Ring substitution pattern, protonation

The structure of hydrol lignin has been examined by NMR spectroscopy. Tables of 8 values for types of protons found in hydrol lignin have been established, and 8 values for various types of aromatic ring substitution patterns have been clarified. 

Acid-catalyzed hydrolysis of A-aryloxaziridines may proceed via cleavage of either the C—O or N—O bonds, depending upon the ring substitution pattern. These reactions are usually discussed in terms of initial protonation at the oxaziridine oxygen atom <77JHC1393>. Acid-catalyzed isomerization to nitrones has also been reported <64HC(19)624>. 

The coupling constants of ortho ( Jhh = 7 Hz), meta Jhh =1-5 Hz) and para protons CJhh I Hz) in benzene and naphthalene ring systems are especially useful in structure elucidation (Table 2.5). With naphthalene and other condensed (hetero-) aromatics, a knowledge of zig zag coupling = 0.8 Hz) is helpful in deducing substitution patterns. 

The most recent application of 1,1-ADEQUATE of which the author is aware is the early 2011 report of Schraml et al.69 The isomeric S-(2-pyrrole) cysteine S-oxide (25) and S-(3-pyrrole)cysteine S-oxide (26) both have AMX proton spin systems with comparable coupling constants that do not allow differentiation of the substitution of the pyrrole ring. The 13C resonances of the two molecules are likewise quite similar and are also not amenable to the unequivocal assignment of the substitution pattern. In contrast, the Vcc derived connectivity information from the 1,1-ADEQUATE spectrum provides an unequivocal assignment of the substitution pattern for the isomeric structures. 

The splitting patterns of the protons in the aromatie region of the H spectrum are frequently used to establish the substitution pattern of an aromatic ring. For example, a trisubstituted aromatic ring has 3 remaining protons. There are 3 possible arrangements for the 3 protons - they can have relative positions 1,2,3- 1,2,4- or 1,3,5- and each has a characteristic splitting pattern. 

More powerful directing groups such as those based on amides and sulphonamides are successful with pyridines as with carboxylic rings, and will not be discussed separately. The enhanced acidity of pyridine ring protons makes the simple carboxylate substituent an ideal director of lithiation in pyridine systems . The pyridinecarboxylic acids 232-234 are deprotonated with BuLi and then lithiated with an excess of LiTMP all the substitution patterns are lithiated nicotinic acid 233 is lithiated in the 4-position (Scheme 113). The method provides a valuable way of introducing substituents into the picolinic, nicotinic and isonicotinic acid series. 

The structures of vanicosides A (1) and B (2) and hydropiperoside (3) were established primarily by one- and two-dimensional nuclear magnetic resonance (NMR) spectroscopy techniques and fast atom bombardment (FAB) mass spectrometry (MS).22 The presence of two different types of phenylpropanoid esters in 1 and 2 was established first through the proton (4H) NMR spectra which showed resonances for two different aromatic substitution patterns in the spectrum of each compound. Integration of the aromatic region defined these as three symmetrically substituted phenyl rings, due to three p-coumaryl moieties, and one 1,3,4-trisubstituted phenyl ring, due to a feruloyl ester. The presence of a sucrose backbone was established by two series of coupled protons between 3.2 and 5.7 ppm in the HNMR spectra, particularly the characteristic C-l (anomeric) and C-3 proton doublets... 

The substitution pattern for ring B is rather more variable but the usual aromatic couplings are observed. The appearance of the signals from ring C protons varies both with the location of the aryl ring B and with the oxidation state of the flavonoid. 

In a recent comprehensive study with respect to the substituent effects of benz anthracene carbocations,280 exclusive protonation at C(7) (102) and C(12) (103, bay region protonation) in the C ring was shown to occur. The relative stability of the resulting carbocations, however, strongly depends on the substitution pattern. Substrates methyl-substituted in the A ring give mixtures of the two cations. Protonation of 5-, 6-, and 7-methyl- and 7-ethyl-substituted compounds, in turn, yields... 

Substitution pattern of the ring-protonated porphyrins H2 (Por) and phthalo-cyanines H2 (Pc). In the quoted tetrasubstitued porphyrins the substituents are in 5,10,15,20 meso positions, in octasubstituted porphyrins they are in 2,3,7,8,12,13,17,18 positions. In tetrasubstituted phthalocyanines the substituents are in 2,9,16,23 positions, in octasubstituted complexes they are in 2,3,9,10,16,17,23,24 positions. Metal complexes are specified by replacing the two H atoms in H2 (Por) or H2 (Pc) by the metal atom with its axial ligands, if any. 

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