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1.
A method based on the differences among the 1H NMR chemical shifts of geminal protons of ring‐F methylene resonances (H2‐23, H2‐24 and H2‐26) is proposed for ascertaining the 25R/25S stereochemistry of ring‐F unsubstituted spirostane‐type steroidal sapogenins and steroidal saponins. Copyright © 2003 John Wiley & Sons, Ltd.  相似文献   

2.
A series of substituted hexahydropyrido [2,1-c] [1,4] oxazin-3(4H)-ones has been synthesised, and the configurations of these bicyclic lactones assigned utilising chemical and spectral data. All the compounds adopt trans-fused conformations and the conformation of the lactone ring is discussed with reference to the magnitude of the geminal coupling constant of the N? CH2? C(O)? O protons, and the vicinal couplings between the angular proton and the methylene protons adjacent to the ring oxygen atom. The lactone ring conformation is shown to differ slightly from the half chair conformation described for some monocyclic δ -lactones. The synthesis and NMR spectra of some related compounds possessing the bridgehead N? CH2? C(O)? O system are discussed and these compounds are also shown to adopt a trans-fused ring conformation.  相似文献   

3.
Examples are given of the conformationally dependent influence of 3-substituents (methyl, n-propyl, allyl, aryl, hydroxy and acyloxy) on the magnetic non-equivalence of the benzylic methylene protons (NCH2Ph) of N-benzylpiperidine as observable by 1H NMR. The results, together with data already reported, emphasise the value of 1H NMR studies of N-benzyl derivatives in stereochemical investigations of substituted piperidines.  相似文献   

4.
Magnetic non-equivalence of methylenic protons in compounds of the type HO2CCH2SR′ and RO2CCH2SR′ were studies and the influence of substitution discussed. Furthermore, it was found that the methylene geminal coupling constant, JAB, is almost independent of the constitution of the compounds and the solvents used. A variation of the magnetic non-equivalence, Δv, of the methylenic protons with the constitution of the compounds and the solvents used is, however, observed.  相似文献   

5.
The 1H and 19F NMR spectra of the CH2FCHF group in FCH2CHFCO2C2H5 were analysed on the basis of an ABCXY spin system. The non-equivalence of the methylene protons is discussed in terms of rotational isomerism. It is concluded that the dominant rotamer is probably considerably distorted from a perfectly staggered orientation. The variation of 3JHF with substituent electronegativity is reviewed.  相似文献   

6.
In phosphinous esters of general formula R? PH(O)OR', the asymmetric phosphorus atom induces magnetic non-equivalence in geminal protons and methyl groups. When alkoxy groups also possess an asymmetric carbon, the presence of the two possible diastereoisomers in equal amounts is proven by 31P and 1H NMR. 31P NMR reveals that the phosphorus atom in phosphonous esters R? P(O·C4H9-s)2 of racemic sec. butyl alcohol, is a centre of pseudo-asymmetry.  相似文献   

7.
13C NMR spectra of ms-tetraphenylchlorins, new aminoalkyl- and hydroxy-pyrroline substituted ms-tetra-phenylchlorins, and ms-tetraphenylisobacteriochlorins are presented and discussed. Significant changes in the chemical shifts of the α-pyrrole, α-pyrroline and meso skeletal carbons are found in chlorins and isobacteriochlorins in comparison to porphyrins. In contrast, the chemical shifts of the β-pyrrole carbons are almost unaffected by the structural modifications in chlorin and isobacteriochlorin. The chemical shifts of the α-pyrrole carbons in the various chlorins and isobacteriochlorins are strongly affected by the substitutents of the pyrroline ring, or by the introduction of an additional pyrroline ring in isobacteriochlorins. The results show that most of the electron density is concentrated in the unreduced part of the molecule, i.e. in the pyrrole rings and, especially, on the α-pyrrole carbons and is transferred or removed through the aromatic pathway by substituents on the pyrroline ring. These observations are supporting evidence that the α-pyrrole, α-pyrroline and meso-carbons are in the aromatic pathway and favour the proposal of a 16 atom dianion as the preferred delocalization pathway in chlorins and isobacteriochlorins.  相似文献   

8.
The three stereoisomers of 2,4,6-tricyanoheptane were separated and their NMR spectra were studied as the three-unit model compounds of polyacrylonitrile. The chemical shifts and coupling constants obtained from the NMR spectra were compared to those of the two-units model compounds. The geminal protons for the isotactic methylene are not equivalent, but the difference of the chemical shifts is smaller than that in the meso two-unit model. The racemic methylene signals appear at higher field than the meso methylene in the heterotactic three-unit model as in the case of the racemic and the meso two-unit models. The signals of the methinic protons in triads appear in the order: isotactic CH, heterotactic CH, and syndiotactic CH from the high field. From the observed values of the vicinal coupling constants, the chain conformations of the model compounds are also discussed.  相似文献   

