Pyran labelled

The high levels of enantioselectivity obtained in the asymmetric catalytic carbomagnesa-tion reactions (Tables 6.1 and 6.2) imply an organized (ebthi)Zr—alkene complex interaction with the heterocyclic alkene substrates. When chiral unsaturated pyrans or furans are employed, the resident center of asymmetry may induce differential rates of reaction, such that after -50 % conversion one enantiomer of the chiral alkene can be recovered in high enantiomeric purity. As an example, molecular models indicate that with a 2-substituted pyran, as shown in Fig. 6.2, the mode of addition labeled as I should be significantly favored over II or III, where unfavorable steric interactions between the (ebthi)Zr complex and the olefmic substrate would lead to significant catalyst—substrate complex destabilization. 

Fig.9. Polyketide products generated via in vitro biosynthesis by the fusion protein DEBS 1+TE. 1 and 2, C-labeled alternate starter unit substrates were used to synthesize the Cj- and CiQ-lactones, respectively. 3 and 4, (2S,3R)-2-methyl-3-hydroxypentanoyl-N-acetylcysteamine (NAC) thioester was used as an intermediate to synthesize the C9-lactone and the 3-oxo analog, respectively. 5, In the absence of NADPH, a pyran-2-one is generated using propionyl-CoA and C-labeled methylmalonyl CoA...

Figure 1 shows heat capacity profiles of human erythrocyte ghosts Incubated with Increasing concentrations of pyran copolymer In 310 Imosm phosphate buffer. The five endothermic transitions of the control sample (bottom curve) are labeled A, B2j C and D, In accord with earlier work by Brandts and coworkers (13-16) and from this laboratory (17-20). The A transition Is assigned quite securely as a partial denaturation of spectrin, the major cytoskeletal protein on the erythrocyte membrane (13). Bj. and B2 appear to Involve proteins and lipids (protein Bands IV.1 and... 

The Importance of divalent Ions shows up most clearly In quantitative binding experiments using C-labeled pyran copolymer. In these experiments, whole ei throcytes - not Isolated membranes - were Incubated at room temperature for 10 minutes with the desired concentration of pyran copolymer In buffer. The cells were then sedimented by centrifugation, washed twice with fresh buffer, disrupted with detergent, and the radioactivity detennined by liquid scintillation counting. 

Triketides are relatively rare. Triacetic acid lactone (4.2) has been detected in Penicillium patulum. It is also produced by fatty acid synthase in the absence of the reductant NADPH. Radicinin (4.3) is a major phytotoxin isolated from Ahernaria radicina (Stemphyllium radicinum) which causes a black rot of carrots. It is also formed by other Ahernaria species. Its pyrano[4,3- ]pyran structure, the identification of which had eluded purely chemical degradative studies, was established in one of the earlier applications of NMR spectroscopy to natural product structure elucidation. The biosynthesis of radicinin from acetate units was studied in 1970 by both radio-isotope methods using carbon-14 and by carbon-13 enrichment studies with NMR methods of detection. This was one of the first applications of this NMR technique to biosynthetic problems. These results established the labelling pattern for radicinin shown in 4.3. 

Hepatic and renal damage represent the major toxicity seen at high dosages in the dog and monkey. In mice, labelled pyran of average MW 17 000 is lost from blood... 

The site of injection of an agent may profoundly effect its action based on the barriers and organs that it encounters via each specific route. It is therefore of interest to compare studies on the organ distribution of -labeled pyran administered intravenously (i.v.) by Munson et al. (unpublished observations) and intraperitoneally (i.p.) by Papamatheakis et al. [34]. 

In vitro, C-pyran and labelled pyran were rapidly ac-... 

A ruthenium-based complex enabled catalytic carbonylative C-H cyclization of 2-arylphenols was achieved by using balloon pressure of CO and Oj (Eq. (7.17)) [23]. Under relatively mild reaction conditions, this methodology produced a variety of 6//-dibenzo [b,d] pyran-6-one derivatives in high yields with broad substrate scope. Competition experiment suggested that electron-rich substrates are more reactive. In addition, experiment with isotopically labeled substrate revealed that the C-H metalation step is reversible. 

In 2006, Liu and coworkers reported a gold-catalyzed intramolecular cycloaddition of diynes with tethered arenes 118 to synthesize l,3-dihydroindeno[2,l-c]pyran or 2,3-dihydro-l//-indeno[2,l-c]pyridinederivatives 120 (Scheme 12.52) [56]. On the basis of deuterium labeling, they believed that the first reaction step is the intramolecular arylation of one alkyne to form vinylgold(I) intermediate 119,... 

When equimolar amounts of unlabeled cis-11 and deuterium-labeled cis-13 aeetals were treated with SnCU, a mixture of four eompounds 12, 14, 15 and 16, with the expeeted trans stereoehemistry in the pyran ring, was obtained in approximately the same yield (determined by mass speetrometrie analysis) (Seheme 5.5). The di-astereomerie ratio was approximately (3 1) in eaeh ease. 

Treatment of [ C]malonic acid with oxalyl chloride in the presence of catalytic amounts of DMF affords [ C]malonyl chloride in nearly quantitative yield. The structure of the reagent was confirmed by acylation of 2-methoxypropene, which provided a 3 1 mixture of phloroglucinol monomethyl ether (428) and 4-hydroxy-6-methyl-2//-[2,4- " C]pyran-2-one (427). Etherification of the latter with dimethyl sulfate followed by ring opening upon treatment with sodium methoxide and recychzation afforded 429, which was combined with the original portion of 428 and etherified to give 1,3,5-trimethoxy-[1,3,5- C]benzene, which was used as a key intermediate in the synthesis of several carbon-14 labeled cholesterol acyltransferase inhibitors . ... 

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