Type reactions Condensation

A further route to the cycloocta[l,2-c 5,6-c ]difuran system makes use of a Perkin-type reaction. Condensation of furan-3,4-dicarboxaldehyde with furan-3,4-diacetic acid in acetic anhydride and triethylamine at room temperature, followed by esterification with methanol and sulfuric acid, gave... 

After every reaction step, the product can be extracted from the organic phase with fluorinated solvents. Excess reagents remain in the organic phase and can be easily removed. The strategy has also been successfully used in a Ugi multicomponent-type reaction condensing derivatised benzoic acid 31 with amines, aldehydes and isonitriles as shown in Scheme 3.3.2. The reaction was carried out in TFE and products 32 were isolated by liquid-liquid extraction. After cleavage of the fluorinated chain with TBAF followed by another extraction, the desired products 33 were isolated in good to excellent yields and with purities >80% (determined by GC). 

The reaction probably proceeds by an initial cyclisation of the acylamlnoacetic acid, followed by a Perkin type of condensation of the aldehyde with the active methylene unit ... 

For monomers of the AB type, reactions (5.CC) and (5.DD) become AB -> and 2AB AbaB, respectively. If kj. and k are the respective rate constants for these reactions, derive an expression which gives the ring to linear ratio in the product as a function of AB concentration and the two rate constants. Criticize or defend the following proposition To obtain a test of Eq. (5.47) without the complications of intramolecular condensations, a series of otherwise identical polymeriztion reactions could be carried out on monomer mixtures at different concentrations. By... 

Formaldehyde condenses with itself in an aldol-type reaction to yield lower hydroxy aldehydes, hydroxy ketones, and other hydroxy compounds the reaction is autocatalytic and is favored by alkaline conditions. Condensation with various compounds gives methylol (—CH2OH) and methylene (=CH2) derivatives. The former are usually produced under alkaline or neutral conditions, the latter under acidic conditions or in the vapor phase. In the presence of alkahes, aldehydes and ketones containing a-hydrogen atoms undergo aldol reactions with formaldehyde to form mono- and polymethylol derivatives. Acetaldehyde and 4 moles of formaldehyde give pentaerythritol (PE) ... 

Reactions. Mahc acid undergoes many of the characteristic reactions of dibasic acids, monohydric alcohols, and a-hydroxycarboxyUc acids. When heated to 170—180°C, it decomposes to fumaric acid and maleic anhydride which sublimes on further heating (see Maleic anhydride, maleic acid, AND FUMARIC acid). MaUc acid forms two types of condensation products linear malomalic acids and the cycHc dilactone or maUde it does not form an anhydride. 

In general, polymers are formed by two types of reactions condensation and addition. The formation of a polyester by polycondensation may be illustrated as follows. 

From Phenol. In the type of condensation discovered by von Pechmaim in 1883, coumarin is formed by reaction of phenol [108-95-2] with malic (34), maleic, or fumaric acids (35—38) in the presence of concentrated sulfuric acid. 

The reaction of 6-amino-5-(l,2-diethoxycarbonylhydrazino)pyrimidines with enamines represents another convenient method for the preparation of pteridines. Fusion of 5-(l,2-diethoxycarbonylhydrazino)-2,4,6-triaminopyrimidine (281) with an excess of mor-pholinocyclohexene leads to 2,4-diaminotetrahydrobenzo[g]pteridine, and with the morpholinoenamine (282) from 17/3-hydroxy-5a-androstan-3-one regioselective condensation to the fused pteridine (283) takes place in almost quantitative yield (equation 101) (71CC83). 6-Amino-5-nitroso- and 6-amino-5-phenylazo-pyrimidines react similarly, imitating the Timmis-type reaction (72CPB1428). 

Another condensation method was investigated by Carothers and co-workers and reported in 1930. They reacted decamethylene dibromide with sodium in a Wurtz-type reaction but found it difficult to obtain polymers with molecular weights above 1300. 

Cyclocondensation reactions with perfluoroalkyl-subsbtuted CO and CN multiple bond systems can be divided into several subgroups, according to the charge pattern of both reactants On the basis of this simple concept, hetero-l,3-dienes should undergo two types of condensation reactions, classified by the number of skeleton atoms of the diene being incorporated into the ring system (equation 10). 

The thermal (or photochemical) decomposition of the azo group gives rise to a radically initiated polymerization. The reactive site F, the transformation site, however, can, depending on its chemical nature, initiate a condensation or addition type reaction. It can also start radical or ionic polymerizations. F may also terminate a polymerization or even enable the azo initiator to act as a monomer in chain polymerizations. 

