Amino phosphonates and their corresponding amino phosphonic acids are known as amino acid mimetics and, therefore, affect the physiological activity of the cell. [1] The synthesis of 1-alkylamino phosphonates by a reaction of a dialkyl phosphite and an imine under thermal conditions had been described in the early 1950s. [2] However, dialkyl phosphites are generally considered to be poor nucleophiles because they exist predominantly in a s 4 l 5 configuration. [3] Dialkyl trimethylsilyl phosphites were presented by Afarinkia et al. as excellent mild phosphonylation agents because the more nucleophilic s 3 l 3 form of the dialkyl phosphite reagent could be obtained by O-silylation with trimethylsilyl chloride (TMSCl) in dichloromethane and triethylamine as the base. [4] Furthermore, exclusive 1,2-addition to a,b-unsaturated imines containing a phenyl group was reported by the same authors using diethyl trimethylsilyl phosphite. [5] During our research into azaheterocyclic phosphonates, [6] we planned to use this phosphonylation reaction with a,b-unsaturated imines 5 a-d derived from cinnamaldehyde. The diethyl trimethylsilyl phosphite was prepared in situ, as described previously. [4] The reaction mixture consisted mainly of the 1,2adduct 9 a ( 31 P NMR: d = 24.44 ppm) and residual starting material after 24 h of reaction with the imine 5 a. However, several small 31 P NMR signals were present and became more abundant upon prolonged reaction times. The minor peaks appeared as two doublets (J = 9.7 Hz) next to two singlets and could be assigned to be the double-addition product 6 a (two diastereomers, ratio: 27:73) after its isolation in very low yield by column chromatography. The same type of product was obtained using imines 5 b-d in increasing amounts proportional to the steric bulk of the N-substituent, which also explains the decreasing yields of 1,2-adducts when more sterically demanding N-substituents were used in the original