The invention relates to a method for producing 1,4-diphenyl azetidinone derivatives from suitably protected β-substituted amino amides, in the presence of silylation agents. Said method uses cyclization catalysts that comprise phosphonium ions as the cation of formula (XII), and ions of the following general formulas (a), (b), (c) or (d) as the anion. In said formulas, the symbols, substituents and indices are defined as follows: Z = C=O, C=S, S=O, SO2 or C=NR20; K = O, S, NR21 or CR22R23; L = NR24 or CR25R26; n = 0 or 1; M = O, C=O, NR27 or CR28R29; Q = O, S, NR30, CR31R32, C=O, C=S, S=O, SO2 or C=NR34; R = CR35 or N; T = CR36 or N; U = CR37 or N; V = CR38 or N; the groups R16 to R19 can represent, for example, aryl or (C1-C15) alkyl, aryl(C1-C10)alkene, and the groups R20 to R32 and R34 to R38 can represent in addition H or heteroaryl; R39 and R40 represent, for example (C1-C6) alkyl. The anion can also be R41O-, R42COO- or Cl-, Br- or I- (combined with Ag2O).