In this paper the importance of C–H？O intermolecular hydrogen bonds and van der Waals forces in crystal packing stabilization of 16α-hydroxyfriedelin (1) and 3-oxo-16-methylfriedel-16-ene (2) is described. Compound 1 is a natural product isolated from the hexane extract of Salacia elliptica branches, whereas compound 2 is obtained from compound 1 after dehydration accompanied by methyl migration of C-17 to C-16. The single-crystal X-ray diffraction experiments for 1 and 2 were carried out at 150？K, and the crystallographic study demonstrated that these compounds crystallize in noncentrosymmetric space groups, with 1 showing an orthorhombic P212121 space group and 2 a monoclinic P21 one. Compounds 1 and 2 are composed of five fused six-membered rings presenting a chair conformation, except for the central ring of 2, which adopts a half-chair conformation. In addition, the intra- and intermolecular parameters were studied using CCDC MOGUL analyses and Hirshfeld surfaces. 1. Introduction Terpenes are well-known secondary metabolites occurring in many plants specimens [1]. This compound class has been widely investigated due to its biological properties, as, for example, antituberculosis [2], nematostatic effects [3], anticancer [4], anti-HIV [5], and anti-inflammatory [6]. The terpenoids derivatives studied here belong to pentacyclic triterpenes (PCTT) class and they are known as 16α-hydroxyfriedelin (1) and 3-oxo-16-methylfriedel-16-ene (2). The triterpene 1 was isolated from the hexane extract of Salacia elliptica branches and its derivative 3-oxo-16-methylfriedel-16-ene (2) was obtained after dehydration accompanied by methyl migration [7]. In a previous report Duarte et al. [7] elucidated the stereochemistry of 1 and 2 using 2D-NMR (NOESY) spectroscopy and mass spectrometry (GC-MS), as well as the 13C-NMR. As part of our ongoing study on X-ray diffraction applied to establish the structural details of pentacyclic triterpenes [8–12], in this paper, we report the crystal structure of the triterpene 16α-hydroxyfriedelin (1) and its derivative 3-oxo-16-methylfriedel-16-ene (2). The X-ray diffraction (XRD) studies of triterpenes have received a meaningful use in order to access both the intra and intermolecular geometry correctly, giving an unambiguous structure determination. Here, we investigate the role of the main intermolecular interactions in the stabilization of the solid state architecture of the 1 and 2 PCTT derivatives building blocks. 2. Experimental Part 2.1. Single Crystal and X-Ray Diffraction Studies The needle-like single crystals of
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