With respect to the initiation of cationic chain polymerizations, the reaction of chlorine-terminated azo compounds with various silver salts has been thoroughly studied. ACPC, a compound often used in condensation type reactions discussed previously, was reacted with Ag X , X, being BF4 [10,61] or SbFa [11,62]. This reaction resulted in two oxocarbenium cations, being very suitable initiating sites for cationic polymerization. Thus, poly(tetrahydrofuran) with Mn between 3 x 10 and 4 x lO containing exactly one central azo group per molecule was synthesized [62a]. Furthermore, N-... 

Historical. PE was first isolated by Tollens who was examining the effect of heat and reagents on formaldehyde. Apparently the crude formaldehyde he was working with contained a small % of acetaldehyde, which accounts for the PE formed. Although Tollens isolated it in 1882, it was not identified as PE until 1888 (Ref 2). Further details of the prepn and props of PE appeared in 1891 (Ref 3). The prepn is essentially a condensation betw 3 moles of formaldehyde and 1 of acetaldehyde to give an intermediate tris(hydroxymethyl)-acetaldehyde which is not isolated. An Intermolecular oxidn/redn then takes place betw this intermediate, and a 4th mole of formaldehyde, giving PE and formic ac (Ref 13, p 2). This type reaction is discussed under Cannizzaro Reaction in the Encycl (Vol 2, C25)... 

Due to the high a-C,H acidity in the alkoxyethylidene complexes 6 (e.g.,piCa=8 (R=Me)) [ 16], transformations via an enolate analog are possible and have been used to introduce additional functionality into the carbene side chain to access various Fischer carbene complexes [3]. The a,/J-unsaturated complex 8 could be obtained from 6 (R=Et) by an aldol-type condensation with benzaldehyde 7 in the presence of triethylamine and trimethylsilyl chloride (Scheme 2) [17]. This reaction proceeds completely diastereoselectively to yield only the trans-isomer. Analogously, binuclear complexes have been prepared from 6 and 1,3-and 1,4-phthaldialdehyde in good yields [17]. This type of condensation has... 

Aldehydes and ketones condense with a-halo esters in the presence of bases to give ot,p-epoxy esters, called glycidic esters. This is called the Darzens condensation. The reaction consists of an initial Knoevenagel-type reaction (16-41), followed by an internal Sn2 reaction (10-13) ° ... 

A second example of step-growth polycondensations with formation of the ole-finic double-bond are Wittig- and Wittig-Horner-type condensations. The Wittig-type polycondensations involve AA/BB-type reactions of aromatic bisal-dehydes with bisphosphonium ylides [99,100] with formation of PPV derivatives (75) and lead to products of only moderate molecular weight (DP 10-20). 

A Friedel-Crafts-type reaction of phenols under basic conditions is also possible. Aqueous alkaline phenol-aldehyde condensation is the reaction for generating phenol-formaldehyde resin.34 The condensation of phenol with glyoxylic acid in alkaline solution by using aqueous glyoxylic acid generates 4-hydroxyphenylacetic acid. The use of tetraalkylammonium hydroxide instead of sodium hydroxide increases the para-selectivity of the condensation.35 Base-catalyzed formation of benzo[b]furano[60]- and -[70]fullerenes occurred via the reaction of C60CI6 with phenol in the presence of aqueous KOH and under nitrogen.36... 

A tandem enzymatic aldol-intramolecular Homer-Wadsworth-Emmons reaction has been used in the synthesis of a cyclitol.310 The key steps are illustrated in Scheme 8.33. The phosphonate aldehyde was condensed with dihydroxyacetone phosphate (DHAP) in water with FDP aldolase to give the aldol adduct, which cyclizes with an intramolecular Horner-Wadsworth-Emmons reaction to give the cyclo-pentene product. The one-pot reaction takes place in aqueous solution at slightly acidic (pH 6.1-6.8) conditions. The aqueous Wittig-type reaction has also been investigated in DNA-templated synthesis.311... 

Although the reaction scheme shows a complete hydrolysis before condensation begins, this is likely not correct as stated earlier. The relative rates and extents of these two reactions will particularly depend on the amount of water added and the acidity of the system (10,11). The high functionality of the triethoxysilane endcapped PTMO oligomer should enhance the incorporation of PTMO molecules into the TEOS network. It was also assumed that the reactivities would be the same between silanol groups from silicic acid and endcapped PTMO. Therefore, no preferential condensation was expected and the deciding factors for which type of condensation (self- or co-) took place would be the diffusivities and local concentrations. 

Besides the Michael addition-initiated domino reactions presented here, a multitude of other anionic domino reactions exist. Many of these take advantage of an incipient SN-type reaction (for a discussion, see above). In addition to the presented SN/Michael transformations [97, 98, 100], a SN/retro-Dieckmann condensation was described by Rodriguez and coworkers, which can be used for the construction of substituted cycloheptanes as well as octanes [123]. Various twofold SN-type domino... 

Base-promoted condensation of ketones with 3-nitroaniline results in the formation of indoles. The reaction proceeds in one pot via a NASH-type reaction and subsequent cyclization (Eq. 9.55).88... 

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