Allenylphosphonates - Useful Precursors of Pyrazoles and 1,2,3-Triazoles

The synthesis of new pyrazoles and triazoles, with or without phosphorus substituents, from allenylphosphonates is described. Thus, Ph3P‐promoted reactions of the allenylphosphonates (OCH2CMe2CH2O)P(O)CH=C=CRR′ [R = H, R′ = Me (1b), R = R′ = Me (1c)] with DIAD/DEAD lead to phosphonopyrazoles by utilizing the –CO2R functionality of DIAD/DEAD for cyclization. The products derived from allenylphosphonates with an α‐phenyl group undergo an unusual but facile P–C bond cleavage to form tetrasubstituted pyrazoles. In the second type of reaction, Me3SiN3 reacts with the allenylphosphonates to form phosphono‐1,2,3‐triazoles in CH3CN at reflux, whereas (β‐azidoallyl)phosphonates were obtained in high yields at room temperature. These latter compounds undergo cycloaddition with activated acetylenes to afford multisubstituted 1,2,3‐triazoles. They were subsequently transformed into diverse N‐substituted 1,2,3‐triazoles through the Horner–Wadsworth–Emmons reaction. The structures of the key compounds were established by X‐ray crystallography.(© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2008) Novel pyrazoles and triazoles with or without phosphorus substituents have been synthesized from allenylphosphonates.