2,3-Dihydroimidazol-2-ones

A related intramolecular N-alkylation leading to saturated nitrogen heterocycles 99 can proceed via dehydration of intermediates 98 (Eq. 39) [96]. Unsaturated nitrogen heterocycles such as pyrroles [97], indoles [98], benzo-azoles [99], 2,3-dihydroimidazol-2-ones [100], and imidazo[l,2-a]pyridines [101] were obtained through similar cyclocondensation reactions. Interesting ruthenium-catalyzed syntheses of quinolines have been achieved by means of cyclocondensations of aniline derivatives with propanediols, aminoalcohols, or... 

Increasing steric bulk at one of the NHC side groups will cause interference with the active site of the complex and generate an amplifying effect on alternation control. A methyl substituted one-carbon spacer like in l-mesityl-3-((rR)-l-phenylethyl)-4,5-dihydroimidazol-2-yhdene is enough to dramatically improve the alternation rate [129]. In Fig. 3.31 the steric impact on monomer coordination for different situations is rationahsed. 

During studies on the use of amidines as azomethine ylide sources, Jones et al. (67-69) reported in a series of papers the application of their general strategy to an asymmetric process. Quatemization of the dihydroimidazole 214 with an a-halo ester followed by DBU-induced ylide formation and subsequent cyclization furnished a range of nitrogen heterocycles in a one pot generation and cyclization protocol (70) (Scheme 3.73). 

Support-bound 1,2-diamines can be readily converted into imidazolidinones by treatment with carbonyl diimidazole [128,129]. The required diamines have been prepared on cross-linked polystyrene by reduction of peptides bound to MBHA resin with borane. Similarly, bicyclic imidazolines have been prepared from triamines and thiocarbonyl diimidazole (Entry 10, Table 14.3). Dehydration of polystyrene-bound monoacyl ethylene-1,2-diamines yields 4,5-dihydroimidazoles (cyclic amidines, Entry 5, Table 13.18). Several groups have reported the synthesis of 2-aminoimidazol-4-ones from resin-bound amino acid derivatives (e.g., Entry 6, Table 15.11). Most of these compounds are, however, unstable, and slowly decompose if dissolved in DMSO (Jesper Lau, private communication). 

One of the emerging applications of 4,5-dihydroimidazole-based compounds is as chiral auxiliaries in metal complexes used for asymmetric synthesis for example, 457 in ruthenium-catalyzed DielsAlder reactions <2001 J(P 1)1500, 2006JOM(691)3445> 458 in diethylzinc addition to aldehydes <2003SL102> 459 in asymmetric intramolecular Heck reactions <20030L595> and 460 in ruthenium-catalyzed epoxidation <2005OL3393> and iridium-catalyzed hydrogenation of imines <2004TA3365>. 

In contrast to 83 and 85, 1,2-dimethyl-4,5-dihydroimidazole (89, n = 2) and 1,2-dimethyl- 1,4,5,6-tetrahydropyrimidine (89, n = 3) underwent direct acylation with two equivalents of aroyl chloride to give diacylenediamines 90 rather than enol benzoates. Their acid hydrolysis with sulfuric acid at water bath temperatures results in cleavage of one of the aroyl groups and benzoyl-substituted 1,1-enediamines 91 are formed in moderate yields (equation 31)103. 

Carbodiimides react with dicarbonyl compounds (or their sulfur analogues) to give imidazoles [48, 49]. When diimmonium salts (19) are treated with guanidines or O-methylisoureas the initial products are 4,5-dihydroimidazoles, but these are readily aromatized by heating in the presence of triethylamine hydrochloride (Scheme 4.3.10). The mildly acidic conditions result in the loss of one of the amino functions from the intermediate [50]. Yields of 2,5-diaminoimidazoles are usually 60-80% overall. 

Homochiral dihydroimidazoM-one 635 was efficiently oxidized to 2-/< t/-butyl-3-methyl-2,3-dihydroimidazol-4-one A -oxide 636 which adds to functionalized olefins to afford the adducts 637 with high diastereoselectivity (Scheme 151) <20040L1653>. This is an efficient method for the synthesis of chiral glycine analogues 638. 

Another way to form a-aminoketones from a-amino acids is via the Dakin-West reaction, in which amino acids are treated with aliphatic acid anhydrides to give ketone amides. Thus, reaction of ( )-phenylalanine with the appropriate aliphatic acid anhydrides followed by acidic hydrolysis afforded keto-amide 1271. Cyclization of 1272 with potassium cyanate gave 5-alkyl-4-benzyl-l,3-dihydroimidazol-2-ones 1273 (Scheme 322) <2002JHC375>. 

