Isocyanates, addition from ureas

Urea derivatives are of general interest in medidnal chemistry. They may be obtained either from urea itself (barbiturates, see p. 306) or from amines and isocyanates. The latter are usually prepared from amines and phosgene under evolution of hydrogen chloride. Alkyl isocyanates are highly reactive in nucleophilic addition reactions. Even amides, e.g. sulfonamides, are nucleophilic enough to produce urea derivatives. 

Aminoimidazole esters have also been used as a source of 2-oxopurines, the additional nitrogen being derived from urea, cyanates and similar compounds. In this manner the amino ester (340) and potassium cyanate furnished a ureidoimidazole which smoothly cyclized to xanthine with dilute alkali (Scheme 142) (42JCS232). Methyl isocyanate similarly gave 1-methylxanthine and related derivatives have also been prepared (49JCS1071,50JCS1884). 

A more detailed quantitative study of the water-o-tolyl isocyanate reaction by Shkapenko et al. 51) showed that at 80° in dioxane solution and in the presence of triethylamine or other catalysts the consumption of the isocyanate was complete within a short period when only approximately half of the theoretical amount of carbon dioxide was released. The evolution of carbon dioxide proceeded from this point on at a slow rate. It was also demonstrated that by heating the reaction mixture to 100°, 30-35% of theoretical CO2 was released, and that this portion of the CO2 was given off by the decomposition of the carbamic acid anhydride formed from the acid and a second molecule of isocyanate. Additional tests showed that 4-5% of the isocyanate formed o-tolyl ammonium-W-o-tolyl carbamate, 18.7% of the water added remained unreacted, and that a trace of the free o-tolyl amine was also present. In addition, the presence of di-o-tolyl urea was proven. 

The compound reacts with alcohols, phenols and amines to form products similar to those obtained from the corresponding reaction with COCI [1358]. Depending upon the conditions and stoicheiometry, carbonates and chloroformates [612,1380], carbamates [1380], isocyanates [1184] and ureas [938] can be obtained with diphosgene, in addition to more complicated products such as carbamoyl chlorides, imidic chlorides, carboxylic acid chlorides, isonitriles and IV-carboxy-Q-amino acid anhydrides [1358]. 

Reviews on the synthesis and chemistry of carbodiimides are given in [1248-1250]. Carbodiimides are mainly synthesized in one of three ways from ureas or thioureas, from isocyanates, or from isocyanides. Several reagents have been employed in carbodiimide synthesis phosgene [1252, 1253], dimethylphosgenimi-nium chloride [1254], tiiphosgene [561, 562], phosphorus pentoxide [1255], phos-phoryl chloride [1256], triphenylphosphine dibromide [758, 1257-1261], triphenylphosphine/tetrahalomethanes [1262, 1263], iminophosphoranes [1264-1277], Mitsunobu reagent [1278, 1279], p-tosyl chloride [1280, 1281], and CDC [1137] oxidative additions have also been used [1282-1284]. 

Without additional reagents Ureas from isocyanates s. 17, 403... 

Without additional reagents Isocyanates and amines from ureas... 

Ethyl carbamate in wine is formed (mostly at the end of fermentation) from urea. The intermediates of its degradation are probably cyanates and cyanic acid (HO-C=N), also known as hydrogen cyanate, which may isomerise to isocyanic acid (H-N=C=0). Iso-cyanic acid can also arise by protonation of the cyanate anion and nucleophilic addition of ethanol to isocyanic acid yields ethyl carbamate. Isocyanic acid also reacts with other nucleophilic reagents, such as water (with formation of ammonia and carbon dioxide), thiols and amino groups of proteins. By catalysis with ornithinecar-bamoyl transferase, citrulline is transformed into ornithine and carbamoyl phosphate, the ethanolysis of which yields ethyl carbamate (Figure 12.39). 

