Synthesis and solid state characterization of molecular rotors with steroidal stators: ethisterone and norethisterone

Abstract

In this article we describe the synthesis and dynamic behavior of two new molecular rotors with 1,4-diethynylphenylene rotators axially linked to two conformationally rigid steroidal norethisterone acetate or ethisterone frames. The resulting 1,4-bis(19-nor-17α-ethynyltestosterone-17β-acetate)benzene (1) and 1,4-bis(17α-ethynyltestosterone)benzene (2) were fully characterized in solution and in the solid state, and the rotational dynamics of the central phenylene were explored with the help of ¹³C NMR with cross polarization and magic angle spinning (CPMAS), and with quadrupolar echo variable temperature (VT) ²H NMR in the case of 1. Splitting of signals from the aromatic ring on the ¹³C CPMAS NMR and a broad quadrupolar spin echo ²H spectrum of polycrystalline samples indicated that the rotation of the central aromatic ring in these compounds was limited at ambient temperature in the solid state. Variable temperature ²H NMR experiments at 350 K in the case of 1-d₄ suggested a 2-fold rotational exchange with upper frequency limit of ca. 10 kHz. Single crystal X-ray analysis of this compound revealed that a crowded environment around the prospective phenylene rotator is responsible of the restricted rotation in the solid state.