Structure proof

Imino-A-4-thiazolines (27) when treated with hydrochloric acid give the A-4-thiazoline-2-ones (28) (Scheme 12). This reaction has been used as a structural proof of the parent compounds (19, 27, 28). 

Nitration of a series of methyl-1,2-benzisoxazoles was studied by Tahkar and Bhawal using fuming nitric acid and sulfuric acid in acetic acid at 100 °C. 3-Methyl-1,2-benzisoxazole gave a mixture of 5-nitro- and 5,7-dinitro-3-methyl-l,2-benzisoxazole, with the 5-nitro isomer predominant. The product obtained from 3,5-dimethyl-1,2-benzisoxazole was the 4-nitro derivative and not the 7-nitro compound as proposed by Lindemann (26LA(449)63). The synthesis of the 7-nitro compound by an alternative method was used as structural proof. Two products were obtained from 3,6-dimethyl-l,2-benzisoxazole and these were the 5-nitro and 5,7-dinitro derivatives. 3,7-Dimethyl-l,2-benzisoxazole was converted into the 5-nitro derivative (Scheme 25) (77lJC(B)l06l). 

There are only few reactions known introducing substituents to the H-bearing nitrogen of oxaziridines. (V-Alkylation of l-oxa-2-azaspiro[2.5]octane (3,3-pentamethylene-oxaziridine 52) with r-butyl chloride to give (53) was carried out for structure proof of (52). This reaction is of no preparative importance, since N-alkylated oxaziridines are easily obtained by ring synthesis. 

All of these methods provide only moderate yields of 17-keto steroids, but some [particularly (b)] have been considered for large scale preparative work. The advent of superior methods now limits the above procedures to structure proof and degradative studies. 

Correct structure proofs were earlier difficult to obtain, usually involving several reaction steps, and the structures were, therefore, assigned by analogy in many cases. However, through Raney nickel desulfurization (cf. Section VII,C), through NMR spectroscopy,and through polarographic analysis such problems are now easily solved. 

It has been shown that 2,3-thiophenedicarboxylic acid is preferentially esterified in the 2-position and the dimethyl ester is preferentially hydrolyzed in this position. The structure proof was difficult to achieve as rearrangements occurred. Thus both isomeric amides (195) and (196) were decarboxylized to the N-methylanilide of 3-thiophenecarboxylic acid (197). The same carbomethoxy benzoyl-thiophene, proved to be 2-carbomethoxy-3-benzoylthiophene (198),... 

Dimethyl acetylenedicarboxylate and 4-methylimidazole are re-ported to yield dimethyl 5-methyl-2- (70) or -4-imidazolylmaleate a fumarate might perhaps have been expected and no structure proof was given. 

Structural proof has been offered by chlorination, thionation, and reduction to yield the pyrido[2,3-d]pyrimidine (74) which has been subjected to NMR analysis. ... 

Tire reaction of the lead dithiolate 18 with elemental sulfur was claimed, without unambiguous structure proof, to give a dithiirane 19 in 75% yield (96ZOR1881). 

Coal, structure of, 517 Coal tar, compounds from, 517 Cocaine, specific rotation of, 296 structure of. 64, 916 structure proof of. 875 synthesis of, 915... 

As an example of the fascinating current progress, Dr. Spencer will describe a beautiful piece of research on the isolation and structure proof of the toxin from Helminthosporium sativum, an important... 

As a result of compelling three-dimensional models and remarkably high levels of precision, it is often assumed that structural elucidation by single crystal X-ray diffraction is the ultimate structural proof. Spatial information in the form of several thousands of X-ray reflection intensities are used to solve the position of a few dozen atoms so that the solution of a structure by X-ray diffraction methods is highly overdetermined, with a statistically significant precision up to a few picometers. With precise atomic positions, structural parameters in the form of bond distances, bond... 

Degradative studies were carried out on cryptopleurospermine (159) and saxoguattine (160) to complete the structure proof of these alkaloids (136,137). These bases were first reduced to diols 178 and 179, respectively, and then oxidized to produce two aldehydes, isovanillin and compounds 49 or 156 from 159 or 160, respectively. 

In their pursuit of modeling Type I copper proteins, Kitajima et al. reported112 a rare, tetrahedrally coordinated complex (105), which displayed an EPR spectrum consistent with the presence of the unpaired electron in the dz2 orbital.1 They also isolated a square-pyramidal DMF adduct (complex (106)). They were successful in providing structural proof of a copper(II) complex (trigonal pyramidal) with C6F5S -coordinated complex (107), with CuN3S chromo-phore.113 The X-ray analysis (poor data set) of a closely similar complex with Ph3CS as the... 

During the course of modeling copper dioxygen chemistry, Kitajima et al. reported the synthesis of a yu-peroxo dinuclear complex with a 3,5-dimethyl-substituted tris(pyrazolyl)borate ligand, which showed remarkable physicochemical similarities to oxy-Hc and oxy-Tyr. Using a 3,5-di-isopropyl-substituted terminal ligand, they provided the first structural proof of the existence of peroxo dicopper(II) core (108) (copper geometry distorted square pyrami-... 

As has already been discussed (Section m.B.3) we were able to demonstrate that the three C3H2 isomers cyclopropenylidene (2), propargylene (36), and vinylidenecarbene (37), interconvert photochemically in low-temperature matrices. Unlike 36 vinylidenecarbene (37) was predicted to be a singlet.108-110 To aid the spectroscopic identification of S-37 we calculated (MP2/6-31G ) IR frequencies and intensities of this species.26 Comparison with the experimental IR spectrum (most intense band at 1952 cm-1) confirmed the allenic structure S-37. For T-37 a completely different IR spectrum was expected. An additional structural proof for S-37 was its reversible transformation into the other two C3H2 isomers S-2 and T-36. 

Other than classical H and 13C NMR spectra which have been reported for these compounds, no detailed studies have appeared in the literature. One X-ray crystal analysis of the TBDMS-protected alcohol analog of acid 368 has been carried out for structural proof. 

A rigorous structural proof of the insecticidal exotoxin (34) from Bacillus thurin-giensis has now been published,107 confirming the a-configuration of the glucosidic bond. The total synthesis of (34) is further confirmation of the correctness of the structural assignment.108 The exotoxin inhibits RNA synthesis in insects and animals and affects the incorporation of orotic acid into nuclear RNA.109... 

The tabulation shows that the t-butyl system is a reasonable model for some equilibrating ions. It fails badly, however, when applied to the norbornyl compounds. The isopropyl system is a poor model for sec-butyl and cyclopentyl ions and is a very poor model for the norbornyl cation. The failure of the models to provide reasonable estimates of the shifts in the tertiary norbornyl cations which are undergoing either VVagner-Meerwein shifts or hydride migration makes it clear that the experimentad shifts in the secondary system cannot be used as structural proofs. Rather they should be regarded as fascinating results to be rationadized in terms of the structure, whatever it may be. 

Various adducts with Lewis acidic elements or ions of the coordination mode I were also described [61], but the first structural proofs were reported by the group of Schmidbaur with the Lewis acids L = S, Se [79, 80], and E [81, 82]. Further adducts of the type I with the main group Lewis acids AlBrs and lnMe3 were reported recently and confirmed by X-ray analyses [71]. 

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