Synthesis and Functional Characterization of Novel RU1968-Derived CatSper Inhibitors with Reduced Stereochemical Complexity

The sperm-specific Ca2+ channel CatSper (cation channel of sperm) controls the intracellular Ca2+ concentration and, thereby, the swimming behavior of sperm from many species. The steroidal ethylenediamine RU1968 (1) represents a well-characterized, potent, and fairly selective cross-species inhibitor of CatSper. Due to its two additional centers of chirality in the amine-bearing side chain, RU1968 is a mixture of diastereomeric pairs of enantiomers and, thus, difficult to synthesize. This has hampered the use of this commercially not available inhibitor as a powerful tool for research. Here, simplifying both structure and synthesis, we introduced novel stereochemically less complex and enantiomerically pure aminomethyl RU1968 analogues lacking the C-21 CH3 moiety. Starting from (+)-estrone, a five-step synthesis was developed comprising a Wittig reaction as the key step, leading to a diastereomerically pure 17β-configured aldehyde. Subsequent reductive amination yielded diastereomerically and enantiomerically pure amines. Compared to RU1968, the novel ethylenediamine 2d and homologous trimethylenediamine derivative 2e inhibited CatSper with similar and even twofold enhanced potency, respectively. Considering that these aminomethyl analogues are enantiomerically pure and much easier to synthesize than RU1968, we envisage their common use in future studies investigating the physiology of CatSper in sperm.