Carbon Reduction Label

Since ivermectin (= 22,23-dihydroavermectin B ) is obtained by catalytic reduction of avermectin B, the same procedure using tritium gas convenientiy affords tritiated ivermectin (22,23- [JT]-22,23-dihydroavermectin B ). The preparation of a tritiated derivative containing a 22,23-double bond starts with the readily available 5-ketone, which is reduced with [JT]-sodium borohydride stereospecificaHy to a 5- [JT]-derivative (40). Carbon-14 labeled avermectins can be obtained by a biosynthetic process using sodium (l- C)propionate as labeled precursor (48). 

The oxygen formed clearly comes from H20 and not from C02, because photosynthesis in the presence of water labeled with lgO produces oxygen labeled with 180, whereas carbon dioxide labeled with 180 does not give oxygen labeled with 180. Notice that the oxidation of the water produces two electrons, and that the formation of NADPH from NADP requires two electrons. These reactions occur at different locations within the chloroplasts and in the process of transferring electrons from the water oxidation site to the NADP reduction site, adenosine diphosphate (ADP) is converted to adenosine triphosphate (ATP see Section 15-5F for discussion of the importance of such phosphorylations). Thus electron transport between the two photoprocesses is coupled to phosphorylation. This process is called photophosphorylation (Figure 20-7). 

Although not catalytic, this system is cyclic in the sense that the W(CO)6 may be converted back to the dianion by photolysis in the presence of NMe3 followed by alkali metal reduction. Labeling studies have shown that the carbon of the sixth carbonyl is obtained from carbon dioxide and not from decomposition of the transition metal carbonyl. 

Carbon-11 labeled BPA, 4, was synthesized from the corresponding aldehyde, 4-boronophenylacetaldehyde, 9. This boronated aldehyde was prepared from commercially available 4-bromophenylacetic acid, 10, in five synthetic steps (Scheme 1). The synthesis was initiated by the borane reduction of acid 10 to the 2-(4-boronophenyl)ethyl alcohol, 11. Alcohol 11 was then carefully oxidized to aldehyde 12. In the next step, 4-bromophenylacetaldehyde, 12, was refluxed with ethylene glycol in the presence of a catalytic amount of/>-toluenesulfonic acid to obtain the corresponding acetal 13.The boronic acid moiety was introduced at the para position of the phenyl ring 1 the reaction with butyllithium followed by triisopropyl borate" to obtain the 4-bronophenylacetaldehyde ethylene acetal, 14. In the final step of the synthesis, acetal 14 was treated with concentrated hydrochloric acid in methanol as solvent to obtain the desired precursor, 4-boronophenylacetaldehyde, 9, for the synthesis of carbon-11 labeled BPA, 4, Scheme 2. 

Ruben et al. first used radiocarbon in studies of the path of carbon in photosynthesis (1940). They found that CO2 fixation in the dark is greater following preillumination. This is in accord with the concept that CO2 fixation occurs by dark reactions which use relatively stable chemical species formed in the light. They discovered that the radioactive product formed from labeled CO2 after preillumination was a carboxylic acid. These findings suggested that the path of carbon reduction in photosynthesis might very well include CO2 fixation mechanisms similar to those found in nonphotosynthetic plant tissues. 

FiU. 6. The carbon reduction cycle of photosynthesis. Solid arrows indicate reactions of the carbon reduction cycle as formulated by Calvin and coworkers. Dashed line represents hypothetical reductive carboxylation reaction discussed in text. Open arrows indicate start of some of the biosynthetic paths leading from intermediate compounds of the cycle. Asterisks indicate approximate relative degree of labeling after a few seconds of photosynthesis. They reflect the results of degradation studies by various workers, as discussed in the text. 

The complete photossnnthetic carbon reduction cycle is shown in detail in Fig. 6. A somewhat idealized distribution of label found experimentally following a short period of photosynthesis with C 02 is indicated by the asterisks. 