In the manufacture of highly resident flexible foams and thermoset RIM elastomers, graft or polymer polyols are used. Graft polyols are dispersions of free-radical-polymerized mixtures of acrylonitrile and styrene partially grafted to a polyol. Polymer polyols are available from BASF, Dow, and Union Carbide. In situ polyaddition reaction of isocyanates with amines in a polyol substrate produces PHD (polyhamstoff dispersion) polyols, which are marketed by Bayer (21). In addition, blending of polyether polyols with diethanolamine, followed by reaction with TDI, also affords a urethane/urea dispersion. The polymer or PHD-type polyols increase the load bearing properties and stiffness of flexible foams. Interreactive dispersion polyols are also used in RIM appHcations where elastomers of high modulus, low thermal coefficient of expansion, and improved paintabiUty are needed. 

There exists ample evidence for the increase in potency that can be achieved by the addition of substituents on the ethylamine fragment at in the sulfonylurea antidiabetic series (see, for example, glyburide in Chapter 2). Reaction of pyrrolidinone (27-1) with the isocyanate from phenethylamine (27-2) leads to urea (27-3). The... 

The nature of a hydrophilic prepolymer permits the addition of large amounts of water. The isocyanate reacts with the water to abstract COt. The amine that also results from the reaction then reacts with an isocyanate group to produce a urea linkage. The reaction continues until the water or the isocyanates are consumed. If provisions are made to trap the CO2 in the mass, a foam is produced. If such provisions are not made, the CO, will bubble away, leaving behind a low gel-strength hydrogel. Careful examination of the resultant molecule might cause one to rename it a polyurethane/polyurea. 

Synthesis From a-Amino Acids and Related Compounds. Addition of eyanates. isocyanates, and urea derivatives to ir-amnio acids yields hydamoin precursors. This method is called the Read synthesis, and c in he considered as the reverse or hydanloin hydrolysis. Thus the reaction of a-attiino acids with alkaline eyanates affords hydamoic acids, which cyclize to hydantoins in an acidic medium. 

A related series of 5-substituted-2-amino-oxadiazole compounds have also been prepared in a one-pot procedure using a microwave-assisted cyclisation procedure (Scheme 6.26)164. Rapid preparation of the pre-requisite ureas from the mono acyl hydrazines and various isocyanates (or the isothiocyanate) was easily achieved by simple mixing. The resulting products were then cyclo dehydrated by one of the two procedures either by the addition of polymer-supported DMAP and tosyl chloride or alternatively with an immobilised carbodiimide and catalytic sulphonic acid. Purity in most cases was excellent after only filtration through a small plug of silica but an SCX-2 cartridge (sulphonic acid functionalised - catch and release) could be used in the cases where reactions required additional purification. 

There are several types of chiral derivatizing reagents commonly used depending on the functional group involved. For amines, the formation of an amide from reaction with an acyl halide [147,148], chloroformate reaction to form a carbamate [149], and reaction with isocyanate to form the corresponding urea are common reactions [150]. Carboxyl groups can be effectively esterified with chiral alcohols [151-153]. Isocynates have been used as reagents for enantiomer separation of amino acids, iV-methylamino acids, and 3-hydroxy acids [154]. In addition to the above-mentioned reactions, many others have been used in the formation of derivatives for use on a variety of packed and capillary columns. For a more comprehensive list, refer to References 155-159. 

Azole approach. A one-pot synthesis of substituted 2,3-dihydro-5-oxo-5H-oxazolo[2,3-b ]quinazolines (273) consists of reacting esters of anthranilic acid with a 2-chloro- or 2-bromoalkyl-isocyanate the initially formed urea (271) is cyclized to the acid salt of the oxazoline (272), which on addition of piperidine as base forms the pyrimidine ring (76S469). In a similar reaction the aziridinyl urea (274), which is available from aziridinyl isocyanate... 

From carbodiimides The aminated polymers (from haloalkylpoystyrenes and potassium phtalimide) were converted into urea with isocyanate and dehydrated to carbodiimides 20 addition of tetramethylguanidine gave the type 5 N-hexalkylbiguanides 9 (Figure 14). 

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