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Acylated crowns

Tusek has reported the Clemmensen reduction of acylated crowns such as shown above. This approach constitutes a useful two-step alkylation procedure, although the yields reported for this sequence are not as high as one might wish. Other examples of closely related acylation approaches can be found in recent work by Tashmukhamedova and coworkers ° ° and by Kauer °. [Pg.27]

AC2O, 18-crown-6, Et3N, 98% yield. The crown ether forms a complex with a primary amine, thus allowing selective acylation of a secondary amine. [Pg.351]

Of the many methods which have been published so far for the substitution of existing crowns, probably the most straightforward are Friedel-Crafts alkylation or acylation reactions. Cygan, Biernat and Chadzynski have reported the successful di-t-butylation of dibenzo-24-crown-8 using t-butanol as alkylating agent s . The crown was heated at 100° for 4 h in the presence of excess t-butanol and 85% phosphoric acid. The product was obtained as a crystalline (mp 52—74°) solid in 93% yield. The alkylated crowns are presumably a mixture of isomers substituted once in each ring as illustrated in Eq. (3.14). [Pg.26]

Even though formic anhydride is not a stable compound (see p. 714), amines can be formylated with the mixed anhydride of acetic and formic acids (HCOO-COMe) °°° or with a mixture of formic acid and acetic anhydride. Acetamides are not formed with these reagents. Secondary amines can be acylated in the presence of a primary amine by conversion to their salts and addition of 18-crown-6. ° The crown ether complexes the primary ammonium salt, preventing its acylation, while the secondary ammonium salts, which do not fit easily into the cavity, are free to be acylated. [Pg.508]

A catalytic effect that may be of the same type as the mimicry of acyl transferase enzymes is reported by Gandour et al. (1978). They describe the rate enhancement of the aminolysis of p-nitrophenyl acetate in the presence of bis(2,6-pyridinyl) crown ethers [328] and [329] in chlorobenzene. The rate of... [Pg.417]

Chirality derived from the readily accessible a-amino acids has been incorporated into the side chains of aza and diaza macrocyclic polyethers. A number of procedures suitable for peptide synthesis have proved (178) to be unsuitable for acylating the relatively unreactive secondary amine groups of aza crown ethers. Eventually, it was discovered that mixed anhydrides of diphenylphos-phinic acid and alkoxycarbonyl-L-alanine derivatives do yield amides, which can be reduced to the corresponding amines, e.g., l-172. By contrast, the corresponding bisamides of diaza-15-crown-S derivatives could not be reduced and so an alternative approach, involving the use of chiral A-chloroacetamido alcohols derived from a-amino acids, has been employed (178) in the synthesis of chiral receptors, such as ll-173 to ll-175, based on this constitution. [Pg.267]

During the past two decades, a relatively few macrocyclic polyethers have played central roles in numerous research programs. Examples are 18-crown-6, dibenzo-18-crown-6, and aza-15-crown-5. 4,13-D1aza-18-crown-6 and its derivatives are compounds of considerable current interest despite the parent s high price and limited availability. 4,13-Diaza-18-crown-6 is a key compound in the study of two-armed macrocycles since it may readily be alkylated or acylated to afford a variety of symmetrical, N,N -disubstituted derivatives. [Pg.116]

The ability of [18]crown-6 derivatives to complex primary alkylammonium ions has been elegantly exploited in the protection of primary amines (80CC300). In the presence of primary amines, secondary amines can be acylated selectively by adding [18]crown-6 and a proton source. This strategy has obvious advantages over normal amine protecting groups which require a deprotection step. [Pg.759]

Under high dilution conditions (81JCR(S)314) bis-acyl chlorides react with diols to generate the corresponding tetralactones (41) (77TL2573). This procedure has most commonly been applied to the synthesis of crown ethers with subunits (79CRV37). Lactams have been prepared similarly from suitable diamines (76TL2603). [Pg.769]

Catalysis of Acyl Transfer Processes by Crown-Ether Supported Alkaline-Earth Metal Ions... [Pg.113]

Earlier work in this field has been thoroughly reviewed [1,2]. However, to illustrate in a sensible and logical way the evolution from simple metal ion promotion of acyl transfer in supramolecular complexes to supramolecular catalysts capable of turnover catalysis, an account of earlier work is appropriate. The following sections present a brief overview of our earlier observations related to the influence of alkaline-earth metal ions and their complexes with crown ethers on the alcoholysis of esters and of activated amides under basic conditions. [Pg.113]

Scheme 5.2 outlines our design of nucleophilic transacylation catalysts based on crown-complexed alkaline-earth metal ions. By virtue of the acidity-enhancing effect ofthe complexed metal ion, dissociation ofthe proton-ionizable function XH should take place under moderately basic conditions. The metal ion assists acyl transfer from a reactant ester to the catalyst and its subsequent transfer from the acylated catalyst to an external nucleophile (solvent), thus restoring the active form of the... [Pg.118]


See other pages where Acylated crowns is mentioned: [Pg.26]    [Pg.555]    [Pg.555]    [Pg.26]    [Pg.555]    [Pg.555]    [Pg.516]    [Pg.34]    [Pg.47]    [Pg.172]    [Pg.281]    [Pg.37]    [Pg.320]    [Pg.134]    [Pg.265]    [Pg.110]    [Pg.337]    [Pg.408]    [Pg.168]    [Pg.92]    [Pg.27]    [Pg.332]    [Pg.164]    [Pg.161]    [Pg.358]    [Pg.125]    [Pg.127]   
See also in sourсe #XX -- [ Pg.26 ]




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Catalysis of Acyl Transfer Processes by Crown-Ether Supported Alkaline-Earth Metal Ions

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