Carbon skeleton determination methylation

Abscisin II is a plant hormone which accelerates (in interaction with other factors) the abscission of young fruit of cotton. It can accelerate leaf senescence and abscission, inhibit flowering, and induce dormancy. It has no activity as an auxin or a gibberellin but counteracts the action of these hormones. Abscisin II was isolated from the acid fraction of an acetone extract by chromatographic procedures guided by an abscission bioassay. Its structure was determined from elemental analysis, mass spectrum, and infrared, ultraviolet, and nuclear magnetic resonance spectra. Comparisons of these with relevant spectra of isophorone and sorbic acid derivatives confirmed that abscisin II is 3-methyl-5-(1-hydroxy-4-oxo-2, 6, 6-trimethyl-2-cyclohexen-l-yl)-c s, trans-2, 4-pen-tadienoic acid. This carbon skeleton is shown to be unique among the known sesquiterpenes. 

Homocysteine (Hey) metabolism is closely linked to that of the essential amino acid methionine and thus plays a central role in several vital biological processes. Methionine itself is needed for protein synthesis and donates methyl groups for the synthesis of a broad range of vital methylated compounds. It is also a main source of sulphur and acts as the precursor for several other sulphur-containing amino acids such as cystathionine, cysteine and taurine. In addition, it donates the carbon skeleton for polyamine synthesis [1,2]. Hey is also important in the metabolism of folate and in the breakdown of choline. Hey levels are determined by its synthesis from methionine, which involves several enzymes, its remethylation to methionine and its breakdown by trans-sulphuration. 

The assembly of the carbon skeletons of these unusual hydrocarbons was first studied in Carpophilus freemani Dobson, through careful GC-MS and Nuclear Magnetic Resonance (NMR) studies of the incorporation of 2H or 13C-labeled precursors (Petroski et al., 1994). Assembly of the carbon skeleton of the aggregation pheromone of C. freemani, (2 , 4 , 6ii)-5-ethyl-3-methyl-2,4,6-nonatriene, involves initiation with acetate elongation with first propionate (to provide the methyl branch), then butyrate (to provide the ethyl branch) and chain termination with a second butyrate (Figure 6.7). At some point, loss of C02 from one of the butyrate units occurs to yield the appropriate hydrocarbon, but Petroski et al. (1994) were unable to determine which of the butyrate units loses its carboxyl group. Bartelt and Weisleder (1996) studied the biosynthesis of 15 additional methyl- and/or ethyl-branched, tri- and tetraenes in the related... 

Thermal decomposition of quaternary ammonium salts and bases is most valuable in structural investigations of amines, particularly heterocyclic secondary amines (Hofmann exhaustive methylation). The course of the elimination (A or B) is determined by the nature of the four alkyl groups on the nitrogen atom. The reaction has found little use in the synthesis of pure olefins. The yields are low even when three of the alkyl groups are methyl radicals. Carbon-skeleton rearrangement does not occur. Thus, the only olefin obtained by pyrolysis of pinacolyltri-methylammonium hydroxide, (CHj),CCH(CHj)N(CHj)j OH, is /-butylethylene (50%). ... 

Kuhn and Dansi determined that Amadori s stable isomer is not a Schiff base but the product of a molecular rearrangement. They also proved that the labile isomer is in fact the i f-substituted glucosylamine. However, they assumed incorrectly (on the basis of C-methyl analyses) that the sugar moiety of the stable isomer contained a branched-carbon skeleton. 

The unusual 17 8-methyl-18-wor-cholestane carbon skeleton of veralkamine has been determined by X-ray analysis of veralkamine hydroiodide (72), confirming the chemical and spectroscopic evidence of its structure. 

Particularly interesting has been the case of Orthosphenia mexicana which yielded five new triterpene methylene quinones with a new carbon skeleton, a greater degree of conjugation than hitherto reported, an extra 14-15 double bond and a rearranged methyl at C-15. Its structure was elucidated by a succession of chemical transformations, spectroscopic methods and X ray diffraction which determined the absolute configuration of this compound [49,50],... 

