Specialized Condensation Reactions

This section includes a few condensation-type reactions that are not readily categorized in previous sections. They are presented to show the variations that are possible with this fundamental strategy. 

One interesting condensation was reported between 2-methylmalonic acid and 1-aminoethanol (4.132), but the yields were poor.59 Refluxing in ethanol followed by reaction with ammonia gave 2-methyl-3-aminobutanoic acid, 4.133.  

Another variation reacted ethyl 3,3-diphenylpropenoate (4.134) with potassium metal in ammonia to give an intermediate bearing a carbanionic carbon P- to the ester moiety. This intermediate was trapped with 2-chloro-N,N-dimethylamino- 

A synthesis of aryl substituted amino acids involved the reaction of maleic acid with aryl aldehydes, in the presence of ammonium acetate, to give P-aryl- -amino acids.6f Specifically, maleic acid reacted with 3-nitrobenzaldehyde (4.136) to generate 3-amino-3-(3-nitrophenyl)propanoic acid, 4.137.  

Functionalized amino acids can be prepared by manipulation of specialized precursors. Reaction of diethyl 2-chloromalonate with sodium phenoxide gave 

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Hemetsberger and coworkers published in 1969 the special condensation reaction of benzaldehydes 76 with ethyl azidoacetate (77a) in the presence of sodium ethoxide to get moderate to good yields of the a-azidocinnamic esters 79a (Scheme 5.12). The temperature and the reaction time had to be controlled careMly, because the strongly alkaline reaction conditions induced the danger of decomposition of the base-sensitive azides 77 and other unwanted side reactions. Thus, Knittel reported some years later that usage of methyl azidoacetate (77b) and sodium methoxide offers several advantages in the synthesis of vinyl azides 79b. Nevertheless, the Hemetsberger-Knittel reaction can only be... 

Synthesis of large heterocycles usually involves condensation reactions of two difunctional molecules. Such molecules tend to polymerize. So far two special techniques have been described above to avoid this important side-reaaion , namely high dilution and use of templates. The general procedure to avoid polymerizations in reactions between difunctional molecules is, of course, the application of protecting groups as described in sections 4.1.2 and 2.6. 

Thiazolium derivatives unsubstituted at the 2-position (35) are potentially interesting precursors of A-4-thiazoline-2-thiones and A-4-thiazoline-2-ones. Compound 35 in basic medium undergoes proton abstraction leading to the very active nucleophilic species 36a and 36b (Scheme 16) (43-46). Special interest has been focused upon the reactivity of 36a and 36b because they are considered as the reactive species of the thiamine action in some biochemical reaction, and as catalysts for several condensation reactions (47-50). 

Apart from some special cases, the ring-closure step in the course of the synthetic pathway is often condensation reaction and, thus, these transformations can be considered as cyclocondensations . Reactions following a few other mechanisms like cycloaddition, electrocyclization, and intramolecular nucleophilic addition of electron pair of an atom to a multiple bond also occur relatively often. Because of the fairly large complexity of the ring systems belonging to this chapter, the treatment according to the heteroatomic combinations seemed the most straightforward. Thus, the fused... 

Carothers and his research group synthesised many polyesters, nearly all of them aliphatic. His basic patent was filed on July 3, 1931 [23], This work, together with his work on condensation polymers in general, proved influential in convincing the scientific community that polymers were indeed macromolecules, as maintained by Staudinger, and not assemblies of small molecules in a special state of association. Staudinger himself was initially reluctant to accept that the polyesters were polymers, since he had defined polymers in such a way as to refer solely to products of addition reactions. He thus excluded products of condensation reactions, where small molecules were eliminated as co-products of the reaction [24],... 

Although the reaction of alkali acetylides with oxirane proceeds slowly in liquid ammonia 15], it is an excellent method for preparing "homo-propargylic alcohols in quantities of 1 mol or more [2). Since oxirane is very volatile, considerable losses could occur if it is allowed to be swept along with the escaping ammonia vapour. One solution is to carry out the reaction under reflux, using a special condenser filled with dry ice and acetone. This would require regular addition of dry ice over a period of at least 12 h. It is much simpler to mix the acetylide... 

Polyurethanes (PU) are a special class of polymers containing the urethane group, (—NH-CO-O-), commonly prepared by a condensation reaction between a diisocyanate and a diol, such as hydroxyl terminated polyether or polyester ... 

Free radical polymerization offers a convenient approach toward the design and synthesis of special polymers for almost every area. In a free radical addition polymerization, the growing chain end bears an unpaired electron. A free radical is usually formed by the decomposition of a relatively unstable material called initiator. The free radical is capable of reacting to open the double bond of a vinyl monomer and add to it, with an electron remaining unpaired. The energy of activation for the propagation is 2-5 kcal/mol that indicates an extremely fast reaction (for condensation reaction this is 30 to 60 kcal/mol). Thus, in a very short time (usually a few seconds or less) many more monomers add successively... 

The ortho di-amino benzene prepared by the reduction of ortho-nitraniline is of special interest because of certain condensation reactions which it undergoes with aldehydes, ketones and nitrous acid. The meta compound also shows a characteristic reaction with nitrous acid. The para compound is readily oxidized and gives characteristic color reactions with ferric chloride and hydrogen sulphide. It is of importance in the preparation of dyes. All of these di-amines are colorless, crystalline solids which can be distilled. 

Free radicals may also be intermediates in poly condensation reactions, in special cases, such as... 

A special kind of condensation reaction occurs as a side reaction during the reduction of o-nitroaniline [120,137] A phenazine derivative is formed. In the reaction (Chapter 18), a double Michael addition of o-phenylenediamine to the intermediate o-quinonedi-imine takes place other nucleophiles might under suitable conditions attack the diimine. 

For silica gels a number of parameters have been demonstrated to have a large effect on the evolution of porosity and subsequently on the resulting silica materials [1]. Almost dense, micro- or mesoporous silica materials can be obtained depending on the experimental conditions in which hydrolysis and condensation reactions of silicon alkoxides are carried out. This is not the case for transition metal alkoxides which are very sensitive to hydrolysis. They do not cillow the adaptation of sol-to-gel transition in order to obtain controlled porous textures. Some years ago special attention was paid to the utilization of amphiphilic systems as reactive media to control hydrolysis and condensation kinetics with transition metal alkoxides [37]. In a more recent work Ayral et al. 

A special problem in the application of the polymer theory is the knowledge of the equilibrium constant of the poly-condensation reaction A n. The values of the dimerization constants in some binary Me0-Si02 systems (M = Ca, Mn, Pb, Fe, Co, Ni) were calculated by Masson (1977). Balta and Balta (1971) found a linear relationship between the logarithm of the equilibrium constant and the ionization potential of the metallic cation, which allows estimation of the equilibrium constant in systems, where the experimental data are missing, e.g. for cations with a ionization potential close to the second ionization potential of Mg, Masson (1977) published for Mn at 1773 K and Pb at 1273 K the values of the equilibrium constant Ku = 0.19 K and Ku = 0.196, respectively. 

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