9.
Solvent effects on the 1H NMR parameters of perhydro-oxazolo[3,4?a]pyridine, perhydropyrido[1,2?c]?[1,3]oxazine and perhydrothiazolo[3,4?a]pyridine derivatives are described. Particularly marked are changes in the proton-proton geminal coupling constants of the NCH2O and NCH2S methylene protons with solvent and these are attributed to reaction field and solvation effects rather than to changes in the position of conformational equilibria.  相似文献   

10.
The 250 MHz 1H NMR spectrum of L -proline is comprehensively analysed by computer simulation for different pH values. A fast endoexo interconversion has already been proposed from chemical shift and coupling constant data. A critical comparison of the vicinal coupling constants leads to the conclusion that in basic solution the equilibrium is shifted towards a more endo conformation. The position of the carboxylic plane in the ring space is given by the relative intramolecular chemical shift of the geminal protons and by the typical patterns of the Hα multiplet. These results are applied to the analysis of the spectra of Pro? NH2 and Ac? Pro? NH2.  相似文献   

11.
Magnetic non-equivalence of methylenic protons in A? CH2? CH3 groups were studied for several types of compounds. Results are compared with respect (1) to the nature of heteroatom A:A = O and A = S and (2) to the structure of the local dissymmetry element: asymmetric carbon, ketal group and allenic system. Influence of substitution is discussed in relation to conformational problems. Furthermore a variation of the methylene geminal coupling constant with substituents of carbon α to OEt is observed.  相似文献   

12.
Some 2-arylsulphonylamido-2-thiono-5-methyl-1,3,2-dioxaphospholanes (1), containing two chiral centres, give NMR spectra in which splittings of the 1H and 13C signals of the 5-methyl substituent and of the 31P signal indicate that they exist in an approximately 70:30% ratio of two racemic mixtures, Z and E respectively, diastereomeric to each other. These splittings were invariably observed for compounds 1a, containing a monosubstituted sulphonylamido group (Y = H) and various substituents at position 4′ of the aryl group, X = H, CH3, OCH3, F, CI and Br. The methylenic protons in position 4 of the phospholane ring are diastereotopic and therefore magnetically non-equivalent. Benzene-induced shifts were measured for the Z isomers of compounds 1a, X = H, F and for compound 1b, containing an N-disubstituted sulphonylamido group (Y = CH3) and X = H; the tentative interpretation of these shifts places the more shielded diastereotopic methylene proton on the same side of the ring as the 5-methyl substituent. The H? P and H? H coupling constants indicate that the preferred conformation of the dioxaphospholane ring should be a ‘twisted envelopey’ shape with the 5-methyl substituent in the equatorial position.  相似文献   

13.
A collection of 33 hydrodipyrrins (9 targets, 21 intermediates, and 3 byproducts) has been prepared. The hydrodipyrrins (dihydrodipyrrins, tetrahydrodipyrrins, and hexahydrodipyrrins) contain a pyrrole ring and a geminal-dimethyl substituted 1-pyrroline (or pyrrolidine) ring. The α-substituents on the pyrrole ring (H, Br, CHO) and pyrroline ring (H, CH3, CH(OR)2, OMe, SMe) provide different reactivity combinations (Nu, E+) and 0, 1, or 2 carbon atoms (which can give rise to the bridging meso-carbons in hydroporphyrins). Straightforward access to various hydrodipyrrins should facilitate development of syntheses of diverse hydroporphyrins.  相似文献   

14.
Small discrepancies between 3H and 1H chemical shifts in organic compounds led to the finding that the ratio of Larmor frequencies ωTH depends on the carbon-hydrogen bond hybridisation. The ‘best’ ratio for ghost referencing of 3H NMR spectra was then determined from measurements on purified partially tritiated TMS as 1.066639738±2 × 10?9. Some 3H isotope effects on chemical shifts are listed and the 3H isotope effect on the methylene geminal coupling constant in benzyl methyl sulphoxide is measured. Use of 2H in the measurement of 2J(HH) coupling constants has inherent disadvantages, overcome by use of 3H substitution.  相似文献   

15.
The chemical shift non-equivalence of the methylene protons adjacent to the quaternary nitrogen in compounds of the type has been investigated. The behaviour of the examined compounds is interpreted in terms of different rotation rates around the three C? N bonds. The vicinal and geminal coupling constants of the non-equivalent methylene protons are also reported and discussed in terms of steric factors.  相似文献   

16.
The 251 MHz 1H NMR spectra of 3-azabicyclo[3.2.2]nonane (1) have been measured from ?40 to ?175°C. Below about ?140°C a conformational process which equilibrates the methylene protons of the CH2NCH2 moiety of 1 becomes slow on the NMR time scale. The free-energy of activation (ΔG#) for this process is 5.9 kcal mol?1. The results can be interpreted either in terms of ring inversion of the seven-membered ring or limited pseudorotation of the six-membered ring. Possible pathways for effecting conformational equilibration in 1 are discussed.  相似文献   

