Urea functionality

BENZENESULFONIC ACID DERIVATIVES As has been discussed previously, substituted -alkylbenzene-sulfonylureas often possess the property of releasing bound insulin, thus sparing the requirement for insulin injections in adult-onset diabetes. A pyrimidine moiety, interestingly, can serve as a surrogate for the urea function. 

Inherent in the reduction of asymmetrically substituted cyclic imides is the problem of regiose-lectivity. Imides, in which one carbonyl group is part of a (thio)carbamate or urea function, usually show complete chemoselectivity for reduction of the other carbonyl group, indicated with an arrow. 

In transition metal complexes, two main coordination modes have been described for urea ligands W-monohapto, or N,0-chelates. In the case of molecules containing more than one urea function, the molecules act as Ar,AT-chelates, so that one of the urea functions always behaves as monohapto-ligand. For example, complexes of Co(III) with N-(2-pyridylmethyl)urea and ethylenediamine have been characterised by X-ray crystallographic analysis (Scheme 7). The urea group is coordinated through only one of its N... 

Both the ureas and thioureas are highly suitable organocatalysts for the asymmetric Strecker synthesis. For example, the thiourea function was replaced by an urea function (note the opposite configurations). The organocatalysts thus obtained showed similar activity and slightly higher enantioselec-tivities with N-allyl benzaldimine (Scheme 39,74% yield with 95% ee for Ri = Bn and R2 = H). Once again, better enantioselectivity (up to 99% ee) was at-... 

Marine organisms have provided very complex molecules possessing a pyr-rolidinone ring as part of a urea function dysidin (105) (140), malyngamide A (106) (141) and pukeleimides A, B, C, D, E, F, and G (107-113) (142, 143). 

Figure 5. Top Tetraurea calixarene monomers 37 and 38 bearing chiral amino acid ester residues (isoleucine and valine methyl esters, respectively) attached to the urea functions. Norcamphor 39 was the chiral guest used to detect the chirality transfer from the outside to the inner cavity.

The urea functionality, a common structural motif in biologically... 

Schazmann B, Alhashimy N, Diamond D (2006) Chloride selective calix[4]arene optical sensor combining urea functionality with pyrene excimer transduction. J Am Chem Soc 128 8607-8614... 

Noticeably propranolol, betaxolol and metoprolol are close or on the borderline for hepatic first-pass effects, whereas talinolol falls markedly below it. Talinolol has been shown to be a substrate for P-glycoprotein [14]. The effect of the urea function is of key importance within this change, as urea lacks a strong type I unit in terms of Seel-... 

In the presence of thiourea catalyst 122, the authors converted various (hetero) aromatic and aliphatic trons-P-nitroalkenes with dimethyl malonate to the desired (S)-configured Michael adducts 1-8. The reaction occurred at low 122-loading (2-5 mol%) in toluene at -20 to 20 °C and furnished very good yields (88-95%) and ee values (75-99%) for the respective products (Scheme 6.120). The dependency of the catalytic efficiency and selectivity on both the presence of the (thio) urea functionality and the relative stereochemistry at the key stereogenic centers C8/C9 suggested bifunctional catalysis, that is, a quinuclidine-moiety-assisted generation of the deprotonated malonate nucleophile and its asymmetric addition to the (thio)urea-bound nitroalkene Michael acceptor [279]. 

M. Shi and Y.-L. Shi reported the synthesis and application of new bifunctional axially chiral (thio) urea-phosphine organocatalysts in the asymmetric aza-Morita-Baylis-Hillman (MBH) reaction [176, 177] of N-sulfonated imines with methyl vinyl ketone (MVK), phenyl vinyl ketone (PVK), ethyl vinyl ketone (EVK) or acrolein [316]. The design of the catalyst structure is based on axially chiral BINOL-derived phosphines [317, 318] that have already been successfully utilized as bifunctional catalysts in asymmetric aza-MBH reactions. The formal replacement of the hydrogen-bonding phenol group with a (thio)urea functionality led to catalysts 166-168 (Figure 6.51). 

Series of various mono,- bi-, and poly-(thio)urea-functionalized (poly)saccha-rides have already been synthesized and studied for molecular recognition of, e.g., dimethyl and phenylphosphate as model compounds for monoanionic and polyanionic phosphate esters, respechvely [111]. Thiourea derivatives such as 195-197 were analyhcally identified to provide double hydrogen bonding mediated host-guest complexes of well-defined dimension and orientations and were also reported to serve as phosphate binders even in the hydrogen bonding environment of water (Figure 6.57) [111]. 

Ureides. Compounds that contain urea functionalities are stable and are not commonly metabolized or hydrolyzed. 

The sulfonylurea hypoglycemic agents, as noted in Chapter 2, also trace their ancestry to the sulfonamides. It is of interest that activity is retained when a substituted 2-amino-1,3,4-thiadiazole replaces the urea function. Reaction of isobutyryl chloride (123-1) with thiosemicarbazone (123-2) leads initially to the transient 1,2-diacyUiydrazine (123-3). This apparently cyclizes spontaneously to thiadiazine (123-4) under reaction conditions. Acylation with p-methoxysulfonyl chloride (123-5) affords the oral hypoglycemic agent isobuzole (123-6) [134]. 

A rhodium catalyst based on the glycine-urea functionalized Xantphos ligand was subsequently used in combination vdth the same support material, as a catalyst for the hydroformylation of 1-octene. In eleven consecutive reactions the catalyst did not... 

Recently, we [53] and others [54] simultaneously reported an example of a complex in which the transition metal dictates the coordination mode, viz. urea-functionalized phosphine 20, which forms a trans palladium complex, complemented by hydrogen bonding ofthe urea fragments (Figure 10.6). Bear in mind that any monophosphine, and even wide bite angle diphosphines, give trans complexes with a hydrocarbyl palladium halide. More interestingly, the urea moieties can function as a host for another halide ion. 

Some Common Pesticides Containing Urea Functional Group Table 2.19.3... 

A large number of nitrogen-containing pesticides are characterized by the carbamate or urea functional group or triazine ring in their structures. These substances constitute the three most common classes of nitrogen-containing pesticides. The structural features of these three distinct classes of substances are shown below ... 

Another interesting and potentially very useful group of calixarene-based anion receptors is represented by systems with appended transition metal complexes of 2,2-bipyridine units. Technically, these systems utilise classical hydrogen bonding interactions of amidic/urea functions hence, from this point of view, they do not differ from many other receptors. On the other hand, the covalent attachment of bipyridine complexes of ruthenium(II) or rhe-... 

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