The structure of pironetin was determined to be (5/ ,6/ )-5-ethyl-5,6-dihydro-6[(jE )-(2/ ,35,4/ ,55)-2-hydroxy-4-methoxy-3,5-dimethyl-7-nonenyl]-2H-pyran-2-one by FAB-MS, H and 13C NMR, COSY, COLOC, DEPT, IR, X-ray crystallographic analyses and adapted Mosher s method to determined the absolute configuration [74]. The biosynthesis of pironetin appeared to involve assembly of acetate, propionate and butyrate precursors. Four fermentation experiments were carried out in order to determine the biosynthetic origin of the carbon skeleton and oxygen atoms of pironetin. [1-13C] Acetate, [2-13C]acetate, [1,2-13C] acetate, [l-13C]propionate, [l-1 C]butyrate and L-[methyl-I3C]methionine were led separately to Streptomyces sp. culture and pironetin was isolated and purified. Analysis of pironetin by 13C NMR showed that pironetin was derived from four acetate units, two propionate units, one butyrate unit and one methyl unit of methionine [78] (Fig. 4). 

In the determination of the carbon skeleton, thoroughly elaborated reaction methods associated with the participation of hydrogen are most often used. In the literature (e.g., ref. 1) the methods for determining the carbon skeleton are considered only on the basis of this reaction. However, for determining the carbon skeleton, other reactions can also be employed successfully, although their field of application is narrower. Thus, methylation is valuable for the identification of hydrocarbons (see Chapter 1). Diazomethane reacts under strong ultraviolet irradiation in the cold in accordance with the following equation ... 

Carbon spectra can be used to determine the number of nonequivalent carbons and to identify the types of carbon atoms (methyl, methylene, aromatic, carbonyl, and so on) that may be present in a compound. Thus, carbon NMR provides direct information about the carbon skeleton of a molecule. Some of the principles of proton NMR apply to the study of carbon NMR however, strac-tural determination is generally easier with carbon-13 NMR spectra than with proton NMR. Typically, both techniques are used together to determine the stmcture of an unknown compound. 

Dehydrohalogenation may give a mixture of products if the halogen is unsymmetrically located on the carbon skeleton. Eor example, 2-bromo-2-methylbutane (6), the substrate you will use in this experiment, yields both 2-methyl-2-butene (7) and 2-methyl-l-butene (8) on reaction with strong base (Eq. 10.5). Because such elimination reactions are normally irreversible under these experimental conditions, the alkenes 7 and 8 do not undergo equilibration subsequent to their production. Consequently, the ratio of 7 and 8 obtained is defined by the relative rates of their formation. These rates, in turn, are determined by the relative free energies of the two transition states, 9 and 10, respectively, rather than by the relative free energies of the alkenes 7 and 8 themselves. 

Prior C-incorporation studies suggested that the carbon skeleton of the antibiotic was derived from a linear polyketide composed of seven acetates, four propionates, and two butyrates. Reversible intraconversion of acetate and butyrate by the organism also resulted in some randomization of these labels. The four C-methyl groups at C-4, C-10, C-12, and C-16 are derived from propionate, and the C-23-methyl group comes from the starter acetate unit of the polyketide. The C study was unable to determine if one, two, or three of the C-ethyl groups were derived from butyrate. 

Cholesterol was isolated m the eighteenth century but its structure is so complex that Its correct constitution was not determined until 1932 and its stereochemistry not verified until 1955 Steroids are characterized by the tetracyclic ring system shown m Figure 26 9a As shown m Figure 26 9b cholesterol contains this tetracyclic skeleton modified to include an alcohol function at C 3 a double bond at C 5 methyl groups at C 10 and C 13 and a C Hn side chain at C 17 Isoprene units may be discerned m var lous portions of the cholesterol molecule but the overall correspondence with the iso prene rule is far from perfect Indeed cholesterol has only 27 carbon atoms three too few for It to be classed as a tnterpene... 

In the production of TNT from the reaction between toluene and mixed acids (nitric/sulfuric), TeNMe forms in amounts between 0.2—0.4% of the total wt of TNT. This TeNMe has been held responsible for several expins which have occurred in TNT plants, causing fatal injuries to personnel and severe damage to facilities. These expins were attributed to the presence of TeNMe in the acid fume lines and the acid storage tanks. Mixts of TeNMe and readily oxidizable materials are known to form very powerful and sensitive expl mixts. Since TeNMe is also isolated from the nitration of Nitrobenzene (NB), the TeNMe formed in the nitration of toluene may arise from the oxidation of the aromatic ring and/or methyl group. In an effort to gain more informa-. tion on the origin of TeNMe from TNT production, radioactive carbon-14 (14C) was used as a tracer to determine the extent to which each of the carbon atoms in the toluene skeleton of the various nitro-substituted isomers contributes to... 

---