4-Ethoxy-1,1,1-trifluoro-3-buten-2-one reacts with phenylmagnesium bromide to afford ethoxy group substitution products, while it reacts with organozinc compounds to yield products arising from 1,2-addition to the carbonyl group. 4-Ethoxy-1,1,1-trifluoro-3-buten-2-one on heating with triethyl phosphite affords [4+2] cycloaddition product, 2,2,2-triethoxy 2,3-dihydro-3-ethoxy-5-trifluoromethyl-1,2λ5-oxaphospholene, which on hydrolysis affords a 2-oxo-2-hydroxy-2, 3-dihydro-3-hydroxy 5-trifluoromethyl-1,2λ5-oxaphospholen. It reacts with amino to give the N-protected amino acids, which is useful for the peptide synthesis.