The reaction of 2,4-di-t-butyl-6-methoxyphenyldithioxophosphorane [MoxP(≡S) 2, 1] with H₂O in the presence of pyridine afforded a novel cyclic phosphonic acid anhydride 2a and 2b in 30% and 22% yields, respectively. The mechanism for the formation of 2 was discussed by ³¹P NMR monitoring. The results from X-ray crystallographic analysis for 2b indicated that it has a six membered phosphorus-containing hetero ring, which bears a 127.7° of the largest bond angle and a 97.9° of the smallest one. On the basis of bond length data, C—P, C—O and P—O bonds show typical single bond characters except C≡C double bond. The two P≡S double bonds are located in the same side of hetero ring, indicating clearly that 2b is a cis isomer. Crystal data for 2b: monoclinic, space group C2/c, a=2.0486(3) nm, b=2.2366(4) nm, c=1.6334(3) nm, β=112.98(1)°, Z=8, V=6.890(1) nm³, D_c=1.261 g·cm^-3, F(000)=2800.00, μ(MoKα)=3.40 cm^-1. The final deviation factor R=0.063, R_w=0.083. The maximum and minimum peaks on the final difference Fourier map correspond to 0.82 and -0.48 ×10³e^-/nm³, respectively.

Key words: 2,4-di-t-butyl-6-methoxyphenyldithioxophosphorane, cyclic phosphonic acid anhydride, ³¹P NMR, reaction mechanism, crystal structure