Question

OCHEM CHAPTER 26 A clever new method of peptide synthesis involving formation of amide bond by reaction of an alpha-keto acid with an n-alkylhydroxylamine, The reaction is thought to occur by nucleophilic addition of N-alkylhydroxylamine to the keto acid as if forming an imine followed by decarboxylation and elimination of water. Show the mechanism. Please show the mechanisms, and I WILL RATE

1 OH DMF R + N R + -R2 + H CO2 H20 20 ZI OH

Answer 1

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Hydroxylamine is used to condense with the ketone group. The lone pair of the nitrogen attacks the ketone and results in the breaking of the C-O double bond while a C-N bond is formed. This event is followed by a proton transfer. In the next step there is a nucleophilic lone pair on the nitrogen. This attacks form a C-n double bond and at the same time -OH leaves we have formed an oxime. However, we note that the oxime has cationic nitrogen. This group is a powerful electron withdrawer. The result is that the carboxy group is destabilized and leaves. The C-C single bond is broken in the process and the pair of electrons in that bond form a C-N triple bond. At the same time the N-o bond is broken, ejecting hydroxide. The hydroxide attacks the electrophilic carbon of the azocarbenium species. This forms a co bond and at the same time breaks one of the pi bonds in the triple bond followed by the electron pair in that pod neutralizing the positive charge on the nitrogen. Finally, tautomerization yields the desired amide. HO OH R- OH Proton Transfer Co@ R Co, R - OH RI он

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