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, Ph(3)p-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 a-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 (beta-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. (C) Wiley-VCH Verldg GmbH & Co. KGaA, 69451 Weinheim, Germany, 2008.