PURPOSE: To industrially advantageously obtain 17-(formamido- sulfonylmethylene)-steroid for an intermediate proparing 21-hydroxy-20-keto-Δ16- steroid for pharmaceuticals, etc., by allowing 17-oxo-steroid to react with sulfonylmethyl isocyanide.

CONSTITUTION: Sulfonylmethyl isocyanide, e.g. p-sulfonylmethylphenyl isocyanide is allowed to react with 17-oxo-steroid, e.g. 3-methoxyandrosta-3,5-dien-17-one at -40°C for 2 hr in THF in the presence of potassium-tert. butoxide, and phosphoric acid, triethylamine and phosphoroxy trichloride are successively added at -35°C. They are stirred at 0°C for 1 hr, poured into iced water and subjected to extraction with methylene chloride. The solvent is removed from the extracted soln. by vacuum evaporation and crystallization is carried out in methanol to obtain the objective compd. of the formula [where R1 is a 1-4 C alkyl or a single bond; R2 is H or a 1-4 C alkyl; R3 is a 1-10 C alkyl, (substd.) phenyl, etc.; and rings A to D may contain double bonds and substituents].