Ekaterina S. Mescheryakova

Stereospecific 7α-alkylation of 20-hydroxyecdysone in a lithium-ammonia solution.

The reaction of 20-hydroxyecdysone with methyl or ethyl iodide or allyl bromide in a lithium-ammonia solution results in stereospecific 7α-alkylation to give 7α-methyl-, 7α-ethyl-, and 7α-allyl-14-deoxy-Δ(8(14))-20-hydroxyecdysones, respectively. By catalytic hydrogenation (Pd-C/MeOH), the 7α-allyl derivative was converted to 7α-n-propyl-14-deoxy-Δ(8(14))-20-hydroxyecdysone.

Intermolecular interactions and chiral crystallization effects in (1,5,3-dithiazepan-3-yl)-alkanoic acids

Crystals of (1,5,3-dithiazepan-3-yl)-alkanoic acids with achiral and chiral amino acid moieties have been obtained, and their structures were studied using a single crystal X-ray technique. Simple crystal systems, namely monoclinic, triclinic, and orthorhombic, were revealed in a series of the studied compounds. The reference 1,5,3-dithiazepan-3-ol forms head-to-head cyclic dimeric associates R22(6) via strong O–H⋯N intermolecular hydrogen bonds. The achiral 1,5,3-dithiazepanes form head-to-head cyclic dimers R22(8) between two carboxylic groups, whereas the co-crystals involve solvent molecules to constitute dimeric pairs R33(11) and R22(8) through O⋯H and N⋯H hydrogen bonds. These dimers further contribute to the aggregation of layers and stacks due to diverse H⋯H and S⋯H contacts. The chiral dithiazepanes form head-to-tail R12(6), R22(7), and R22(8) cyclic dimers between the carboxylic group of one molecule and the dithiazepane moiety of the other one through H⋯H, S⋯H, and O⋯H intermolecular contacts. Hirshfeld analysis has shown that S⋯H (9.2–19.8%) and O⋯H (5.4–18.2%) intermolecular hydrogen bonds as well as weak H⋯H contacts (40.2–64.0%) are predominant in all the compounds to form crystal packing.

What is responsible for conformational diversity in single-crystal tetraoxazaspiroalkanes? X-Ray, DFT, and AIM approaches

The conformational mobility of large heteroatomic cycles (>6) in organic compounds studied in the liquid state is a major challenge. This is due to low energy barriers between the conformational isomers (1–2 kcal mol−1), which make it difficult to find the most important factors in their preference. Eleven single crystal tetraoxazaspirocycloalkanes are considered in this work with single-crystal X-ray diffraction, DFT, and AIM calculations. Natural bond orbital analysis at the B3LYP/6-31G(d,2p) level of theory and topological analysis of the electron density within Baders theory of “Atoms in Molecules” showed that the tetraoxazocane cycle in spiro-adamantanetetraoxazocanes and tetraoxazaspiroalkanes possesses a twist–boat–chair conformation due to the lower interaction energy of LP(O6) → σ*(C5–N4) and the large contribution of the π-component to the C3–O2 bond. The tetraoxazocane cycle in isopropyl-methyl-tetraoxaza-spiro-tridecanes adopts a chair–chair conformation due to the higher interaction energy of LP(O6) → σ*(C5–N4) and the smaller contribution of the π-component. A key factor, which determines the large O1–O2–O6–O7 pseudo-torsion angles (102–112°) in spiro-adamantanetetraoxazocanes and tetraoxaza-spiro-alkanes, is the absence of intramolecular C–H⋯O and C–H⋯π contacts and vice versa, which provide smaller O1–O2–O6–O7 pseudo-torsion angles (0.6–4°) in isopropyl-methyl-tetraoxaza-spiro-tridecanes. The molecules of spiro-adamantanetetraoxazocanes and tetraoxazaspiroalkanes in single crystals are stabilized via C–H⋯O and C–H⋯H–C weak intermolecular interactions. The molecules of isopropyl-methyl-tetraoxaza-spiro-tridecanes are packed in crystals with C–H⋯O unusual bifurcated one-component and multicomponent contacts.