Hydroxyketones ring closure

In addition to the examples of coumarin syntheses given in the reviews mentioned above and in the treatise on heterocyclic compounds (B-51MI22400), more recent studies have made use of the Perkin synthesis. These include the use of substituted phenoxyacetic acids to prepare 3-phenoxycoumarins (78CI(L)628> and the synthesis of chlorocoumarins from chlorosalicylaldehydes (81T2613). The use of DBU in place of sodium acetate was necessary to effect the ring closure of a number of o-hydroxyketones (78BCJ1907). 

Complex reactions involving insertion of a P-X bond, O transfer from C to P, and ring closure occur when PhPCla reacts with jS-hydroxyketones . In contrast, ketene reacts with PBra to yield only a single product involving insertion into the alkene linkage . This result parallels the addition of SF5CI into ketene . 

Combretazolone 22 was constructed via reaction of an appropriate a-hydroxyketone with PMB-isocyanate, subsequent cyclization and final removal of the PMB group in refluxing trifluoroacetic acid. Cyclopentenone analogues were prepared via sequential malonate alkylation-acylation followed by ring closure and decarboxylation. 

Ring closures of hydroxyketones Vinylcyclopropyl ketones from y,d-ethylene- -liydroxyketones by stereospecific cyclodehydration... 

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). 

The formation of the pyrrolo[l,2-a]quinoxaline stmcture in this reaction can be represented in one of two ways (Scheme 3.38) (a) isomerization of the 2-hydroxy-1,5-diketone 118 to the 5-hydroxy-l,4-diketone 121 with the subsequent formation of the o-aminophenylpyrrole 122 and closure of the dihydroquinoxaline structure 123 (b) reaction of the a-hydroxyketone fragment with 1,2-DAB with the formation of the hydroquinoxaline derivative 125 and subsequent closure of the dihydropyrrole ring and isomerization of the 3,3a-dihydropyrrolo[l,2-a]quinoxaline structure 124 to the more stable 4,5-dihydropyrrolo[l,2-a]quinoxaline structure 123. 

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