13C labeling from

To distinguish adjacent 13C labels from natural abundance isotopes, proton-detected 13C-NMR spectra (HMBC) will show cross peaks associated with the double label that are split into doublets. In contrast, natural abundance 13C will show single cross peaks. 

FIGURE 31 -10 Cerebral amino acids and carbohydrates incorporate 13C label from infused glucose. The top panel shows a 13C NMR spectrum obtained from a gray-matter-rich volume in the human head. (From reference [141].) The right panel shows label incorporation into brain glycogen and glucose in humans. (From reference [142].) The stack plot illustrates the rate of label incorporation into many compounds and carbons in the rat brain. (From reference [ 143].) In all studies, glucose labeled at the 1 or 6 position was administered intravenously. 

Figure 27 Four possible pathways for ABA biosynthesis. Open and closed circles show the 13C label from [1-13C]-d-glucose in the mevaloic acid pathway and the MEP pathway, respectively. DAP, dihydroxyacetone phosphate DXP, 1-deoxy-xylulose-5-phosphate FDP, farnesyl diphosphate GAP, glyceraldehyde-3-phosphate GGDP, geranylgeranyl diphosphate HMG-CoA, 3-hydroxy-3-methylglutaryl CoA IDP, isopentenyl diphosphate MEP, 2-C-methyl-D-erythritol-4-phosphate.

Stationary culture of M. extorquens AMI maintained at 20 °C in alkaline culture medium (pH = 8) containing glycine, [13C]methanol and borate produced L-[3-13C]ser-ine. The 13C-label from [13C]methanol has been incorporated into the C(3) of serine without significant dilution. 

MMA polymerization is one of the most studied systems and was thought to be explicable, within experimental error, in terms of Bemoullian statistics. Moad et ai.jb have made precise measurements of the configurational sequence distribution for PMMA prepared from 13C-labeled monomer. It is clear that... 

The novel highly substituted spiro[4.4]nonatrienes 98 and 99 are produced by a [3+2+2+2] cocyclization with participation of three alkyne molecules and the (2 -dimethylamino-2 -trimethylsilyl)ethenylcarbene complex 96 (Scheme 20). This transformation is the first one ever observed involving threefold insertion of an alkyne and was first reported in 1999 by de Meijere et al. [81]. The structure of the product was eventually determined by X-ray crystal structure analysis of the quaternary ammonium iodide prepared from the regioisomer 98 (Ar=Ph) with methyl iodide. Interestingly, these formal [3+2+2+2] cycloaddition products are formed only from terminal arylacetylenes. In a control experiment with the complex 96 13C-labeled at the carbene carbon, the 13C label was found only at the spiro carbon atom of the products 98 and 99 [42]. 

Other miscellaneous imines that underwent photoreaction with chromium alkoxycarbenes include iminodithiocarbonates [33],the mono-N-phenylimine of benzil and the bis-JV-phenyl imine of acetoin [20]. By preparing the chromium carbene complex from 13CO-labeled chromium hexacarbonyl, /J-lactams with two adjacent 13C labels were synthesized [34]. 

Our studies of quinone methides and related species using 13C labeling and spectral global fitting started in the late 1990s and have continued to the present with two papers on prekinamycins slated for publication in 2009. In this section, we summarize what we learned from both our published and unpublished studies. 

SCHEME 7.22 Products arising from prekinamycin reductive activation at pH 7.5. The 13C labels are designated with asterisks ( ). 

The electron impact mass spectrum of methyl isobutyrate, 27, contains a signal for a [M—CH3]+ ion. 2H- and 13C-labelling clearly establishes that the eliminated methyl radical originates exclusively from the intact PCH3 group (6)12,13,1 . 

Chidthaisong A, Watanabe I. Methane formation and emission from flooded rice soil incorporated with 13C-labeled rice straw. Soil Biol. Biochem. 1997 29 1173-1181. 

Fig. 9. Pulse microreactor system for use with 13C-labeled hydrocarbons. D, E, and J are microreactors J contains the catalyst to be used for hydrocarbon skeletal reaction D and E are used, when necessary, to generate the required reactant hydrocarbon from a non-hydrocarbon precursor (e.g., alcohol dehydration in D and olefin hydrogenation in E) reactant injected at C. F is a trap which allows the accumulation of products from several reaction pulses before analysis G is a G.P.C. column, K a katharometer. Traps H collect fractions separated on G for subsequent mass spectrometric study. When generating reactant hydrocarbon in D and E, a two-step process is preferable in which, with J below reaction temperature, the purified reactant hydrocarbon is collected in H, and this is recycled as reactant with D and E below reaction temperature but with J at reaction temperature. After C. Corolleur, S. Corolleur, and F. G. Gault, J. Catal. 24, 385 (1972).

Organometallic formation may result from a chain mechanism [Eqs. (21)-(23) and (18)—(20)] and/or radical displacement [Eqs. (21)-(23), alone]. The reaction of 13C-labeled mercuric cyclohexanoate with cyclohexylcarbonyl peroxide (1 1) gave mainly unlabeled organomercu-rial, which was derived from radical displacement (122). Decarboxylation by a chain mechanism was reported for the syntheses of organomercuric carboxylates of straight chain alkyls [R = Me(CH2) , n - 0-8, 10, or 15 (123-131)], branched alkyls [R = Me2CH(CH2) , n = 0 or 2 (132) or Me3C(CH2) , n = 0-2 (133)], substituted alkyls [R = cyclopentylmethyl... 

By using an elegant 13C-labeling study that involved incubation of Phoma sp. with 1-13C and 1,2-13C acetate, Oikawa et al. [9], were able to isolate the proposed biosynthetic intermediate phomactatriene (or Sch 49026), with 13C incorporation from singly labeled acetate units as indicated by in Fig. 8.3. Phomactatriene is strikingly reminiscent of taxadiene, a biosynthetic intermediate for Taxol . The net biosynthesis for both involves geranylgeranyl diphosphate (GGDP) cyclization [9]. It is noteworthy that prior to isolation of phomactins, the only known related structure is cleomeolide, a diterpene from the herb Cleome viscosa [10] that remarkably resembles phomactin H. 

Fig. 2 (a) DRAMA pulse sequence (using % = t/2 = rr/4 in the text) and a representative calculated dipolar recoupled frequency domain spectrum (reproduced from [23] with permission), (b) RFDR pulse sequence inserted as mixing block in a 2D 13C-13C chemical shift correlation experiment, along with an experimental spectrum of 13C-labeled alanine (reproduced from [24] with permission), (c) Rotational resonance inversion sequence along with an n = 3 rotational resonance differential dephasing curve for 13C-labeled alanine (reproduced from [21] with permission), (d) Double-quantum HORROR experiment along with a 2D HORROR nutation spectrum of 13C2-2,3-L-alanine (reproduced from [26] with permission)... 

---