17.
In the 1H NMR spectra of the 1‐vinylpyrroles with amino‐ and alkylsulfanyl groups in 5 and 2 positions, an extraordinarily large difference between resonance positions of the HA and HB terminal methylene protons of the vinyl group is discovered. Also, the one‐bond 1J(Cβ,HB) coupling constant is surprisingly greater than the 1J(Cβ,HA) coupling constant in pyrroles under investigation, while in all known cases, there was a reverse relationship between these coupling constants. These spectral anomalies are substantiated by quantum chemical calculations. The calculations show that the amine nitrogen lone pair is removed from the conjugation with the π‐system of the pyrrole ring so that it is directed toward the HB hydrogen. These factors are favorable to the emergence of the intramolecular C–HB???N hydrogen bonding in the s‐cis(N) conformation. On the other hand, the spatial proximity of the sulfur to the HB hydrogen provides an opportunity of the intramolecular C–HB???S hydrogen bonding in the s‐cis(S) conformation. Presence of the hydrogen bond critical points as well as ring critical point for corresponding chelate ring revealed by a quantum theory of atoms in molecules (QTAIM) approach confirms the existence of the weak intramolecular C–H???N and C–H???S hydrogen bonding. Therefore, an unusual high‐frequency shift of the HB signal and the increase in the 1J(Cβ,HB) coupling constant can be explained by the effects of hydrogen bonding. Copyright © 2013 John Wiley & Sons, Ltd.  相似文献   

18.
meso and dl Dimers (ArCHOR)2 where R is Me, Et, iPr, tBu, cyclohexyl and 1-adamantyl may readily be differentiated by their NMR spectra; the benzylic protons of the meso isomer always absorb at a slightly higher field than those of the dl isomer in each of the solvents used. Differences in chemical shift are discussed in terms of preferences in conformer distribution. The formation of equal amounts of both dimers from the corresponding radical ArCHOR shows that steric and polar factors are not important in influencing the dimerization. Magnetic non-equivalence due to the presence of asymmetric centres was found in some of the compounds discussed above.  相似文献   

19.
Some new phosphoramidates were synthesized and characterized by 1H, 13C, 31P NMR, IR spectroscopy and elemental analysis. The structures of CF3C(O)N(H)P(O)[N(CH3)(CH2C6H5)]2 ( 1 ) and 4‐NO2‐C6H4N(H)P(O)[4‐CH3‐NC5H9]2 ( 6 ) were confirmed by X‐ray single crystal determination. Compound 1 forms a centrosymmetric dimer and compound 6 forms a polymeric zigzag chain, both via ‐N‐H…O=P‐ intermolecular hydrogen bonds. Also, weak C‐H…F and C‐H…O hydrogen bonds were observed in compounds 1 and 6 , respectively. 13C NMR spectra were used for study of 2J(P,C) and 3J(P,C) coupling constants that were showed in the molecules containing N(C2H5)2 and N(C2H5)(CH2C6H5) moieties, 2J(P,C)>3J(P,C). A contrast result was obtained for the compounds involving a five‐membered ring aliphatic amine group, NC4H8. 2J(P,C) for N(C2H5)2 moiety and in NC4H8 are nearly the same, but 3J(P, C) values are larger than those in molecules with a pyrrolidinyl ring. This comparison was done for compounds with six and seven‐membered ring amine groups. In compounds with formula XP(O)[N(CH2R)(CH2C6H5)]2, 2J(P,CH2)benzylic>2J(P,CH2)aliphatic, in an agreement with our previous study.  相似文献   

20.
Meso-tetracinnamylporphyrin (H2tcp) has been synthesized and characterized by UV–Vis, 1H NMR and IR spectroscopies. Protonation of H2tcp with HCl is accompanied by a blue shift (77 cm−1) of the Q(0,0) band, probably indicating a decrease in the efficiency of the π-electron donation from the meso-substituents to the a2u orbital in comparison with the corresponding dication of meso-tetraphenylporphyrin (H2tpp). The unusual NH signal multiplicity observed in 1H NMR spectrum of H2tcp shows evidence of a long distance coupling between the vinyl protons and NH ones. This long range coupling can occur between the remote centers using the a2u orbital as a connector. Mntcp(OAc) shows higher catalytic activity (ca. 1.2-fold higher) than Mntpp(OAc) in olefin epoxidation with tetra-n-butylammonium hydrogen monopersulfate (TBAO). The low ratio (∼1.5) of cis- to trans-stilbene oxide, in competitive oxidation of cis- and trans-stilbene with TBAO, strongly suggests the involvement a common high-valent manganese oxo complex as the reactive intermediate responsible for oxygen atom transfer.  相似文献   

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