Methoximes

The 3-methoxime (74) provides a base-stable protecting group for A " -3-ketones. It is removed in low yield by conversion to the corresponding semicarbazone. 

Hint To distinguish these compounds without elemental composition or standards for GC retention time, split the GC effluent to a FID, a nitrogen-phosphorus detector, and the mass spectrometer, simultaneously. Using this splitter system, it is easy to determine if the GC peak contains nitrogen. Also, the analyst can differentiate between azobenzene and benzophenone by using the methoxime derivative. 

For low molecular weight aliphatic acids, try TMSDEA reagent. Otherwise, use MSTFA, BSTFA, or TRI-SIL BSA (Formula P). For analysis of the keto acids, methoxime derivatives should be prepared first, followed by the preparation of the trimethylsilane (TMS) derivatives using BSTFA reagent. This results in the meth-oxime-TMS derivatives. 

Add 100 pi 1 of MSTFA reagent to less than 1 mg of dry extract. Heat at 60° for 15-20 min. If necessary, add 250 /a 1 of acetonitrile or other suitable solvent. For additional structural information, prepare the methoxime-TMS derivative to determine if one or more carbonyl groups are present. 

Add 250 /a 1 of methoxime hydrochloride in pyridine (MOX1 reagent to less than 1 mg of dry extract. Let this solution stand at room temperature for 2 hours. Evaporate to dryness with clean, dry nitrogen. Add 250 /a1 of MSTFA reagent and let stand for 2 hours at room temperature. 

This derivative is useful for determining the presence and number of keto groups as well as for protecting the ketone from enolization. Some diketones that polymerize readily, such as 2,3-butanedione. should be freshly distilled and the methoxime derivatives should be prepared. 

Preparation of methoxime derivatives Add 0.5 ml of MOX reagent to the sample. Heat at 60° for 3 hours. Evaporate the reaction mixture to dryness with clean, dry nitrogen. Dissolve in the minimum amount of ethyl acetate. Some solids will not dissolve. 

Molecular ion The MW of aliphatic ketones can be determined from its prominent molecular ion. In general, the intensity of the molecular ions of ketones is greater for C3-C8 than for CVC u. A molecular ion is usually observed for methoxime derivatives. 

B. Preparation of Methoxime (MO)-TMS Derivatives (especially for hydroxyketosteroids, which may decompose under the given GC conditions unless the MO-TMS derivatives are prepared.) Add 0.25 ml of methoxime hydrochloride in pyridine to the dried extract. Let stand for 3 hours at 60° or overnight at room temperature. Evaporate to dryness with clean, dry nitrogen. Add 0.25 ml of TRI-SIL TBT and 0.25 ml of pyridine to the dried reaction mixture. Cap tightly and heat at 60° for a few hours or overnight at room temperature. 

Derivatives 1 ml of urine adjusted to pH 8 with NaHC03 solution. Add methoxime hydrochloride or ethoxime hydrochloride. Dissolve and mix thoroughly, and then saturate the solution with NaCl. Adjust the solution to pH 1 with 6N HC1. 

In the mass spectrum of 2-hexanone (Figure 20.1), the molecular ion is apparent at m/z 100, which can be confirmed by preparing the methoxime derivative. The compound type is verified by the presence of m/z 43 and 58. 

The Oxamyl in this extract is then determined by gas chromatography using on-column reaction with trimethylphenyl ammonium hydroxide, the derivative so formed being determined by a flame photometric detector operated in the sulphur mode. Both Oxamyl and Oxamyl oxime in the soil react with trimethylphenyl ammonium hydroxide to form the same methoxime derivative (CH3)2NCOC(SCH3) -NOCH3. 

An oximine (i.e., a substituted oxime) analogue of alprenoxime was examined in an attempt to overcome the problem of low stability in aqueous solution. To this end, the methoxime analogue of alprenolol (11.74, R = Me) was prepared and evaluated [109]. Stability in solution was greatly improved at neutral pH. Topical administration to rabbits produced a decrease in intraocular pressure that had the same onset and intensity as that produced by alprenolol, but that lasted longer. Alprenolol was, indeed, formed in eye tissues as a metabolite, with the peak concentration reached 30 min after topical administration of the methoxime. 

All evidence presented here indicates that the methoxime-to-ketone reaction is enzymatic, but the nature of the reaction is unclear. Hydrolysis by an uncharacterized hydrolase, as postulated by the authors, is a valid assumption. However, one should not dismiss another possibility, namely oxidative O-demethylation to the oxime, followed by spontaneous oxime-to-ketone hydrolysis, and stereospecific reduction, as explained above. 

Figure 16. Total ion current plot for the GC/MS analysis of selected aldehyde o-methyl oximes in the presence of fipyfold excess hydrocarbons. (Methoxime doublet peaks are from chromatographic resolution of syn and anti conformers.)...

System (4) has been reported for the simultaneous determination of cortisol and cortisone in human plasma by stable-isotope dilution-MS [178]. For the determination, capillary GC-MS with H5 -cortisol and H5 -cortisone (as internal standards) was used. The method used a SPB-1 fused-silica capillary column (7 m x 0.25 mm) and helium as the carrier gas (at 29.4 Pa), with 70 eV EIMS being used for selective-ion monitoring. The concentrations were determined from the peak height ratios of the [M-31] fragment ions of the methoxime-TMS derivatives of cortisol, cortisone, and their internal standards. The sensitivity of the method was 200 pg per injection for both cortisol and cortisone. 

Mason and Slover187 studied the separation of monosaccharides as the O-trimethylsilyl derivatives of the oximes, as an alternative method to analyzing for sugars in foods, and Clayton and H.G. Jones188 used trimethylsilylated D-glucose oxime. Both groups of workers referred to the instability of these derivatives, as noted in Section III (see p. 23). Laine and Sweeley189 proposed a similar method that used the methoxime derivatives. 

Acylation of the amine with the methoxime from aminothiazole-glyoxylate has a similar effect on broadening the antibacterial spectmm in the 3-methyl series. In this case, the amine group on the starting thiazole (21-3) is first protected by conversion to (28-2) by alkylation with thriphenylmethyl chloride. Condensation of the acid by the mixed anhydride method with 7-ADC A (26-6) leads to the corresponding amide. The trityl group is then removed to afford the antibiotic cefetamet (28-3) [34]. 

Mass spectra of methoxime (MO)-TMS derivatives of steroids typically give the following fragments M-31 (loss of oxime), sequential 90 losses (trimethylsilanol), loss of the primary TMS group (M-103), and combinations thereof. Often the ion chosen for monitoring is one formed by the above fragmentations. A detailed de-... 

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