In the laboratory of E.B. Pedersen, several 2-methylsulfanyl-1H-imidazoles were prepared and tested for their activity against HIV-1 These compounds can be regarded as novel non-nucleoside reverse transcriptase inhibitors. The required a-aminoketone hydrochloride building blocks were prepared using the Dakin-West reaction. L-Cyclohexylalanine was dissolved in excess pyridine and propionic anhydride and was kept at reflux overnight. The resulting a-propionylamino ethyl ketone was hydrolyzed with concentrated hydrochloric acid and the a-aminoketone hydrochloride was heated with one equivalent of potassium thiocyanate in water to afford 4-cyclohexylmethyl-5-ethyl-1,3-dihydroimidazole-2-thione. This material was then advanced to 4-cyclohexylmethyl-1-ethoxymethyl-5-ethyl-2-methylsulfanyl-IH-imidazole. 

The formation of oxetanes by photochemical (2 + 2)-cycloaddition of carbonyl compounds, such as aldehydes, ketones, and quinones, with carbon-carbon double bonds has been reported for various heterocyclic compounds. Maleic anhydride,142 isocoumarin (and its derivatives),143,144 benzol 61 thiophene 1,1-dioxide,144 l,3-dihydroimidazol-2-ones,131,132... 

Stabilized Cu(I) in the form of its iV-heterocyclic carbene (NHC) complex, e.g., (SIMes)CuBr (SIMes = iV,7V -f>is(2,4,6-trimethylphenyl)-4,5-dihydroimidazol-2-ylidene), and the eyclohexyl analog [(ICy)2Cu]PF6, catalyzes click reactions very well in aqueous /-butanol, and even better in water alone. Low conversions were noted in nonaqueous solvents such as tetrahydrofuran (THF), /-BuOH, and dichloromethane (DCM). Starting from an alkyl bromide, triazoles could be smoothly generated by m situ conversion to the corresponding azide (aqueous NaNs) followed by copper-catalyzed cycloaddition. This is but one example of the potential for combining several steps in a single flask that culminates with a click reaction vide infra). The... 

In a related fashion, asymmetric amination of ( )-cinnamic acid yields L-phenylalanine using L-phenylalanine ammonia lyase [EC 4,3,1,5] at a capacity of 10,000 t/year [1274, 1601], A fascinating variant of this biotransformation consists in the use of phenylalanine aminomutase from Taxus chinensis (yew tree), which interconverts ot- to p-phenylalanine in the biochemical route leading to the side chain of taxol [1602], In contrast to the majority of the cofactor-independent C-0 and C-N lyases discussed above, its activity depends on the protein-derived internal cofactor 5-methylene-3,5-dihydroimidazol-4-one (MIO) [1603], Since the reversible a,p-isomerization proceeds via ( )-cinnamic acid as achiral intermediate, the latter can be used as substrate for the amination reaction. Most remarkably, the ratio of a- vs, 3-amino acid produced (which is 1 1 for the natural substrate, R = H) strongly depends on the type and the position of substituents on the aryl moiety While o-substituents favor the formation of a-phenylalanine derivatives, / -substituted substrates predominantly lead to p-amino analogs, A gradual switch between both pathways occurred with m-substituted compounds. With few exceptions, the stereoselectivity remained exceUent (Scheme 2,215) [1604, 1605],... 

Superoxide production from paraquat in a pig pulmonary microvascular endotheUal cell suspension was demonstrated by Tampo et al. (1999) using 2-methyl-6-(p-methoxyphenyl)-3,7-dihydroimidazol[l,2-a]pyrazin-3-one, a chemiluminescence probe, to detect superoxide anions. Increased rates of superoxide production from paraquat, which were sensitive to superoxide dismutase, required the presence of reduced nicotinamide adenine dinucleotide phosphate (NADPH) in the reaction medium, and occurred instantaneously after the addition of NADPH, which is impermeable to cell membranes. NADH as an electron donor was not as effective, and xanthine or succinate had no influence. Paraquat was anaerobically reduced in the presence of NADPH and 2-methyl-6-(p-metho-xyphenyl) - 3,7 - dihydroimidazol[l,2 - ajpyrazin- 3 -one to yield a one-electron reduced radical, and the reduction was inhibited by NADP". Diphenyleneio-donium, an inhibitor of flavoprotein reductase, also markedly inhibited both paraquat reduction and superoxide production. 

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