Revisiting the absolute chirality and polymorphism of (–)‐Istanbulin A

• Published in: Acta crystallographica. Section C, Crystal structure communications Crystal structure communications, 2020-09, Vol.76 (9), p.914-920
• Main Authors: Arancibia, Luz, Naspi, Mariana, Pucci, Graciela, Rodriguez, Maricel, Di Salvo, Florencia
• Format: Article
• Language: English
• Subjects: absolute configuration; Antiinfectives and antibacterials; antimicrobial activity; Chirality; crystal structure; Fungicides; Istanbulin; natural product; Natural products; NMR; Nuclear magnetic resonance; Polymorphism; Senecio filaginoides; terpenoid
• ISSN: 2053-2296; 0108-2701; 2053-2296; 1600-5759
• DOI: 10.1107/S2053229620011493

The terpenoid (−)‐Istanbulin A is a natural product isolated from Senecio filaginoides DC, one of the 270 species of Senecio (Asteraceae) which occurs in Argentina. The structure and absolute configuration of this compound [9a‐hydroxy‐3,4a,5‐trimethyl‐4a,6,7,8a,9,9a‐hexahydro‐4H,5H‐naphtho[2,3‐b]‐furan‐2,8‐dione or (4S,5R,8R,10S)‐1‐oxo‐8β‐hydroxy‐10βH‐eremophil‐7(11)‐en‐12,8β‐olide, C15H20O4] were determined by single‐crystal X‐ray diffraction studies. It proved to be a sesquiterpene lactone showing an eremophilanolide skeleton whose chirality is described as 4S,5R,8R,10S. Structural results were also in agreement with the one‐ and two‐dimensional (1D and 2D) NMR and HR–ESI–MS data, and other complementary spectroscopic information. In addition, (−)‐Istanbulin A is a polymorph of the previously reported form of (−)‐Istanbulin A, form I; thus, the title compound is denoted form II or polymorph II. Structural data and a literature search allowed the chirality of Istanbulin A to be revisited. The antimicrobial and antifungal activities of (−)‐Istanbulin A, form II, were evaluated in order to establish a reference for future comparisons and applications related to specific crystal forms of Istanbulins. The natural product (−)‐Istanbulin A was isolated from Senecio filaginoides DC and the chirality is described as 4S,5R,8R,10S based on single‐crystal X‐ray diffraction studies. (−)‐Istanbulin A corresponds to a polymorph of a previously reported form of (−)‐Istanbulin A (form I) and this is the first time that polymorphism has been reported in the Istanbulin family.