NMR spectra were acquired on Bruker Avance 600 and 300 spectrometers at 293, 303, and 333 K; the chemical shifts δ were measured in ppm relative to the solvent (1H: DMSO-d6, δ 2.50 ppm; 13C: DMSO-d6, δ 39.50 ppm). Splitting patterns are designated as s, singlet; d, doublet; t, triplet; q, quartet; m, multiplet; dd, double doublet; ddd, double double doublet; dt, doublet triplet. The coupling constants (J) are in Hertz. The structures of compounds were established using 1D NMR (1H, 13C) and 2D NMR (1H-1H COSY, 13C-1H HMBC, 13C-1H HSQC) spectroscopy. Infrared spectra were measured on a FT-IR spectrometer in KBr pellets. High-resolution mass spectra (HRMS) were measured using electrospray ionization (ESI) in positive ion mode (interface capillary voltage 4,500 V); the mass range was from m/z 50 to 3,000 Da; external/internal calibration was performed using an electrospray calibrant solution. A syringe injection was used for solutions in CH3CN (flow rate 3 ml/min). Nitrogen was applied as a dry gas and the interface temperature was set at 180°C. Melting points were measured on a Boetius capillary melting point apparatus and are uncorrected. Analytical thin-layer chromatography (TLC) was carried out on silica gel plates (silica gel 60 F254 aluminum supported plates); the visualization was accomplished with an UV lamp (254/365 nm) and using chemical staining with [Ce(SO4)2/H2SO4]. Column chromatography was performed on silica gel 60 (230–400 mesh, Merck). Androst-5-en-3β-ol-17-one-3β-acetate, 17β-hydroxy-5α-androstan-3-one, estrone, phosphorus oxychloride, guanidine salts, and acetimidamide hydrochloride were commercially available and were used as purchased. The spectroscopic data for steroidal chlorovinyl aldehydes 2a–c are consistent with those reported previously (Komkov et al., 2015; Volkova et al., 2016). All reactions were carried out in freshly distilled and dry solvents.