Mass keto groups

By far the most important reducing system for the batchwise application of vat dyes is sodium dithionite (Na2S204) in a solution of sodium hydroxide. Obviously the theoretical concentrations required will depend on the number of keto groups in the dye molecule and on its relative molecular mass and concentration, but the reaction can be represented as in Scheme 12.20 for an anthraquinonoid dye with two keto groups. The effect of air oxidation on alkaline... 

On reduction, spireine afforded di- and tetrahydro derivatives. The location of two keto groups in spireine was revealed by H-NMR and mass spectral analysis of deuterated spireine and tetrahydrospireine. When spireine was heated with selenium at 340°, a compound with molecular formula C20H27NO2 was obtained. Structure 182 was proposed for this compound on the basis of spectral data. Since the C-19 imine bond is usually unstable and cannot be isolated in that form, we suggest that the imine bond is present at C-20 rather than at C-19 in the selenium degradation product (C2oH27N02). Thus, structure 183 should be considered for the latter. Each of the structures considered for spireine has unusual features. The exocyclic double bond in 181 bears some resemblance to lycoctamone (184), a rearrangement product of lycoctonine. 

Another silylation procedure consists in a 30-min treatment with acetonitrile—BSTFA (1 1) at 20°C, when the enolized keto group is also silylated, as demonstrated by IR and mass spectra [185]. SE-30 or OV-17 were utilized and the temperature was programmed from 100 to 200°C. By condensation with diamines with various substituents, derivatives possessing specific properties can be obtained. 

This strongly levorotatory secondary base contains a trisubstituted double bond and, similar to kurchiphylline, exhibits a carbonyl maximum at 1742 cm i in the IR-spectrum. The presence of the 16-keto group was confirmed by the mass spectrum in which the fragments at m/e 298 and m/e 213 were identical with those reported as characteristic 61) of this keto group. The structure LXIX was proved by correlation with hola-mine (3a-aminopregn-5-en-20-one). On Wolff-Kishner reduction acetyl-... 

The mass spectra of ajmaline type alkaloids can be divided into three main groups namely, ajmaline (17) and similar compounds widi a H-2P orientation, ajmalidine (15) and other compounds with a 17-keto group, and compounds with a H-2a configuration [e.g. quebrachidine (36)]. For the general features of the mass spectra of ajmaline alkaloids, and the determination of structures with the aid of mass spectra, see Refs. 198-202. 

Buxazidine-B (145), also isolated from B. sempervirens, possesses a primary alcohol group and a keto-group. It has been assigned structure (145) on the basis of i.r. and mass spectra, as well as certain chemical reactions which are not described. This structure, if it is correct, would be particularly interesting as it corresponds to a I6-oxo-dihydrocyclomicrophylline which, in alkaline medium, easily gives (146), containing one nitrogen atom and the 17-en-16-one system. 

The odd-electron ion is formed by a McLafferty rearrangement on the alkyl side of the keto group, while the ion of mass 143 arises by simple cleavage a to the carbonyl. The common elimination of methanol from these ions demonstrates that each contains the methyl ester function. 

Trimethylsilylation in combination with methyloxime (MO) formation is the most popular derivatization method for steroids [62, 63], and the literature contains many examples of corresponding fragmentation pathways and details of the sy and anti isomers formed with non-sterically-hindered keto groups. A typical steroid MO-TMS derivative mass spectrum is reproduced in Figure 7 for pregnenolone [64], together with an indication of the origins of the major ions. 

Sodium borohydride reduction is carried out with 1 mg reagent in 0.5 ml isopropanol. The rates of reduction differ with the position of the keto group (108). Treatment with lithium aluminum hydride in diethyl ether results in formation of C-24-ols from bile acid methyl esters. This may be valuable in chromatographic identification work, especially if combined with mass spectrometry. 

Although keto groups do not have to be derivatized this is sometimes of advantage for mass spectrometric reasons. For example, the presence of a 3-keto group can be established by conversion into a 1,1-dimethylhydrazone. 0-methyloximes are simple to prepare and reaction rates are different depending on the position of the keto groups. 

Common to spectra of ketonic bile acid methyl esters lacking hydroxyl groups is that the peak at mje M-13 cl) has a considerably higher relative intensity than in spectra of derivatives of hydroxylated bile acids. The influence of keto groups on fragmentations in the side chain is also seen in the relatively pronounced loss of 32 mass units (cj) from the molecular ions of 3-keto- and 7-ketocholanoates (Fig. 9). Derivatives of hydroxylated bile acids usually give an ion at mje M-3 by loss of the ester methoxyl group. 

When a 3-keto group is the only substituent in a cholanoic acid structure the mass spectrum clearly indicates its position. The mass spectrum of the ethyl ester of 3-keto-5/3-cholanoic acid is shown in Fig. 9 and it is seen that a major fragmentation is loss of carbons 1-4 with the carbonyl group n, M-70). The ion formed is found at mje 332 and 318 in spectra of the ethyl... 

One way to study the presence of a 3-keto group is to convert it to an enol trimethylsilyl ether which gives pronounced peaks at mje 142 and 143 (cf. 23). The latter ion is the base peak in the spectrum of an enol silyl ether of methyl 3-keto-12a-hydroxy-5/3-cholanoate. An intense peak is also seen at mje 316, i.e., A/-(90- -142). In view of the preferred loss of the side chain following loss of a 12-trimethylsiloxy function the fragment of mass 142 is unlikely to represent the side chain with C-16 and C-17. A prominent peak at mje 201 (Af-[90-f 1424-115]) indicates that it represents carbon atoms 1-4. 

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