Rearrangement urea derivatives

Rearrangement of 0-acyi hydroxarrac acxl derivatives with base or heat to amines or urea derivatives (via isocyanates) or rearrangement of carboxylic acids via their hydroxamic acxis to amines... 

Beckmann rearrangement of amidoximes to urea derivatives in the presence ol acids (benaene suMonyl chloride). 

AT-Substituted amidines 74 are transformed via the carbodiimide intermediate 75 into urea derivatives 76 as shown in Scheme 34 [141]. This reaction has been performed with a variety of aliphatic and aromatic substituents and in addition to (diacetoxyiodo)benzene 3 other hypervalent iodine compounds can also be used for this rearrangement. Phenyl substituted amidines 74 (R = Ph) can lead to cyclized products of type 77. 

Perchloro-lA3,3A3-diphosphetane 48 reacts with urea derivative 60 to form tricyclic species 61, which can be rearranged to the symmetric isomer 62 <2002ZFA1903>. 

Conventional chemistry converted carbinol 277 into acyl azide 278, which was treated under Curtius rearrangement conditions to obtain either amine 279 or urea derivative 280, the respective precursors of guanine derivative 281 and thymidine analogue 282. 

Formycin-iV(6)-oxide (110) rearranged to give the urea derivative (111) on irradiation at >290 nm in aqueous solution <84JCS(Pl)242i>. Irradiation of compound (112) gives the dimer (113) (Equation (6)) <86H(24)3467>. 

MethoxyphenyI)-3-carbonyIamino-N-t-butyI-2-oxo-1,2-dihydro-1,8-naphthyridine, (III), was converted into the corresponding 3-urea derivative, (IV), using the Hoffman rearrangement and is described (1,2). The procedure was also performed using 3-carbonylamino-2-oxo-pyridine (3). 

Carboxylic acid azides give rise to three different reactions under different conditions. Azide coupling (equation 9) was the earliest method in peptide synthesis and is still one of the most important in fragment condensation and preparation of cyclic peptides due to its almost complete lack of racemization. At elevated temperatures a frequent side reaction is the Curtius rearrangement. Trapping of the intermediate isocyanate with amines (equation 10) gives urea derivatives and with carboxylic acids rearranged amides are obtained (equation 11). ... 

Perfect retention of the geometrical configurations has been observed in the Curtius rearrangement of cis- and franr-crotonyl azides (10), which respectively give cis- and franr-propenyl isocyanates (11), characterized as their urea derivatives (12 Scheme 8). The cis isomers (10b) and (11b) tend to isomerize thermally to the trans isomers (10a) and (11a) respectively. 

Beckmann rearrangement of amidoxlmes to urea derivatives hi the presence of acids (benzene sutfonyt chloride). 

Similar difficulties should be expected in cyclization by the acid azide method, where Curtius rearrangement generates a cyclic urea-derivative instead of the desired cyclic peptide ... 

A sequential one-pot synthesis of unsymmetrical aryl urea derivatives was developed by arylation of N-substituted cyanamides under metal-free conditions, followed by a second N-arylation under copper-catalyzed conditions [96]. Chen and Chen devised the arylation of pyridine N-oxides with At21X via initial O-arylation followed by a 1,3-radical rearrangement to o-pyridinium phenolates, and arylation of pyridine N-amidates delivered o-pyridinium anilines (Scheme 5d) [97]. 

Scheme 33 Urea derivatives 124 via an oxidative tandem aryl-aryl coupling/rearrangement process...

Condensation of the vinylogous carbamate (40) with acetaldehyde and isocyanic acid afforded the pyrimidine derivative (41) which was converted to the urea derivative (42) by Curtius rearrangement in 65% overall yield. Treatment of (42) with acetic acid at 50° afforded exclusively the desired product (39), whereas treatment with trifluoroacetic acid gave a mixture of (39) and stereoisomer (43). The stereochemistry was established by comparing the coupling constants involving H-5 and H-6 with those of saxitoxin (1). Different cyclization mechanisms were suggested for the acetic acid and trifluoroacetic acid catalyzed cyclization to account for the different stereochemistry of the products. 

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