O-Methylisourea

Add 1.5 pi of the o-methylisourea hemisulfate solution to the peptide solution with mixing. 

The group of Lindau has demonstrated the effective O-alkylation of carboxylic acids using a polymer-supported O-methylisourea reagent [123], Under conventional conditions, complete esterifications were observed only after refluxing for several hours in tetrahydrofuran, and the acidic work-up required limited the scope of applicable substituents. In contrast, employing microwave heating led to complete esterifications within 15-20 min, with only 2 equivalents of the polymer-bound... 

The interaction (575) of O-methylisourea and N-methylglycine occurs normally at low temperatures to yield the expected creatine (XXXII) (21%). At 40—50° the production of 1-methyl-l-biguanide acetic acid (XXXIII) in low yield has been reported, but the mechanism of its formation ist not clear. 

A new variant of the Sn(ANRORC) substitutions was found in reactions of A-methylpyrimidinium salts with bifunctional nucleophiles, such as S-methylisourea, O-methylisourea, and cyanamide. 

To a flask containing 60 gm (1.0 mole) of urea is added 126 gm (1.0 mole) of methyl sulfate, and the stirred reaction mixture is warmed for 7 hr at 40°C. Stirring is continued until the reaction mixture is clear. The O-methylisourea is isolated as the picrate salt by adding 350-400 ml of 2 M lithium picrate in 300 ml of ethanol. The resulting mixture is boiled on the steam bath for 5 min. Most of the picrate dissolves but precipitates on slowly cooling to 0°C. The picrate is filtered, washed with ethanol, and air-dried to afford 182 gm (60%), m.p. 177°C. 

O-Methylisourea hydrogen sulphate (2-methylpseudourea sulphate) [29427-58-5] M 172.2, m 114-118 , 119 . Recrystd from MeOH-Et2O (327g of salt dissolved in IL of MeOH and 2.5L of Et20 is added) [Fearing and Fox JACS 16 4382 1954 ]. The picrate has m 192° [Odo et al. JOC 23 1319 1958]. iV-Methyl maleimide [930-88-1] M 111.1, m 94-96 . Crystd three times from ethyl ether. 

The synthesis of this ring system may be achieved by building the triazole onto a preformed pyrimidine ring. Cyclocondensation of ketoesters 330 with O-methylisourea (331) gave the pyrimidine 332, whose acylation gave the /V-acyl derivative 333, which can be cyclized with hydrazines to give 334 (89GEP3839711) (Scheme 64). 

Guanidination of all 10 t-amino groups with O-methylisourea causes loss of all enzymic activity toward RNA (103). The product has a sedimentation constant and alkaline spectral titration behavior which indicate no marked conformational change. Extensive guanidination can occur with no loss of activity. Only the last one or two groups to be modified are related to the activity loss. Allewell (120) found a residual 1% activity toward C > p after exhaustive reaction of RNase-S. The product was crystallized in a new space group. 

Cyclizations of urea derivatives of allylamines with selenium reagents have been examined recently (equation 62 and Table 18).98 Cyclization of the allylic ureas produces franj-2-oxazoline derivatives with high stereoselectivity when the double bond is internal (Table 18, entry 2). Although O-methylisourea derivatives can be cyclized to dihydroimidazoles [see Section 1.9.3.2.2(ii)], cyclization with phenylsel-enenyl trifluoromethanesulfonate and trifluoromethanesulfonic acid produces 5,6-dihydro-1,3-oxazines (6-endo products), even when the double bond is monosubstituted (entry 3). 

The same cyclization of allylic O-methylisoureas provides imidazolines or 5,6-dihydro-l,3-oxazines (when catalyzed by benzeneselenenyl triflate).3... 

OME-reagent Dissolve 50 mg O-methylisourea hemisulfate (94.0%, Acros Organics, Geel, Belgium store at room temperature) in 51 pE water. Prepare always a fresh solution and use immediately. 

Scheme 2. Reaction of O-methylisourea with the e-amino group of a lysine. The formed guanidino group of homoarginine has a higher proton affinity than the original amino group, resulting in better protonation efficiencies in the MALDI and ESI process.

Number of residues/mole of protein. b Determined by protein guanidination with O-methylisourea. ... 

Guanidylated cytochrome c (G-cytc) was synthesized from native cytochrome c (N-cytc) and O-methylisourea (Figure 14.9) [19]. Almost all of the lysine residues of cytochrome c were substituted with a homoaiginine moiety as seen in Table 14.2, but... 

The reaction of o-methylisourea hydrogen sulfate with ethyl acetoacetate and aldehydes provided dihydrop3Timidines 47 (Scheme 11) which underwent... 

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. 

In a typical procedure (Kimmel 1967), an aqueous solution of a protein (10 mg/ml) is mixed at 4°C with an equal volume of O-methylisourea, adjusted to pH 9.5 with NaOH. The reaction is allowed to proceed for 4-5 days at 4°C, and is then terminated by dialysis against ice-cold deionized water. If lysine modification is found to be incomplete, the reaction may be attempted either at a higher temperature-(e.g. 25°C), or at pH 10.5-11 at 4°C. 

In our view, iodine inactivation in the case of rat intestinal alkaline phosphatase causes blocking of the —SH group, since other evidence for the presence of this group is strong. The c-amino group of lysine is considered to be essential not merely because tyrosine can be reasonably excluded, but because rat intestinal alkaline phosphatase is inhibited by the lysine-specific o-methylisourea reagent and because the pZm-pH plot shows a discontinuity at pH 9.6 (G5), the pilf of the e-amino group of lysine (C14). 

Modification of its lysyl residues with O-methylisourea has no effect (Geschwind and Li, 1957), but acetylation which removes positive charges from the enzyme, affects strongly the enzymic activi-... 

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