Buchanan (1953) reported that sucrose is photosynthesized from fructose-6-phosphate of the carbon reduction cycle. One molecule of fructose-6-phosphate is converted to glucose-6-phosphate, which reacts with uridine triphosphate (UTP) to form uridine diphosphoglucose (UDPG). UDPG is always labeled with during short periods of photosynthesis with The UTP was... 

A study of the kinetics of the labeling of alanine show that its rate of labeling reaches a maximum as soon as the intermediates of the carbon reduction cycle are saturated with C . Since no secondary products of carbon photosynthesis such as sucrose are approaching saturation at this time (3-5 minutes), it appears that alanine is formed directly from intermediates of the cycle. Presumably, alanine is formed from PGA by the transamination of pyruvic acid derived from phosphoenolpyruvic acid which in turn is derived from PGA [Eq. (28)]. 

It will be noted that the adhesion drops off with increasing conductivity of the oil. An increase in electrical conductivity favors charging of the particles and repulsion from the surface, thus reducing the quantity of adherent particles (see Section 33). Hence, in studying the operating mechanism of additives that reduce adhesion, a study was made of the behavior of a model disperse material (carbon black labeled with radioactive thallium) in an electric field [337]. The use of the additives gave a reduction in the quantity of adherent carbon particles in the electric field by a factor of approximately 20, in comparison with the quantity adhering to the surface when no additive was used. 

Y.-Y. Liu, E. Thom, and A. A. Liebman. Selective reduction of a carboxy-group with dibor-ane. Synthesis of specifically carbon-14 labelled D,L-2-amino-4-(2-aminoethoxy)butanoic acid. Ganad. J. Ghem., 1978, 56, 2853. 

C. Leng, J. Wu, Q. Xu, G. Lai, H. Ju, F. Yan, A highly sensitive disposable immunosensor through direct electro-reduction of oxygen catalyzed by palladium nanoparticle decorated carbon nanotube label. Biosens. Bioelectron. 27 (2011) 71-76. 

The reduction of ethyl bromo[ " C]acetate can be stopped at the aldehyde level when the reaction is performed with DIBAL-H at low temperature. This has been demonstrated for doubly carbon-13-labeled material. The resulting bromo[ C2]acetaldehyde (1021 was not isolated but trapped with Ph3P to give the corresponding Wittig reagent 103. which was used for two sequential C2 extensions of the aldehyde 104 in the synthesis of... 

LiAlH4 reduction of carbon-14-labeled diethyl malonate, followed by halogenation of the resulting 1,3-diol produces l,3-dibromo[ C]propane (Figure 6.126), a valuable precursor for the synthesis of [ C ]glutaronitrile, [ C ]glutaraldehyde and [ C ]glutaric acid . ... 

For example, condensation of the 5-cyano ketone 146 with methyl [ C ]cyanoacetate gave (Figure 7.36) a-[ C]cyanobenzo[a]quinolizin-2-yUdeneacetate derivative 147 in 52% yield. Reduction of the double bond followed by selective de-methoxycarbonylation provided the dinitrile, a key intermediate in the synthesis of carbon-14-labeled 4a-hydroxy-allo-berbane (148) . Despite loss of half of the label during formation of the final ring by Thorpe-Ziegler condensation, this route was considered superior to the alternative longer route. 

An alternative procedure, which, however, has only been exploited so far for the synthesis of carbon-13-labeled material, avoids die drastic reaction conditions of the above sequence. It begins with the condensation of ethyl [ C]cyanoacetate with urea to afford [ C]cyanoace-ty lurea 173 (Figure 7.41). Partial reduction of 173 and treatment of the intermediately generated 3-phenylamino[3- " C]acryloylurea with ethanolic HCl gave [4- C]uracil 174 in 17% yield. Under basic conditions 173 cyclizes in nearly quantitative yield to 6-amino[6- C]uracil 175. which serves as a key intermediate for thepreparation of uric acid labeled with isotopiccarbon. ... 

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