Synthesis and Reactivity of Estra‐1,3‐5(10)‐trienes with Heterocyclic Four‐Membered Rings in 14,15‐Position3‐Methoxy‐estra‐1,3,5(10),14‐tetraen‐17β‐ol acetate1and ‐17α‐ol acetate2react with chlorosulfonyl isocyanate in stereospecific manner and good yields to 3‐methoxy‐14β,15β‐[3′,4′‐(N‐chlorosulfonyl‐2′‐oxo‐azetidin)]‐estra‐1,3,5(10)‐trien‐17β‐ol acetate3and ‐17α‐ol acetate4, respectively. Hydrolysis of3and4in the presence of reducing agents affords the N‐unsubstituted oxo‐azetidines5and6, reduction of which gives the azetidines9and10. Cleavage of the azetidine ring yields the amino acid derivatives11and15, from which the oxooxetanes12and13and the hyd