Conformational studies of Angiotensin II and analogues in dimethyl sulfoxide by 1H NMR: Lability and intramolecular interactions of the tyrosine hydroxyl and histidine imidazole NH protons

1H NMR studies in dimethyl sulfoxide illustrate that the tyrosine hydroxyl proton and the histidine imidazole NH proton in [Sar1]ANGII and analogues are labile at neutral pH but not at acid pH. This is attributable to intramolecular interactions of these groups with negatively charged groups. Methylation or elimination of the Tyr hydroxyl in [Sar1]ANGII and analogues, invokes a small but consistent deshielding effect on the His C-2 and C-4 protons, suggesting an interaction between the Tyr hydroxyl and the His ring. Nuclear Overhauser Effect (NOE) enhancement experiments in {Sar1]ANGII and the Sarmesin analogue [Des1, Tyr(Me)4]ANGII, support the Tyr/His interaction and confirm the presence of a trans His-Pro peptide bond.

29Si and 13C NMR spectra of some substituted bis(trimethylsiloxy)benzenes

NMR spectra of a few 4-substituted 1,2-bis(trimethylsiloxy)benzenes and positional isomers of bis(trimethylsiloxy)benzene were measured under standard conditions that approach those of infinite dilution. Under these conditions somewhat smaller values of steric or proximity deshielding effects on 29Si chemical shifts are found but the trends established earlier are confirmed; the deshielding effect of the ìsecondî ortho positioned trimethylsiloxy group in a tris(trimethylsiloxy)benzene derivative on 29Si chemical shifts is somewhat smaller. Also, overcrowding of three trimethylsiloxy groups probably drives the outer groups to assume, at least for a part of their time, coplanar conformation which is not encountered in bis(trimethylsiloxy) derivatives. The sensitivity of 29Si chemical shifts to para substitution is in 1,2-bis(trimethylsiloxy)benzenes equal to that in 2-methoxytrimethylsiloxybenzenes.

Beiträge zur Chemie des Bors, 124 [1] Tris(trimethylsilyl)silyl-borane und Tris(trimethylsilyl)silyl-borate / Contributions to the Chemistry of Boron, 124 [1] Tris(trimethylsilyl)silyl Boranes and Tris(trimethylsilyl)silyl Borates

The reactions of several boron halides, methoxides, alkyls and hydrides with tris(tri-methylsilyl)silyllithium, which was isolated as the solvate [(CH3)3Si]3SiLi · 3C4H8O, 1(TMSSLi) have been studied. (CH3)2B−Si[Si(CH3)3]3 and 9−[(CH3)3Si]3Si-9-BBN, were isolated, while [(CH3)3Si]3Si-B(tC4H9)2 could only be detected by 11B NMR. In addition TMSS-B[N(CH3)2]2, (TMSS)2BNMe2 and (TMSS)2BOCH3 were prepared. The methoxy-boranes (CH3)3-nB(OCH3)n add to 1 forming silylborates; however, no OCH3/Si[Si(CH3)3]3 substitution occurs. The hydrogen bridge in 9-BBN is cleaved symmetrically. The results can be explained by the basicity and the steric requirements of the TMSS group. The TMSS group exhibits a deshielding effect at the boron nucleus relative to the (CH3)3Si group for silylboranes as well as for silylborates.

Sporidesmins. XIX. The diazotization and coupling in acid of p-Nitro- and 2,4-Dinitro-aniline to veratrole or guaiacol: 1H N.M.R. of the products in deuterochloroform and deuteroacetone

p-Nitrobenzenediazonium chloride in conc. HCl coupled to guaiacol (2-methoxyphenol) to give 4-hydroxy-3-methoxy-4'-nitroazobenzene, but did not couple to veratrole (1,2-dimethoxybenzene). 2,4-Dinitroaniline, diazotized in conc. HCl with no urea added, yielded 2-chloro-4,5-dimethoxy- 2',4'-dinitroazobenzene with neat veratrole. However, 2,4-dinitroaniline, diazotized in conc. HCl and then treated with urea, yielded 2-chloro-4-nitrobenzenediazonium chloride which again coupled in acid with guaiacol to give 2-chloro-4'-hydroxy-3'-methoxy-4-nitroazobenzene but not with veratrole in acetic acid. Veratrole added to 2,4-dinitrobenzenediazonium phosphate or trichloroacetate in acid media yielded 3',4'-dimethoxy-2,4-dinitroazobenzene. These 2,4-dinitrobenzenediazonium salts coupled to guaiacol giving 4'-hydroxy-3'-methoxy-2,4-dinitroazobenzene. p-Nitrobenzenediazonium and 2-chloro-4-nitrobenzenediazonium salts in acid gave the Gomberg reaction with veratrole forming the corresponding biphenyls. The 1H n.m.r. spectra of all the products obtained in this investigation and the deshielding effect of deuteroacetone on four of the compounds are discussed. The deuterochloroform spectrum of 4'-hydroxy-3'-methoxy-2,4-dinitroazobenzene at 60 MHz shows an anomalous set of 26 or more lines that are not attributable to a quinone hydrazone structure.

Substituent effects on the 13C chemical shifts of monosubstituted twistanes

The 13C chemical shift data for some hydroxy, chloro, bromo, and oxo twistane derivatives are presented and compared to the shifts observed in corresponding adamantanes. The substituent effect at the α and β carbons is more pronounced in twistanes than in adamantanes. The substituent shift induced at an antiperiplanar γ carbon is shown to depend on the presence or absence of 1,3-diaxial hydrogen–hydrogen interactions between the substituted and γ carbons. If such an interaction is present the effect is of shielding whereas if it is absent, the effect is of deshielding. The deshielding effect appears to occur via a through bond interaction.

Additivity of deshielding effects in the 1H N.M.R. spectra of some exo-Triazatricyclo[5,2,1,02,6]decenes and exo-Azatricyclo[3,2,1,02,4]octanes

The presence of an endo-chloro substituent at C8 (or C9) of 5-phenyl-exo-3,4,5-triazatricyclo-[5,2,1,02,6]dec-3-enes and at C 6 (or C 7) of 3- phenyl-exo-3-azatricyclo[3,2,1,02,4]octanes causes significant deshielding of the endo-bridgehead protons. The through-space deshielding effect is remarkably additive in cis,endo-dichloro analogues.

The stereochemistry of organometallic compounds. XIV. trans-Effects in the 13C N.M.R. spectra of Dicarbonyltriphenyl-phosphine- and -arsine-(arene)chromium compounds

The 13C chemical shifts of some triphenyl-phosphine and -arsine dicarbonyl(arene)chromiums are reported. The shifts are discussed in terms of a deshielding effect on the arene ring carbons trans to the Group 5 atom in the preferred conformations of these molecules.