Oligomerization intermolecular

The nature of the product strongly depends on the length of the hydroxy acid generally when the hydroxyl group is remote the yield of lactone drops significantly. For example, 10-hydroxydecanoic acid [1679-53-4] does not produce any decanoUde instead, the reaction proceeds by intermolecular oligomerization, and a complex mixture of di-, tri-, tetra-, and pentalactones results (90). However, when Pseudomonas sp. or Candida iylindracea]i 2Lses are incubated with 16-hydroxyhexadecanoic acid [506-13-8] hexadecanoUde is the predorninant product (91). 

Note that such cis-dienic structures can be present only if intermolecular oligomerization of nitrile groups has taken place.) The Diels-Alder product interrupts conjugation, thus reducing color formation. In the particular case of maleic anhydride, and under the relatively rough conditions of stabilization, this type of reaction should lead to crosslinking, by cross-anhydridization ... 

Olive and Olive (1981) [200] presented evidence that intermolecular oligomerization leading to cross-linked structures takes place and at higher temperatures, could be more important than intramolecular reaction leading to ladder polymer. 

Control of the concentration of polymer during the cyclization is also vital in order to yield high purity macrocyclics. More concentrated solutions favor the bimolecular oligomerization reaction while more dilute solutions favor the unimolecular cyclization reaction (Figure 1). As a result, the key cyclization reaction is typically carried out in a vast excess of solvent to discourage intermolecular oligomerization. 

The inefficiency of the final Friedel-Crafts cyclization was attributed to the presence of two non-equivalent carboxylic acids in 38 activation of the carboxylate adjacent to the fiiran might lead to intermolecular oligomerization processes. We reasoned that if only the desired position was available for activation, a more efficient cyclization could be realized. This strategy required a method for differentiation of the two carboxylic acids, which was achieved as shown in Scheme 14. 

Lipase-catalyzed intermolecular condensation of diacids with diols results in a mixture of macrocycUc lactones and liuear oligomers. Interestingly, the reaction temperature has a strong effect on the product distribution. The condensation of a,(D-diacids with a,(D-dialcohols catalyzed by Candida glindracea or Pseudomonas sp. Upases leads to macrocycUc lactones at temperatures between 55 and 75°C (91), but at lower temperatures (<45°C) the formation of oligomeric esters predorninates. Optically active trimers and pentamers can be produced at room temperature by PPL or Chromobacterium viscosum Upase-catalyzed condensation of bis (2,2,2-trichloroethyl) (+)-3-meth5ladipate and 1,6-hexanediol (92). 

All the described properties of such a s-fraction of poly(NVCl-co-NVIAz) synthesized at the temperature above the PST of the reacting system allowed us to draw the conclusion that the chains of this type had the comonomer sequence, which at the temperatures above the conformation transition facilitated the formation of polymer particles, where H-blocks are in the interior shielded by the P-blocks against additional intermolecular association. Such a behaviour of this copolymer in aqueous media is close to that of oligomeric proteins similar to casein [46] possessing a rather hydrophobic core surrounded by the polar segments. 

There have been several efforts geared toward reducing the oligomeric nature of the group 2 metallocene compounds. Reduction of the nuclearity has been accomplished via the use of sterically demanding cyclopentadienides and intermolecular Lewis bases (38-40). Hanusa and Williams have used bidentate amines to coordinatively saturate... 

The Sn6P6 cages 19c and 19d are accessible by two different Bronsted acid-base reaction pathways Reaction of lc and Id, respectively, with two different stannanediyl derivatives furnished in 80-89% yield red-black crystals of the aggregates (Eq. 12) (39). The tin(II) phosphandiides are somewhat related to the previously described oligomeric bis (phosphaneyl) stannanediyls of the type PkSn, which easily form intermolecular aggregates (50, 51) or remain monomeric, if the phosphorus atoms bear very crowded organosilyl substituents (52). 

Imipenem (5.46) has not completely fulfilled such expectations [122], Indeed, this compound is unstable in both acidic and alkaline media. In weakly acidic solutions, imipenem undergoes complex oligomerization, a reaction initiated by the intermolecular attack of the carboxy group on the /3-lactam Fig. 5.17) and yields, finally, a diketopiperazine compound. In weakly alkaline solution, an intermolecular reaction between the /3-lactam and (iminome-thyl)amino group was observed Fig. 5.18). This reaction proceeds via an unstable dimer that breaks down to thienamycin (5.45) and a /3-lactam ring-opened compound bearing a Ai-formyl group [123],... 

Fig. 5.17. Acid-catalyzed oligomerization of imipenem (5.46) starting with an intermolecular attack by the carboxy group on the ji-lactam bond [123]...

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