Nucleophilic ring cleaving reactions

Rhodium-Catalyzed Nucleophilic Ring Cleaving Reactions of Allylic Ethers and Amines... 

Deacylations are known. C-Acyl groups in 1,3,4-thiadiazoles are cleaved by sodium ethoxide in ethanol (68AHC(9)165). Imidazole-2-carbaldehyde behaves similarly, yielding imidazole and ethyl formate this reaction involves an ylide intermediate. 3-Acylisoxazoles (405) are attacked by nucleophiles in a reaction which involves ring opening (79AHC(25)147). 

Penicillin interferes with the synthesis of the bacterial cell wall. A nucleophilic OH group of the glycopeptide transpeptidase enzyme cleaves the P-lactam ring of penicillin by a nucleophilic acyl substitution reaction. The opened ring of the penicillin molecule remains covalently bonded to the enzyme, thus deactivating the enzyme, halting cell wall construction, and killing the bacterium. Penicillin has no effect on mammalian cells because they are surrounded by a flexible membrane composed of a lipid bilayer (Chapter 3) and not a cell wall. 

The single most important reaction of cyclopropenium ions involves their capture by nucleophiles Such reactions give the corresponding cyclopropene which may survive the reaction conditions i4c,27o, 536 eliminate to yield a triafulvene (Section II.G), ionize to a new cyclopropenium ion " " , or ring cleave With unsymmetrically substituted cyclopropenium ions mixtures of cyclopropenes are often produced (equation 110) although regioselective processes are known . 

Thermal and chemical stability toward ring-cleavage reactions have not been extensively studied. Reactions with strong nucleophiles have been reported to yield ring-cleavage products, and benzosilacyclobutane is cleaved by water 108). Halogens also cleave carbon-metal bonds. 

The relative selectivity for nucleophilic reaction at one of the three amide bonds in 176 has been examined by Bruice and co-workers.146,147 Primary and secondary amines generally yield the deacylated compound 177 while oxygen bases (e.g., formate, acetate, hydroxide ion) provide the ring-cleaved product 178 Eq. (63)1. 

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