respectively, and the upper molecular layer density increases with compression of the monolayer after collapse. A "Ries mechanism" for constant area collapse and a "folding and sliding mechanism" for constant pressure collapse have been proposed.     

钙离子对表面活性剂单层膜聚集结构的影响

采用分子动力学模拟研究了气液界面上钙离子对阴离子表面活性剂十二烷基苯磺酸钠单层膜聚集结构的影响.结果表明,单层膜结构与表面覆盖度及Ca2＋离子存在与否均有关系.Ca2＋离子能够压缩表面活性剂极性头使聚集结构排列更加紧密,均力势体现了Ca2＋离子与极性头之间的结合能力强弱,二者之间的相互作用与稳定的溶剂分离极小值有关,而Ca2＋离子需要克服一个溶剂能障才能与之发生相互作用,并引起极性头周围水分子结构的重排.模拟表明,分子动力学方法可以在分子水平上研究无机盐离子对表面活性剂单层膜水化结构的影响,解释无机盐离子在界面膜中的动力学行为.     

Acid-catalyzed cyclization of acyliminium ions derived from allenamides. A new entry to protoberberines

Under mild acidic conditions, N-(ω-phenylalkyl)allenamides can yield stabilized acyliminium ions which through intramolecular aromatic electrophilic substitution furnish 1-vinylisoindolines and isoquinolines. Stereoelectronic and entropic effects in these cyclizations have been evaluated by DFT computations. The 1-vinylisoquinolines obtained have been employed as key intermediates in the synthesis of the protoberberine skeleton.