Thiamine (8,) pyrophosphah ir recagnlzed as co-carboxylase, Bi monophosphate and Bt triphosphate are also well known. However, nothing has yet been reparted on the 81 phosphorous derivatives substituted at other positions of the hydroxyethyl group. In the preceding papers, the authors reported that the reaction of Bi with aldehydes (1) or amines (2) occurred at the Bt-thiarole (th)-Crposltion, in the different modes of HET type products, via nucleaphilic &I-co&no or pseudo Bt type intermediate, respectively. Recently, Ramirez et al. (3) reported that the unique acyloxycarbene was held responsible for the formation of the phthalide from phthalic anhydride and trlethyl phosphite, and added diethyl phosphite captured the acyloxycarbene to give diethyl phthalide phosphonate. From this fact the authors are interested in the reaction . of dialkyl phosphite with Bi. After having been saturnted with Cq, diethyl hydrogenephosphite (III) was added ta B+k (I) in toluene or EtOH affording colorless crystals(V), m.p. 133-135". V was also obtained from the reaction of Bi-chloride (II)with III in the presence of NEta or other suitable bases. V showed analytical data for CieHn04N4SP corresponding to the 1: 1 adduct of Bi and diethylhydrogenephosphite. From the W spectrum l<$l mp (a): 234.5 (9,875), 275 (5,25@] it was estimated that 2-methyl4cunino-Cpyrimidinyl group might still remain. IR spectrum showed absorption bands at 3390,3375,1668 (NH_3, 1252 (PO),and at 1042 cm" (P-O-C). NMR spectrum (T values) showed peaks at 2.00' (1 H, pyrimidine (pm)-Cd-H), 3.9g (2H, pm-CCNHJ, 7.60s (3H, pm-C+&), and at 8.so' (3H, th-C,X&) which was extremely higher than that expected as th-CCC& signal, in addition, typical signals corresponding to the hydroxyethyl group were deformed and shifted. Fram the obove spectral considemtions, it was supported that the cyclization occurred similar to the case of preceding oldehydes or amines to form tetmhydrofumn ring by the reaction of the hydroxyethyl group with th-C&-double bond. Accordingly, the structure of V was confirmed to be diethyl 2_C3-(2methyl4ominopyrimidiIr5_yl)methyl-~-methyl~~ydrofum[2~-d] thiazole}phosphonate.