Imidazole ring closure reactions

Triacanthine (17) has been isolated from Holarrhena mitis Details on the structure and total synthesis of eritadenine (18), a hypocholesterolemic alkaloid from Lentinus edodes, have been reported.An unsuccessful attempt to improve the imidazole ring-closure reaction in a total synthesis of eritadenine, which has been previously discussed, was also reported in this study. Concentrations of both caffeine and theobromine in the cocoa bean were shown to increase most markedly in the time period from flowering to harvest. ... 

Imidazole-fused dithiazines were synthesized by Brzozowski and Saczewski <2002JME430>. The ring-closure reaction of the acetal 181 took place in sulfuric acid to yield the tricyclic product 182 in almost quantitative yield. 

Reviews of the coordination chemistry of this type of ligand have been published for pyrazoles24 and imidazoles,25 and to a lesser extent for the triazoles26 and tetrazoles.27 Many azole ligands are commercially available or are easily synthesized by a variety of condensation and ring-closure reactions.4-6... 

It is not strictly necessary to introduce the 1,1 -binaphthyl backbone. For axial chirality, the biphenyl scaffold is sufficient, provided that rotation around the phenyl-phenyl axis is sufficiently hindered. Hoveyda combined this reduced axial chiral motif with additional central chirality in the imidazole backbone (O and C ) [6,7], Synthetically, the task is accomplished by Buchwald-Hartwig amination of enantiomerically pure (H ,21 )-diphenylethylenediamine with 1-methoxy-I -iodo-biphenyl and subsequent reaction with mesityl bromide to introduce the bulky wingtip group on the second amino group of the chiral starting material. Ring closure reaction with triethyl orthoformate and hydrolysis of... 

Ring Closure Reactions Heravi et al. studied the synthesis of 2,4,5-trisubstituted imidazoles through three-components coupling of l,2-diketone/l,2-hydroxyketone, aromatic aldehydes, and ammonium acetate using MCM-41 as catalyst under solvent-free conditions [96]. MCM-41 efficiently catalyzes the reaction affording the desired product in good yields in a relatively short time (Scheme 22). 

An alternative method involves reaction of an a-acylaminoketone (12) with a primary amine and subsequent ring closure of the resultant Schiff s base (13) with phosphoryl chloride. This enables the introduction of a 1-substituent as in (14) to be carried out efficiently, and if the amine were replaced with a monosubstituted hydrazine, the imidazole derivative (IS) resulted (78LA1916). 

Sulfur may also be alkylated under appropriate conditions. On formation, the N-/3-chloroethylthiazolidine-2-thione (307) underwent spontaneous ring closure to the tetrahy-drothiazolo[2,3-6]thiazolylium chloride (308) (71CHE1534) and a similar reaction occurred in the imidazole series. 

Isoxazoles (478) in the presence of base undergo ring opening to a-ketonitriles (479). When the reaction was carried out in the presence of hydrazines, 5-aminopyrazoles (480) were obtained. The reaction is also a convenient source of imidazoles, For example, when the 1,2-benzisoxazole (481) was treated with phenylhydrazine, decarboxylation initially occurred with subsequent ring closure to (482) (see Chapter 4.16). 

The Suzuki coupling of soluble polyethylene glycol (PEG)-bound bromothiophene 71 and p-formylphenylboronic acid provided biaryl 72 [56]. Due to the high solubilizing power of PEG, the reaction was conducted as a liquid-phase synthesis. Treatment of 72 with o-pyridinediaminc resulted in a two-step-one-pot heterocyclization through an imine intermediate. Nitrobenzene served as an oxidant in the ring closure step. Finally, transesterification with NaOMe in MeOH resulted in l//-imidazole[4,5-e]pyridine 73. 

In a further step, the authors performed a ring closure to the corresponding imidazole moiety affording isochromeno[3,4-tf imidazol-5-ones it was not possible to afford the l//-isochromeno[3,4-tf]imidazol-5-ones directly from the 3-amino-4-(arylamino)-l//-isochromen-l-one precursors, but the reaction from purified iso-chromenone was successful with yields of up to 96% [21]. 

The ring closure 319 - 320 has not been effected experimentally, though there are many reactions that could lead to 1,4-diazacyclo-octatetraenes 319 where the product isolated is a pyrrolinoll,2-a]-imidazole 320 (this does not necessarily implicate 319 as an intermediate). Equations (32) and (33) are typical examples. 

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