Unusually cyclized triterpenoids: occurrence, biosynthesis and chemical synthesis

Covering: 2009 to 2021Biosynthetically, most of the syntheses of triterpenes follow the cascade cyclization and rearrangement of the acyclic precursors viz., squalene (S) and 2,3-oxidosqualene (OS), which lead to the very well known tetra- and pentacyclic triterpene skeletons. Aside from these, numerous other triterpenoid molecules are also reported from various natural sources and their structures are derived from "S" and "OS" via some unusual cyclization operations which are different from the usual tetra- and pentacyclic frameworks. Numerous compelling advances have been made and reported in the identification of these unusual cyclized mono-, di-, tri- and tetracyclic triterpenes between 2009 and 2021. Besides a dramatic increase in the newly isolated uncommon cyclized triterpenoids, substantial progress in the (bio)-synthesis of these triterpenes has been published along with significant progress in their biological effects. In this review, 180 new unusual cyclized triterpenoids together with their demonstrated biogenetic pathways, syntheses and biological effects will be categorized and discussed.

Biological activities of meroterpenoids isolated from different sources

Meroterpenoids are natural products synthesized by unicellular organisms such as bacteria and multicellular organisms such as fungi, plants, and animals, including those of marine origin. Structurally, these compounds exhibit a wide diversity depending upon the origin and the biosynthetic pathway they emerge from. This diversity in structural features imparts a wide spectrum of biological activity to meroterpenoids. Based on the biosynthetic pathway of origin, these compounds are either polyketide-terpenoids or non-polyketide terpenoids. The recent surge of interest in meroterpenoids has led to a systematic screening of these compounds for many biological actions. Different meroterpenoids have been recorded for a broad range of operations, such as anti-cholinesterase, COX-2 inhibitory, anti-leishmanial, anti-diabetic, anti-oxidative, anti-inflammatory, anti-neoplastic, anti-bacterial, antimalarial, anti-viral, anti-obesity, and insecticidal activity. Meroterpenoids also possess inhibitory activity against the expression of nitric oxide, TNF- α, and other inflammatory mediators. These compounds also show renal protective, cardioprotective, and neuroprotective activities. The present review includes literature from 1999 to date and discusses 590 biologically active meroterpenoids, of which 231 are from fungal sources, 212 are from various species of plants, and 147 are from marine sources such as algae and sponges.

Meroterpenoids: A Comprehensive Update Insight on Structural Diversity and Biology

Meroterpenoids are secondary metabolites formed due to mixed biosynthetic pathways which are produced in part from a terpenoid co-substrate. These mixed biosynthetically hybrid compounds are widely produced by bacteria, algae, plants, and animals. Notably amazing chemical diversity is generated among meroterpenoids via a combination of terpenoid scaffolds with polyketides, alkaloids, phenols, and amino acids. This review deals with the isolation, chemical diversity, and biological effects of 452 new meroterpenoids reported from natural sources from January 2016 to December 2020. Most of the meroterpenoids possess antimicrobial, cytotoxic, antioxidant, anti-inflammatory, antiviral, enzyme inhibitory, and immunosupressive effects.

Two new iridal-type triterpenoids from Iris forrestii

Two new iridal-type triterpenoids, named forrestins A and B (1 and 2), were isolated from Iris forrestii Dykes by comprehensive chromatographic methods. The structures of 1 and 2 were elucidated by interpretation of extensive spectroscopic data including 1D and 2D NMR and HRESIMS.

Iridal-Type Triterpenoids with a Cyclopentane Unit from the Rhizomes of Belamcanda chinensis

Ten new iridal-type triterpenoid derivatives, polycycloiridals K-T (1-10), with a cyclopentane moiety originating from the cyclization of the homofarnesyl side chain were isolated from the rhizomes of Belamcanda chinensis. Their structures were established by spectroscopic data, a single-crystal X-ray diffraction analysis, TDDFT quantum mechanical calculations, and chemical derivatization. Compounds 2 and 10 exhibited 33.4% and 32.9% inhibitory activities, respectively, against PTP1b (protein tyrosine phosphatase) at a concentration of 10 μM. A putative biosynthetic pathway is proposed.

Isolation of naturally occurring novel isoflavonoids: an update

This review covers the literature concerning the isolation and identification of new naturally occurring isoflavonoids from Leguminosae and non-Leguminous species between 2012–2017.

New iridal-type triterpenoid derivatives with cytotoxic activities from Belamcanda chinensis

Eight new iridal-type triterpenoid derivatives, including two noriridals with ether bridge (1-2); two iridals lactone (3-4), four monocyclic iridals (5-8), together with five known iridals (9-14) were identified from the rhizome of Belamcanda chinensis. Their structures were elucidated on the basis of comprehensive spectroscopic methods and electronic circular dichroism (ECD) calculations. Bioassay results showed that belamcanoxide B (1) exhibited moderated cytotoxic activities against HCT-116 and MCF-7 cell lines with IC₅₀ values of 5.58 and 3.35 μM.

Belamchinenin A, an unprecedented tricyclic-fused triterpenoid with cytotoxicity from Belamcanda chinensis

Herein we report an unprecedented tricyclic-fused triterpenoid, which features a rigid and half-caged tricyclic nucleus. The absolute configuration of the tricyclic nucleus of 1 was unequivocally assigned by ECD calculation.

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A personal selection of 32 recent papers is presented covering various aspects of current developments in bioorganic chemistry and novel natural products such as macrophilone A from Macrorhynchia philippina.