Central intermediate

Three possible mechanisms may be envisioned for this reaction. The first two i.e. 1) Michael addition of R M to the acetylenic sulfone followed by a-elimination of LiOjSPh to yield a vinyl carbene which undergoes a 1,2 aryl shift and 2) carbometallation of the acetylenic sulfone by R M followed by a straightforward -elimination, where discarded by the authors. The third mechanism in which the organometallic reagent acts as an electron donor and the central intermediates is the radical anion ... 

A biomimetic oxidation with perfluorinated porphyrin complexes [(F20TPP) FeCl] showed high catalytic activity with secondary alcohols with over 97% yield in all cases [144]. Furthermore, this catalyst is able to oxidize a broad range of alcohols under mild conditions with wCPBA as terminal oxidant. Here, an a-hydroxyalkyl radical species is proposed as central intermediate. 

The above-described experiments, calculations, and theroretical considerations showed that there is no theoretical or experimental evidence whatsoever for the 5a-C-centered radical 10. All relevant reactions can be traced back to occurrence and reactions of o-QM 3 as the central intermediate. The three reactions commonly cited to support the occurrence of the chromanol methide radical 10 in vitamin E chemistry (Figs 6.6-6.8) are actually typical processes of the o-QM intermediate (Figs 6.9-6.11). 

Figure 8.16 Reaction of a serine (3-lactamase with sulbactam. The central intermediate can go on to form products, can transiently inhibit the enzyme in a quasi-reversible fashion, or can irreversibly inactivate the enzyme.

Acetyl-CoA as a central intermediate in the metabolism of all carbon compounds can be dissimilated to generate biologically useful energy or assimilated and used for growth and multiplication. But the shortest and quickest way to store this carbon skeleton is synthesis of poly(3HB) via formation of aceto-acetyl-CoA (Fig. 1). Since the enzymes involved in the metabolic route to poly(3HB) are unspecific, the synthesis of other homopolyesters and heteropolyesters is possible. Such analogues are formed if appropriate prefabricated substrates (which merely need to be activated and incorporated) are offered. Compounds of this type are called related substrates. 

Assuming a reactive oxonium ylide 147 (or its metalated form) as the central intermediate in the above transformations, the symmetry-allowed [2,3] rearrangement would account for all or part of 148. The symmetry-forbidden [1,2] rearrangement product 150 could result from a dissociative process such as 147 - 149. Both as a radical pair and an ion pair, 149 would be stabilized by the respective substituents recombination would produce both [1,2] and additional [2,3] rearrangement product. Furthermore, the ROH-insertion product 146 could arise from 149. For the allyl halide reactions, the [1,2] pathway was envisaged as occurring via allyl metal complexes (Scheme 24) rather than an ion or radical pair such as 149. The remarkable dependence of the yield of [1,2] product 150 on the allyl acetal substituents seems, however, to justify a metal-free precursor with an allyl cation or allyl radical moiety. 

Both (2.S>naringenin and its dihydroflavonol derivative are central intermediates in flavonoid biosynthesis (see Fig. 5.4), acting as branch-point metabolites,... 

M). stand for central, intermediate, and peripheral metal, respectively, and BL stand for inner and outer bridging ligand, respectively. 

Central intermediates in our strategy were the known ortho-acylated hydroxy)... 

ATP + (d)CMP = ADP + (d)CDP (<4> formation of a ternary complex, addition of substrates is random [5] <1> reaction proceeds by a sequential mechanism, a ternary complex of the enzyme with both substrates is formed as the central intermediate in the reaction [12] <3> reaction mechanism is sequential and nonequilibrium in nature, substrates bind to the enzyme in a random order, substrate binding is cooperative [14] <7> the mechanism is analogous to the phosphoryl transfer mechanism in cAMP-dependent protein kinase that phosphorylates the hydroxyl groups of serine residues [16] <8> random bi-bi mechanism [17])... 

Three groups have independently developed solid-phase synthetic approaches toward the quinoxalinones.5,8,9 In our approach, the central intermediate la was reacted with a variety of oc-amino acids 60 using the aqueous solvent system [0.5 M NaHC03(aq)/acetone 1 1] described above for the synthesis of 1,5-benzodiazepinones (Scheme 9).6... 

