Synthesis of 2H-1,2-oxaphosphorin 2-oxides

Two novel heterocyclic dienes, 2-ethoxy-3-phenyl-2H-oxo-1,2-oxaphosphorin 2-oxide (3) and 5-bromo-2-ethoxy-3-phenyl-2H-oxo-1,2-oxaphosphorin 2-oxide (4), are described. They were prepared in four steps starting from 2-ethoxy-3-phenyl-1,2-oxaphosphorinane 2-oxide (12a,b) (trans and cis) by a bromination–dehydrobromination sequence. Free radical bromination of 12a,b with NBS/AIBN furnished two isomeric bromides 13a,b. Isomer 13b gave 2-ethoxy-5,6-dihydro-3-phenyl-2H-1,2-oxaphosphorin 2-oxide (14) on treatment with LiCl/DMF. Isomer 13a underwent COEt bond cleavage followed by HBr elimination to give the phosphonic acid 17. Treatment of 14 with NBS/AIBN afforded isomeric 5-bromo-2-ethoxy-5,6-dihydro-3-phenyl-2H-1,2-oxaphosphorin 2-oxides (15a,b) and 5,5-dibromo-2-ethoxy-5,6-dihydro-3-phenyl-2H-1,2-oxaphosphorin 2-oxide (18), separated by chromatography. Dehydrobromination of 15a,b, and 18 with an excess of Et3N in toluene at 80–95 °C provided the target dienes 3 and 4, respectively.