Homologation processes

For the preparation of 19-nor-B-homosteroids Tadanier s acetolysis procedure appears to be the method of choice. The mild reaction conditions employed with each of these procedures allows the maintainance of a variety of functional groups, e.g., double bonds, ethers, hydroxyls and the like during the homologation process. 

Singular characteristics of the ruthenium catalysts are the capability of direct activation of different substrates (alcohols, ethers, formic,orthoformic and other carboxylic acid esters) at a low iodine concentration, and a high flexibility toward carbonylation and/or homologation processes for the substrates used. The catalytic activity of the ruthenium catalysts moreover do not strongly decrease, as occurs with Co or Rh systems, by passing from methyl to higher oxygenated alkyl derivatives. 

Monometallic ruthenium, bimetallic cobalt-ruthenium and rhodium-ruthenium catalysts coupled with iodide promoters have been recognized as the most active and selective systems for the hydrogenation steps of homologation processes (carbonylation + hydrogenation) of oxygenated substrates alcohols, ethers, esters and carboxylic acids (1,2). 

Industrial development of homologation processes for dimethyl ether, methyl formate, methyl acetate and acetic acid is likely only if they are integrated into a comprehensive plant for the production of C2 derivatives from syn-gas (Scheme 5). 

Higher alcohol formate esters do not appear here because an equivalent amount of methanol is consumed in the homologation process which produced them.) In reactions using trifluoroethanol as solvent, the observed... 

Under some conditions the secondary reactions can be suppressed, and the concentrations of primary products then increase linearly as the reaction proceeds. For example, Fig. 3 gives the results of a reaction in which the homologation process has been inhibited by added tri-w-butylphosphine. A similar inhibition of the homologation reaction was found in experiments carried out in 2,2,2-trifluoroethanol solvent (36). This alcohol is itself resistant to homologation under these conditions, but does undergo transesterification with methyl formate. 

The pivotal step in carbon homologation processes using carbon dioxide is the insertion of carbon dioxide into metal-carbon bonds. Hence, this insertion reaction is of paramount importance in the utilization of carbon dioxide as a source of chemical carbon. The generalized reaction is schemat-... 

The reaction is illustrated here by the overall conversion of the dicarboxylic acid, sebacic acid, into dodecanedioic acid by a bis-homologation process (Expt 5.130). 

Other major homologation processes are the nitroaldol reaction, arising from the reaction between a nitronate anion and a carbonyl compound, and a Michael addition reaction. 

Again, much efficiency was gained by switching from alkoxy to siloxycyclopropanes . Dibromocarbene addition to silyl enol ethers generates cyclopropanes which open to a-bromo a,j5-unsaturated carbonyl compounds on thermolysis or treatment with acid in methanol (equation 137) . It has been shown that this homologation process also works for siloxycyclopropanes obtained by addition of other carbenoids (equation and that it is useful for terpene preparation . ... 

In general, all types of boron-stabilized carbanions are readily alkylated by primary alkyl halides. The situation for the i ylation of dimesitylboryl compounds is sununarized in Scheme 2. These reactions represent highly efficient homologation processes. Oxidation of the tertiary all l organoboranes is slow, but use of 4-methoxy-2,6-dimethylphenyl groups instead of mesityl groups renders the products sensitive to solvolysis. ... 

The reactions of a-TMS-substituted boranes with aldehydes proceed with eliminadon of the TMS group to give alkenylboranes as in equations (45) and (46). Such products can be hydrolyzed to alkenes or oxidized to aldehydes. The intermediate in the case of the dimesi lborane reaction was directly oxidized to aldehyde in 95% overall yield, a good formyl homologation process. ... 

Table 1 includes new literature on catalyst systems and reaction conditions up to 1994. As can be seen from this table, a large range of different catalyst combinations, promoters, and solvents has been investigated. However, the overall conversion and selectivity remain unsatisfactory. It seems to be impossible to achieve both a high conversion of methanol and a high selectivity to ethanol at the same time. Thus, the results of all the efforts have not justified a commercial realization of the homologation process up to now. 

Carbonylation. A, 0-Acetals in which the amino nitrogen atom is acylated are converted into 0-chelated palladium species by replacing the oxy group (e.g., PhO group). Incorporation of CO completes the homologation process. ... 

Substituted allylic alcohoh. The synthesis from esters by a two-step branching homologation process involves nucleophilic attack of an (alkoxysilyl)methylmagnesium chloride (2 equiv.) and oxidative desilylation. 

Hoppsigen http //pbil.univ-lyonl.fr/databases/hoppsigen.html Human, mouse homologous processed pseudogenes... 

QXXRW (associated with regions Ul, U2, U3, and U4, respectively), a motif characteristic of processive P glycosyltransferases (Saxena et al. 1995). Cellulose synthases undoubtedly share a common ancestry. Therefore, unlike the process of crystallization, synthesis of the P-l,4-glucan homopolymer in bacteria and eukaryota is a homologous process that forms an evolutionary link between all cellulose producing organisms. 

Asymmetric boronic esters synthesized by hydroboration chemistry have been converted into asymmetric carboxylic acid derivatives having carboxyl carbon in place of the boron atom via homologation with (dichloromethyl)lithium and oxidation [7]. This sequence is relatively inefficient because of the several steps required. If the asymmetric boronic ester is generated via catalytic hydroboration with catecholbo-rane, the homologation process is a more efficient way of introducing the carboxyl carbon [72],... 

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