Structural assignment

Structural assignments (constitutions and configurations,of substituents) are based on chemical conversions to known compounds and on spectroscopical measurements, mainly NMR. Not each single compound shall be discussed but rather some characteristic examples and coherences. 

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By assuming that the heat of combustion of the cis isomer was larger than the trans structural assignments were made many years ago for the stereoisomenc 2 3 and 4 methylcyclohexanols This assumption is valid for two of the stereoisomenc pairs but is incorrect for the other For which pair of stereoisomers is the assumption incorrect Why" ... 

The UV spectra have been used in studies of protonation and related covalent hydration, structural assignments and tautomerism (see appropriate Sections), as well as in studies of bridgehead addition to 5-deazapterins (79MI21500, 78TL2271) and related 5-deazaflavin derivatives (80JA1092). 

Studies on covalent hydration of N-heterocycles (67AG(E)919,76AHC(20)117) have revealed the diagnostic value of alkyl substituents in structural assignments due to their steric hindrance effects in addition reactions. C-Methyl substituents are therefore also considered as molecular probes to solve fine-structural problems in the pteridine field. The derivatives... 

The fragmentation pattern of isoxazoles on electron impact has been well studied. It has been used as an important tool for the structural assignment of isoxazoles obtained from the reaction of chromones with hydroxylamine 79MI41600, 77JOC1356). For example, the structures of the isoxazoles (387) and (388) were assigned on the basis of their fragmentation patterns. Ions at mje 121 (100%) and mje 93 (19.8%) were expected, and indeed observed, for the isoxazole (388), and an ion at mje 132 (39.5%>) was similarly predicted and observed for the isoxazole (387). 

I-Cyano-3-phenylurea, first obtained by the alkaline hydrolysis of 5-anilino-3- -toluyl-l,2,4-oxadiazole, has been prepared by tlic condensation of phenyl isocyanate and the sodium salt of cyanamide. However, in these publications an incorrect structural assignment for the product was made. 1-Cyano-3-phenyl-urea is obtained also, together with other products, by warming gently l-cyano-3-phenylthiourea with caustic soda in the presence of ethylene chlorohydrin, or by gradually adding caustic )otash to a boiling solution of 1-phenyldithiobiuret and ethylene clilorohydrin in ethanol. ... 

A class of 5,6,15- and 5,14,15-trioxygenated metabolites of arachidonate has been described by Samuelsson et al. Two of these compounds, termed lipoxins A and B, have recently been assigned the structures shown below. 1 Outlined below is a synthesis of a putative biosynthetic precursor of these compounds. Syntheses of the structures assigned to lipoxins A and B have also been accomplished.3.4... 

A number of azacrown compounds were reported by Lockhart and coworkers in 1973. In a typical case, 2-aminophenol was heated at reflux with an equivalent of tet-raethylene dichloride for 48 h. A mixture of the monoaza-15-crown-5 (5) and the mono-aza-12-crown-4 (4) shown in Eq. (4.4) were obtained. Structural assignments were based on H-NMR, IR and mass spectral data. 

The bromination of 4,5-j -dihydrocortisone acetate in buffered acetic acid does not proceed very cleanly (<70%) and, in an attempt to improve this step in the cortisone synthesis, Holysz ° investigated the use of dimethylformamide (DMF) as a solvent for bromination. Improved yields were obtained (although in retrospect the homogeneity and structural assignments of some products seem questionable.) It was also observed that the combination of certain metal halides, particularly lithium chloride and bromide in hot DMF was specially effective in dehydrobromination of 4-bromodihydrocortisone acetate. Other amide solvents such as dimethylacetamide (DMA) and A-formylpiperidine can be used in place of DMF. It became apparent later that this method of dehydrobromination is also prone to produce isomeric unsaturated ketones. When applied to 2,4-dibromo-3-ketones, a substantial amount of the A -isomer is formed. 

Careful spectroscopic (infrared and NMR) examination of these compounds should result in clarifying the structural assignments. 

The addition of p-quinone to enamines normally produces furan derivatives, especially when the enamine possesses a 3 hydrogen (see Section III. A). 1,2 Cycloaddition is claimed to take place to give a cyclobutane derivative when p-quinone and an enamine with no jS hydrogens are allowed to react at low temperatures (51). However, little evidence is reported to verify this structural assignment, and the actual structure probably is a benzofuranol (52). Reaction of a dienamine (formed in situ) with p-quinone in the presence... 

The mass spectra of enamines (215,216), and optical rotatory dispersions (217) of optically active enamines have also been used for structural assignments. 

Ultraviolet absorptions ofvinylogous lactams were found by MOLCAO calculations and compared with experimental values (663). Infrared spectroscopic studies of vinylogous amides (664) and some fifty vinylogous urethanes (665) allowed configurational and structural assignments. The effect of enamine-imine equilibrium in a series of benzophenone derivatives was established (666) and the effect of structure on enamine basicity studied (667). 

Raney nickel desulfurization of the macrocyclic ketones or their reduction products confirmed the structures assigned to these compounds and simultaneously offers new methods for the synthesis of macrocyclic alicyclic compounds. =°... 

The ultraviolet spectra of many substituted aminopyridines were reported in 1959 with rather vague reference to their tautomerism however, the gross structure assigned to some of the sulfonamido compounds has been criticized. ... 

The early literature on the reactions of the indole Grignard reagents with the simple diacid chlorides, in particular with carbonyl chloride and oxalyl chloride (see Section III,C,4,b), is both conflicting and confusing and much of the work reported warrants repetition since the evidence presented in support of many of the structural assignments made is not entirel3 convincing. 

P3Timidines, and pyrido[4,3-< ]pyrimidines. The experimentally determined values have been used for studies of covalent hydration, structural assignments, - and tautomerism. -... 

Nuclear magnetic resonance spectra of all four parent compounds have been measured and analyzed.The powerful potentialities of NMR as a tool in the study of covalent hydration, tautomerism, or protonation have, however, as yet received no consideration for the pyridopyrimidines. NMR spectra have been used to distinguish between pyrido[3,2-d]pyrimidines. and isomeric N-bridgehead compounds such as pyrimido[l,2- ]pyrimidines and in several other structural assignments (cf. 74 and 75). 

Buckles et al. suggested tentative structural assignments for 53a and 53b and their respective benzamido acids on the basis of ultraviolet spectral data and by comparison of physical properties with those of model compounds. They pointed out that it is not possible to establish structural relationships from configurations of the diastereomeric 2-benzamido-3-methoxy-3-phenylpropionic acids (54), each of which, on treatment with acetic anhydride, give mixtures of the azlactones. Similar observations have been made by others. ... 

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