Assignment of Structure

Quite often, the absorption data of the derivatives or degradation products of a compound of unknown structure have lead to the ultimate structure determination of the parent compound. Recently, the degradation product of vitamin Big, 5 6-dimethylbenzimmazoIe, was identified by its ultra-violet absorption spectrum4. The structure of vitamin Bx (thiamine) was elucidated by comparison of the spectra of its sodium sulphite fission products with the spectra of model compounds11. The crude structure of vitamin Kx was found by the use of spectral data. The absorption spectrum of vitamin IC, has the following bands 249 my (e 19,000), 260 mju. (f 18,000) and 325 my 

Vitamin A gives a blue colour with antimony trichloride in chloroform. This blue solution showed two well-defined maxima around 585 and 620 my and another weak band around 695 my, With fiver oil from fresh-water fish, the 695 my was prominent and the 620 my band was not observed. This indicated the presence of two different species of vitamin A, Ax and A., (il and m)s. The near ultra-violet absorption spectra of viiamins Ax and A2 are also different. The longer wavelength band of A2 is probably due to an additional double bond in the conjugated.system, HI. The 287 my baud of vitamin Aa has been ascribed to the diene partial chromophore 93. 

The spectrophotometric analysis of vitamin A has now replaced the tedious biological method of assay. Other important examples where ultra-violet spectroscopy has replaced the biological methods of assay are vitamin D (four peaks around 280 m/x) and vitamin K. 

Electronic absorption spectroscopy did not provide any direct evidence for the characterization of penicillins, since they show little absorption in the. near ultra-violet or visible regions which can be associated with chromo-phoric groups. However, ultra-violet absorption spectra were useful in solving the structures of some of the degradation products7. On reaction with alcohols, penicillins give a-esters of penidlloates, IV, which are quantitatively converted to penamaldates, V, in the presence of alcoholic mercuric OO 

Although the correct structural formula had been intuitively proposed earlier, the structure of strychnine was founded on a firm basis only when Woodward 6t al.8 discovered that strychnone obtained from the oxidation of IP-strychnine gave ultra-violet absorption maxima resembling those of N-acylindoles. Strychnine, oh the other hand, gives bands similar to N-acyldihydroindoles. Woodward and McLamore recently established the structure of another alkaloid, semperivinine, on the basis of its ultra-violet absorption spectrum. (For absorption data of alkaloids. see references 94 and 95.) 

---

Retrosynthetic analysis of antheridic acid produced a totally different plan of synthesis from that which had been employed for the structurally related target gibberellic acid. The synthesis of antheridic acid, which included a number of novel steps, allowed definitive assignment of structure and revised stereochemistry at C(3). 

HOAt, 7-aza-l-hydroxybenzotriazole HATU (CAS Registry No. 148893-10-1), A-[(dimethylamino) (3//-1,2,3-triazolo(4,5- )pyridin-3-yloxy)methylene]-A-methyl-methanaminium hexafluorophosphate, previously known as G-(7-azabenzotriazol-l-yl)-l,l,3,3-tetramethyluronium hexafluorophosphate. [Note Assignment of structure to HATU as a guanidinium species rather than as a uronium species, i.e., attachment of the (Mc2NC=NMe2) unit to N3 of 7-azabenzotriazole 1-A-oxide instead of to the O, is based on X-ray analysis (ref. 33b)]. 

O. Ponzio, Oazz. Chim. Ital. 36 II, 316 (1906). The authors of Ref. 58 were responsible for the assignment of structure 47 to the product, on very slender evidence. 

Heating phenyl isocyanate or its derivatives in the presence of Lewis acids reportedly gives diazocinediones 42 however, the assignment of structure 2 to the product from phenyl isocyanate is questionable as the melting point (mp 279-280 C)42 differs markedly from the generally agreed literature value (mp 334 C)43 (see also Section 1.5.1.2.3.). 

Structure of Oxy-F Compound F is extremely unstable and is difficult to obtain at a level of purity suitable for NMR studies. However, an oxidation product, Oxy-F, formed when F is left standing at — 20° C, is considerably more stable than F and can be purified to a sufficiently high level of purity. Oxy-F is nonfluorescent and shows absorption maxima at 237 nm and 275 nm (shoulder). The high-resolution FAB mass spectrum indicated the molecular formula of Oxy-F to be C33H3809N4Na2 [m/z 703.2363 (M + Na)+ and 681.2483 (M + H)"1"]. The H and 13C NMR data allowed the assignment of structure 7 to oxy-F (Fig. 3.2.6 Nakamura et al., 1988). 

The theory of valency is equivalent to the theory of structural chemistry. This theory has now passed through the first stage of its modern development, that of the formal assignment of structures to chemical substances, and is entering upon its second stage, the development of a system permitting the prediction of the approximate thermodynamic stabilities corresponding... 

Storage products derived from complex glycoproteins have been identified in the brains of Springer spaniels suilering from a progressive nervous disorder. F.a.b.-m.s. of the native compounds and their acetylated and permethylated derivatives allowed the assignment of structures 26, 27, and 28. 

Comparison of the NMR data of 18,54, and 55 in Table V and VI provides the main evidence leading to the assignment of structures 54 and 55 for hydrakouminol and neodihydrokoumine, respectively ... 

Because of the novel structure assigned to this compound, additional confirmatory data could prove useful in order to ensure that the original assignment of structure was correct. 

The paper devoted to the phosphetan synthesis26 describes optimum conditions for the cyclization process leading to (22), and explains the effects of other conditions in terms of competing reactions, such as alkene polymerization or addition of hydrogen chloride. Assignment of structure (23) to the product from hept-l-ene (24)... 

Other structural applications of ultraviolet spectra include the detection of the open-chain tautomers of 1-cyanotriazoles and assignments of structure to the isomers formed by Dimroth rearrangement of 1 -anilinotriazoles. ... 

Incorporating partial structures (a) and (b) into the ring and consideration of the isoprene rule leads to the assignment of structure IV as the basic skeleton for compound III. 

The UV PE spectra of furazan and benzofurazan have been recorded and interpreted with the aid of ab initio MO calculations. X-ray photoelectron spectroscopic studies for benzotrifuroxan support the NMR assignment of structure as (15) rather than hexanitrosobenzene. The radical anions of benzofurazan and benzofuroxan, generated by alkali metal or electrochemical reduction have been studied by ESR spectroscopy. 

Trioxolanes remain the most studied ring system by microwave spectroscopy and recently, 1,2,4-trithiolane also became the subject of attention. In all cases, isotopically labelled derivatives were made which have very different rotational constants. These aid assignment of structures and also provide useful tools for looking at the mechanism of the ozonolysis reaction. Rotational constants for the parent compounds and their calculated dipole moments are given in Table 3. 

Not all of the possible absorption frequencies of the paraffin molecule are of equal value in the assignment of structure. The C—C bending vibrations occur at very low frequencies (below 500 cm-1) and therefore do not appear in our spectra. The bands assigned to C—C stretching vibrations are weak and appear in the broad region of 1200-800 cm-1 they are generally of little value for identification. 

In the absence of crystallographic or NMR data, predictive techniques based on protein primary sequences can be used to elaborate crude 3D models. Such models will suggest that certain amino acid residues are involved in forming the active (receptor) site. The assignment of structural or functional roles to particular residues can be tested by site-directed mutagenesis, and the model can be further refined by consideration of SAR among ligands. 

---