Ugi four-component reaction template

Benzaldehyde and methylamine form an imine; acetic acid protonates it, tert-butyl isocyanide adds to the iminium ion (a nitrilium ion), acetate traps it and the Mumm rearrangement moves the acetyl group from O to N.

Mechanism step by step

Aldehyde, amine, carboxylic acid and isocyanide combine to an α-acylamino amide

1. Imine formation – The amine adds to the C=O and water is lost (see Imine formation)
2. Protonation – The carboxylic acid protonates the imine: an iminium ion and acetate
3. α-Addition of the isocyanide – The isocyanide carbon lone pair attacks the iminium carbon: a nitrilium ion
4. Acetate adds – Addition to the nitrilium carbon gives an O-acyl imidate
5. Mumm rearrangement – The amine nitrogen adds to the ester C=O: a five-membered tetrahedral intermediate
6. Collapse: acyl now on N – The C=O reforms and the ring C–O bond breaks: O → N acyl transfer
7. Proton transfer – The imidate takes the N–H proton: the second amide

Key points

• Four components, one pot (usually MeOH, room temperature): only water is lost, so the reaction is highly atom-economical.
• The isocyanide carbon is both nucleophile and electrophile (α-addition): it bonds to the iminium carbon and then to the carboxylate oxygen.
• The Mumm rearrangement (O → N acyl transfer) is irreversible and pulls all the earlier equilibria through.
• Product: a peptide-like bis-amide with a new stereocentre (racemic unless a chiral component is used); varying the four inputs builds combinatorial libraries.

Drawn with 169 atoms, 21 arrows, 11 shapes – every one editable in MolDraw.

Sources

• Kürti & Czakó, Strategic Applications of Named Reactions in Organic Synthesis, Elsevier 2005 Textbook
• Smith, March's Advanced Organic Chemistry, 8th ed., Wiley 2020, ch. 16 Textbook