Pseudo-high-dilution conditions

Cyclic aryl ether ketones have been prepared from l,2-bis(4- uoroben2oyl)ben2ene and bisphenols under pseudo high dilution conditions. These materials undergo ring-opening polymeri2ation in the presence of an anionic catalyst (87). 

These sulfur-containing macrocycles had been prepared under pseudo-high dilution conditions, but Hay and coworkers demonstrated that the cyclodepolymerization of aromatic disulfide polymers was an efficient and easier... 

Pyrrole-substituted a,co-diamines 517 varying in length and number of heteroatoms between the amine functionalities were obtained from the corresponding acid and appropriate diamine under standard amide-forming conditions in yields of 18-70%. The amines 517 were reacted with l,l -ferrocenedimethanol 518 under pseudo high-dilution conditions to form 7 r 7-ferrocenes 519, the macrocyclic anion receptors, in 20-35% yields (Equation 122) <2001JOM(637)343>. 

Synthesis of cyclic oligomers of PEK under pseudo-high-dilution conditions... 

The synthesis and characterization of an expanded Cu(I)-catenate 4.62 and the first demetalated conjugated [2]catenane 4.63 by using quaterthiophene instead of terthiophene units was accomplished by the same group. Cyclization of 4.59 under pseudo-high-dilution conditions surprisingly led to monomacrocycle 4.60 (50% yield) instead of the expected [2]catenate (Scheme 1.55). Apparently, closure of the first macrocycle generates steric constraints disentangling the second crescent-shaped precursor. 

The pseudo-high-dilution conditions of sohd phase minimize production of... 

Another unique approach to sohd-supported metathesis was examined by Schreiber and coworkers toward the production of homodimeric compounds via an intra-site cross metathesis. This strategy used of a highly loaded (l-2mmol/g), lightly cross-linked (1% DVB) resin 28 to minimize the pseudo-high dilution conditions on the resin surface and facilitate metathesis reactions between immobihzed molecules yielding 29 (Scheme 6.7). Again, the ElZ ratios of the homodimeric products 30 were identical to those prepared in solution, as the solid support had no influence on the ratio of stereoisomers. This approach... 

Akaji and Kiso demonstrated another interesting example of a Heck cyclization toward the macrocyclization of peptides. The cyclization reaction was faciUtated by the pseudo-high dilution conditions afforded by the sohd support to reduce dimer or ohgomer formation. An iodobenzylamine was coupled to Fmoc-Asp(OfBu)-OH, which was then deprotected and immobilized on a sohd support. Standard Fmoc-based peptide synthesis was then performed and the peptide was capped with an acrylic acid monomer. The Heck macrocyclization was then performed on both the sohd support and in solution phase. Interestingly, based on the HPLC analysis of the reaction mixture, the cychzation on the solid support was generally completed within 2 h (affording a 30% overall yield), while the corresponding solution-phase reaction required approximately 8 h (the yield of the solution-based reaction was not reported). 

Oxidation of [l ]paracyclophanes with thioether linkages to sulfones has been investigated extensively by Colquhoun et Reacting 4,4 -sulfonylbis-(benzenethiol) (13.13) with 4,4 -dichlorodiphenylsulfone (12.14) under pseudo high-dilution conditions - i.e., slow addition of both starting materials to the reaction mixture - leads (Scheme 12.5) to the formation of macrocycles 12.12a, 12.12b, and 12.12c with 4, 6, and 8 phenylene units, respectively. Given that, with a direct nucleophilic aromatic substitution mechanism, only cyclic products with 4 n = l, 2, 3,. ..) phenylene units should be able to form directly in this reaction, it is surprising that the [l ]paracyclophane 12.12b with six phenylene units is produced as the major macrocycle. 

Malesevic, M., Strijowski, U., Bachle, D., and Sewald, N. (2(X)4) An improved method forthe solution cyclization of peptides under pseudo-high dilution conditions. Journal of Biotechnology, 112,73-77. 

Currently it is possible to obtain low-molecular-weight cyclic oligomers completely selectively with respect to linear oligomers by applying pseudo-high-dilution conditions to a hydrolysis/condensation reaction of BPA bischloroformate, catalyzed by EtsN in biphasic dichloromethane/NaOHaq. (Scheme 14). 

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