Four New Limonoids from the Barks of Toona ciliata

Four new limonoids, toonayunnanaes F − I (1 − 4), and six known compounds (5 − 10) were isolated from the barks of Toona ciliata. Their structures were elucidated by thoroughly analyzing of NMR and HRMS data, and single-crystal X-ray diffraction of 1. The oxetane ring moiety in 1 was rare in limonoids and other natural products. Compound 1 showed nitric oxide (NO) inhibitory effect with an IC50 38.45 ± 0.41 µM in lipopolysaccharide (LPS)-activated RAW 264.7 macrophages. Graphic Abstract

Selective preparation of tetrasubstituted fluoroalkenes by fluorine-directed oxetane ring-opening reactions

The selective ring-opening reaction of fluoroalkylidene-oxetanes was directed by the presence of the fluorine atom, enabling a two-step access to tetrasubstituted fluoroalkenes with excellent geometry control. Despite its small van der Waals radii electronic, rather than steric influences of the fluorine atom governed the ring-opening reaction with bromide ions, even in the presence of bulky substituents.

Intramolecular Ring-Opening of Oxetanes: Access to Functionalised Hydroxymethyl 2,3-Dihydroimidazo[1,2-c]quinazolines

An intramolecular oxetane ring-opening was developed, affording novel 2-(hydroxymethyl)-2,3-dihydroimidazo[1,2-c]quinazolines from N-(3-methyloxetan-3-yl)quinazolin-4-amines under mild conditions. The resulting medicinally relevant tricyclic scaffolds were synthesised in good yields with diverse substituents. Moreover, reaction optimisation led to the development of a one-pot procedure.

Rubescins F–H, new vilasinin-type limonoids from the leaves of Trichilia rubescens (Meliaceae)

Three new limonoids, designated as rubescins F (1), G (2), and H (3), together with two known compounds of this type, TS1 (4) and trichirubine A (5), were isolated from methylene chloride/methanol extracts of Trichilia rubescens leaves. The structures of these compounds were elucidated based on 1D and 2D nuclear magnetic resonance (NMR) analysis and complemented by electrospray ionization high-resolution mass spectrometry results and by comparison to data of related compounds described in the literature and ab initio calculations. Rubescin F (1) is the first limonoid from Trichilia spp. with an oxetane ring between C-7 and C-14, which seems to be formed by the isomerization of TS1 (4). The γ-hydroxybutenolide rubescin G (2) is a potential precursor of trichirubine A (5), whereas rubescin H (3) is the first example of a triterpenoid with a single bond between C-7/C-14, forming a cyclopropane ring. The absolute configuration of these limonoids was derived from biosynthetic considerations and ab initio calculations of NMR and optical rotation dispersion data.

A mild catalytic synthesis of 2-oxazolines via oxetane ring-opening: rapid access to a diverse family of natural products

A new catalytic protocol for the expedient synthesis of oxazolines from oxetanes is developed.