Simulation of the mutarotation of arabinose closely followed the work on galactose, except that four components plus a central intermediate were included, and the / -pyranose was the starting anomer. The components were the a-pyranose (kt, k2), / -pyranose (k3, k4), a-furanose (k5, k6), and / -furanose (k7j kg). Table III shows sets of values for the k s which yield the experimentally observed half-times for the approach of the components to their equilibrium levels. 

Computations were carried out on the IBM 1620 and the Univac 1108 computers. Programs (Fortran) were prepared for the cases where n, the number of components in addition to the central intermediate, is 2, 3, 4, and 5. These programs are available on request from the authors. 

The procedure illustrated here may be applied directly to any system of n components which are interconverted via a single, central intermediate. A similar approach may be used to integrate the differential equations for any network of unimolecular reactions, plus a central intermediate. Their procedure involves using the Laplace-the kinetic analysis of reaction networks, including the three components... 

Sterols and Cholesterol. Natural sterols are crystalline C76 C1(1 steroid alcohols containing an aliphatic side chain at C17. Sterols were first isolated as lionsaponifiable fractions of lipids from various plant and animal sources and have been identified in almost all types of living organisms. By far, the most common sterol in vertebrates is cholesterol (8). Cholesterol serves two principal functions in mammals. First, cholesterol plays a role in the structure and function of biological membranes.. Secondly, cholesterol serves as a central intermediate in the biosynthesis of many biologically active steroids, including bile acids, corticosteroids, and sex hormones. 

Lipid synthesis is unique in that it is almost exclusively localized to the surface of membrane structures. The reason for this restriction is the amphipathic nature of the lipid molecules. Phospholipids are biosynthesized by acylation of either glycerol-3-phosphate or dihydroxyacetone phosphate to form phosphatidic acid. This central intermediate can be converted into phospholipids by two different pathways. In one of these, phosphatidic acid reacts with CTP to yield CDP-diacylglycerol, which in bacteria is converted to phosphatidylserine, phosphatidylglycerol, or diphos-... 

Michael reactions of chiral lithioenamines of p-oxo esters with dimethyl alkylidenemalonates were studied.173-176 Especially the a-alkyl-substituted compounds (161) and (163), derived from L-valine t-butyl ester, afford, after hydrolysis, the adducts (162) and (164), respectively, diastereoselectively and with high ee (Scheme 57).175 In the presence of TMS-C1, even weaker acceptors like acrylates or MVK were shown to react.176 A somewhat related diastereoselective 1,4-addition, followed by a Pictet-Spen-gler-type cyclization, allows the preparation of compound (165 Scheme 58),177 a central intermediate for the synthesis of several alkaloids.177-178... 

Nucleoside diphosphate disaccharides and peptides are the anabolic precursors of pseudomurein, the peptidoglycan of archaebacteria [65,66], The central intermediate is a UDP-activated glycopentapeptide consisting of a pentapeptide which is covalently linked to the disaccharide N-acetyltalosaminuronic acid (Pl-3)iV-acetylglucosaminide. In eubacteria a pentapeptide is attached to UDP-activated iV-acetylmuraminic acid (1) (Fig. 5). 

Irradiation of 2,2-dimethyl chromene through Pyrex using a 550-W Hanovia lamp initiates a retro 4 + 2 reaction to form the extended quinone methide 4, which reacts with methanol to form a pair of methyl ethers (Scheme 6A).18 Flash photolysis of coniferyl alcohol 5 generates the quinone methide 6 (Scheme 6B) by elimination of hydroxide ion from the excited-state reaction intermediate.19 The kinetics for the thermal reactions of 6 in water were characterized,20 but not the reaction products. These were assumed to be the starting alcohol 5 from 1,8-addition of water to 6 and the benzylic alcohol from 1,6-addition of water (Scheme 6). A second quinone methide has been proposed to form as a central intermediate in the biosynthesis of several neolignans,21a and chemical synthesis of neolignans has been achieved... 

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