Rearrangement condensation reactions

Moving from rearrangements, condensation reactions were also presented. Condensation reactions occur when two reactive species condense with one another forming a new compound. The first was the aldol condensation (Scheme 8.9). Later, a more complex application of the aldol condensation was presented in the form of the Robinson annula-tion (Scheme 8.10). For both of these reactions, the underlying lessons relate to the ability to induce reactions and incorporate substitutions at carbon atoms adjacent to carbonyl groups. Similar reactivities of such carbon atoms can be utilized for alkylation (SN2) and acylation (addition-elimination) reactions as illustrated in Scheme 8.11. 

Benzilic acid rearrangement Benzoin reaction (condensation) Blanc chloromethylation reaction Bouveault-Blanc reduction Bucherer hydantoin synthesis Bucherer reaction Cannizzaro reaction Claisen aldoi condensation Claisen condensation Claisen-Schmidt reaction. Clemmensen reduction Darzens glycidic ester condensation Diazoamino-aminoazo rearrangement Dieckmann reaction Diels-Alder reaction Doebner reaction Erlenmeyer azlactone synthesis Fischer indole synthesis Fischer-Speior esterification Friedel-Crafts reaction... 

This particular polymer is a fibre-forming material (Perlon U). Although in many respects this reaction resembles the formation of polyesters and polyamides it is not a condensation reaction but involves a transfer of hydrogen atoms and thus may be considered as an example of rearrangement polymerisation. 

Besides simple condensation reactions, valence isomerization reactions, in particular, arc used for the synthesis of unsaturated, eight-membered-ring azaheterocycles. These isomeriz-ations mainly involve rearrangements of nitrogen-containing bicyclo[4.2.0]octatriene or semi-bullvalene systems. 

The production of such high concentrations of radicals leads to a very unstable situation and if the radicals are not stabilized via H-donation, they undergo a variety of undesired reactions such as condensation, elimination or rearrangement (7). Neavel has shown that at short times ( 5 min) a vitrinite enriched bituminous coal can be converted to 80% pyridine soluble form in even non-donor reaction solvents (naphthalene) (8). But if reaction times are extended, the soluble products revert to an insoluble form via condensation reactions. Such condensation reactions were... 

Some benzimidazoimidazopyridines can be prepared from the reaction of the sulfoxide 294 with acetic acid. This unusual acid-induced rearrangement, condensation, and oxidation sequence gives the dimer 295, the proposed pathway being as shown in Scheme 80 <1987JOC4582>. In a similar way, 296 can be converted into 297 upon treatment with acid <1999PHA734> (Equation 50). The product (probably a mixture of isomers) is described as violet in color, but it is obtained in amounts too small for its potential as a colorant to be assessed. 

We have intentionally selected example reactions (Figs. 29-33) that would not usually be immediately obvious to a chemist. The examples chosen have all been concerned with rearrangements of various types, since their courses are frequently difficult to predict. It remains to emphasize that the reactivity functions contained in EROS perform perfectly well with other types of reaction. This is true, for example, with reactions that a chemist could derive directly from an analysis of the functional groups in a molecule. Thus, EROS predicts addition reactions to carbonyl compounds, nucleophilic substitutions, and condensation reactions, to name just a few examples. In all these reaction types, the possibility of assigning a quantitative estimate to the reactivity at the various sites via the reactivity functions is of particular merit. It... 

Condensation reactions Organic pyrolysis Structural rearrangement... 

Fig. 6.25. Simplified mechanism of two degradation reactions between peptides and reducing sugars occurring in solids, a) Maillard reaction between a side-chain amino (or amido) group showing the formation of an imine (Reaction a), followed by tautomerization to an enol (Reaction b) and ultimately to a ketone (Reaction c). Reaction c is known as the Amadori rearrangement (modified from [8]). b) Postulated mechanism of the reaction between a reducing sugar and a C-terminal serine. The postulated nucleophilic addition yields an hemiacetal (Reaction a) and is followed by cyclization (intramolecular condensation Reaction b). Two subsequent hydrolytic steps (Reactions c and d) yield a serine-sugar conjugate and the des-Ser-peptide...

Although ketones are reported to give poor yields in such condensation reactions, Maag and co-workers have used this step in a synthesis of a rearrangement product of bicyclomycin (78JA6786). The route involves two successive aldol condensations, the first with an aldehyde and the second with a ketone the latter step (Scheme 27) proceeds in 41% yield. 

In the area of reaction energetics. Baker, Muir, and Andzehn have compared six levels of theory for the enthalpies of forward activation and reaction for 12 organic reactions the unimolecular rearrangements vinyl alcohol -> acetaldehyde, cyclobutene -> s-trans butadiene, s-cis butadiene s-trans butadiene, and cyclopropyl radical allyl radical the unimolecular decompositions tetrazine -> 2HCN -F N2 and trifluoromethanol -> carbonyl difluoride -F HF the bimolecular condensation reactions butadiene -F ethylene -> cyclohexene (the Diels-Alder reaction), methyl radical -F ethylene -> propyl radical, and methyl radical -F formaldehyde -> ethoxyl radical and the bimolecular exchange reactions FO -F H2 FOH -F H, HO -F H2 H2O -F H, and H -F acetylene H2 -F HC2. Their results are summarized in Table 8.3 (Reaction Set 1). One feature noted by these authors is... 

The N-methyl derivatives of 1 -aryl-1,4-dihydro-3(2//)-isoquinolinones (e.g., 54) can be lithiated in position 4 with butyllithium. The subsequent reaction, e.g., with p-nitrobenzaldehyde, gives the 4-p-nitrobenzylidene derivative. In this case, the condensation reaction is not followed by H-l rearrangement [86ACH(121)263]. 

The reaction of 2-(diphenylmethylene)thietan-3-one with 1,2,4,5-tetrazines in KOH-MeOH-THF gives 4//-pyrazolo[5,l-c]thiazines. This novel condensation reaction proceeds via the intermediacy of an 8-(diphenylmethylene)-2//-l,4,5-thiadiazocin-7(8H)-one, which undergoes a multi-step rearrangement including a rare anti-Michael addition.30... 

The kinetics of the nonenzymatic glycosylation of hemoglobin to form hemoglobin Ale have been analyzed in detail by Higgins and Bunn (31). Assuming a bimolecular reaction for the condensation reaction and first-order kinetics for the Amadori rearrangement, they determined the rate constants for the glycosylation of the N-terminal valine of the 8-chain ... 

The benzoin condensation reaction proceeds via a nucleophilic substitution followed by a rearrangement reaction. 

Such condensation reactions are also promoted by certain trTvalent phosphorus compounds, e.g. triphenyl phosphite (2) or diphenyl ethylphosphonite (3), or to a lesser extent by pFosphonate esters, e.g. diphenyl n-butylphosphonate (3). "Bates reagent," p-oxobi s[tri s(cTi methyl ami no)phosphoni urn] bi s-tetra-f1uoroborate (2) may also be used to activate the carboxyl function towards amide bond formation during peptide synthesis (4) and to bring about the Beckmann rearrangement of ketoximes (F). 

We wish to report a new approach to condensation reactions of hydroxy compounds related to the Ritter reaction, the Beckmann rearrangement and peptide formation based on easily accessible pentaco-ordinate spirophosphoranes of the type (3) (6 - ). 

The equations indicate that solvolysis is also possible. However, in most cases the reactions give only small yields. Many side reactions, cleavage of the Si-Si bond, rearrangements, and condensation reactions to polymers take place. 

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