1,2-Condensation dimer

Figure 5. The molecular structure of the 1,2-condensation dimer of CgHi,(NH)[NP(NEt2)2KPNEt2) (XV), as the trisulfide derivative. Ethyl groups on N5, N6, N7, N8 and N9 omitted for clarity.

The resinification induced by 7-alumina11 seems to proceed by a somewhat different mechanism, probably because of the higher temperatures involved. Side reactions are more prominent from the beginning and it has been suggested, but not proved, that the C-3 and C-4 positions of the ring are vulnerable under these conditions to substitution reactions. A new compound, viz. 4-(2-furfuryl)-2-pentenoic acid-7-lactone, which is an isomer of the condensed dimers, was identified among... 

Another important reaction in synthetic chemistry leading to C-C bond formation is the Michael addition. The reaction typically involves a conjugate or nucleophilic 1,4-addition of carbanions to a,/l-unsaturated aldehydes, ketones, esters, nitriles, or sulfones 157) (Scheme 21). A base is used to form the carbanion by abstracting a proton from an activated methylene precursor (donor), which attacks the alkene (acceptor). Strong bases are usually used in this reaction, leading to the formation of byproducts arising from side reactions such as condensations, dimerizations, or rearrangements. 

When 2-aminopyrimidine is treated with 1,3-dichloroacetone and sodium cyanide, an unusual condensation dimer (49) is formed along with the anticipated product (48). The... 

Fig. 9D.9 Absorption spectra in the visible region of a catechin-Mv3glc direct condensation dimer as a function of pH (adapted from Salas et al. 2004b with permission from Elsevier)...

In this chapter, we include certain natural products, such as typical anthraquinones and naphthoquinones, which are present, along with condensed dimeric structural drivatives such as dianthrones, in many medicinal plants. In addition, we deal with the much more infrequent OT-r/io-quinones. 

So far we have considered only self-condensations —dimerization reactions of a single carbonyl compound. These form only a tiny fraction of known aldol reactions. Those that occur between two different carbonyl compounds, one acting as a nucleophile in its enol or enolate form, and the other as an electrophile, are called cross-condensations. They are more interesting than self-condensations, but working out what happens needs more thought. 

When the interaction is intermolecular, condensation (dimerization, polymerization) or redistribution products may be formed. An intramolecular reaction may give rise to rearrangement of the hydrocarbon skeleton (isomerization) or group migrations. 

Further, it should be noted that the same macrocycle as in Figure 1 could be obtained by using condensation dimers of acetone, such as diacetone alcohol or mesityl oxide (8). It was also found that several kinds of a,0-unsaturated ketones and 0-hydroxyketones may be used to prepare macrocyclic diamino-diimines (8 ). This observation is very important, because by using suitable ketones it becomes possible to introduce desired substituent(s) into the three carbon atom linkage. In fact, a variety of diamino-diimine macrocycles has been prepared and used as ligands in recent years (9 - 14 ). However, almost all of the macrocycles were synthesized under reaction conditions not requiring metal ions such as nickel(II). 

The fully condensated dimer ions dominate these are obtained by retaining all boron and bridging hydrogen atoms and eUminating all terminal H atoms. The only reactions which occur with formation of a neutral boron-containing product (BH3) leading to borane ions BsH ( =3,4,5,6) are endothermic and appear in the case of diborane. They can only be detected by double-resonance experiments if the primary ions have sufficient internal energy. 

A tailored modifier does not always have to be deliberately added to a crystallizing system it may already exist, e.g., as a synthesis by-product of a chemical reaction. If its presence causes a crystal habit problems, it must be removed or deactivated. On the other hand, it may have a beneficial effect. These are both commonly encountered cases in the manufacture of organic chemicals. A simple, but industrially important, example is the effect of biuret on the crystallization of urea (Davey, Fila and Garside, 1986). In the synthesis of urea (NH2CONH2) from ammonia and carbon dioxide a small amount of biuret (NH2CONHCONH2), a condensation dimer, is formed. The presence of biuret is actually beneficial because from pure aqueous solution urea crystals form as elongated [001] needles that are difficult to process. Biuret retards growth in the... 

A number of non-condensed dimers are formed and only some of these have the right steric conformation to undergo an addition dehydrogenation reaction to give fully condensed molecules with only six-membered rings involving the most reactive position 9 for this to... 

The mechanistic pathways for the reactions of monovinylsilanes are well-established [19,21] however, the general catalytic scheme for condensation (dimerization) of divinylsubstituted silicon compounds (as a particular example of its oligomerization) is given in Scheme 1 [22]. 

The third of the reactions described above (i.e., the Guerbet reaction of primary alcohols) allows for the self-condensation/dimerization of alcohols to give the... 

A condensation dimer of 5-hydroxy-3-methyl-2(5H)-furanone and a 2-ring analog of strigol were also active as growth inhibitors. 

The Onset of Condensation Dimer Formation in the CRESU Experiment... 

Aldol condensation includes reactions of aldehydes or ketone producing /3-hydroxyaldehydes or /3-hydroxyketones by self-condensation (dimerization) or mixed condensation. A general formula of the reaction may be drawn as follows. 

The 3.8-nonadienoate 91, obtained by dimerization-carbonylation, has been converted into several natural products. The synthesis of brevicomin is described in Chapter 3, Section 2.3. Another royal jelly acid [2-decenedioic acid (149)] was prepared by cobalt carbonyl-catalyzed carbonylation of the terminal double bond, followed by isomerization of the double bond to the conjugated position to afford 149[122], Hexadecane-2,15-dione (150) can be prepared by Pd-catalyzed oxidation of the terminal double bond, hydrogenation of the internal double bond, and coupling by Kolbe electrolysis. Aldol condensation mediated by an organoaluminum reagent gave the unsaturated cyclic ketone 151 in 65% yield. Finally, the reduction of 151 afforded muscone (152)[123]. n-Octanol is produced commercially as described beforc[32]. 

Chiefly in an hydrophobic medium, a base can extract the proton on position 2 leading to a reactive intermediate (able to give subsequent condensation) that could be an ylid (35, 36) or a carbene (37), though no dimer has ever been isolated as is the case with benzothiazolium (32, 38). Two mechanisms have been proposed for explaining the particular reactivity of thiazolium ... 

The polymer described in the last problem is commercially called poly (phenylene oxide), which is not a proper name for a molecule with this structure. Propose a more correct name. Use the results of the last problem to criticize or defend the following proposition The experimental data for dimer polymerization can be understood if it is assumed that one molecule of water and one molecule of monomer may split out in the condensation step. Steps involving incorporation of the monomer itself (with only water split out) also occur. 

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