The dichapetalins and dichapetalin-type compounds: structural diversity, bioactivity, and future research perspectives

Covering mainly from 2013 up to 2023 with relevant references to work done before 2013First reported in 1995, the dichapetalins and analogous compounds constitute a novel class of natural dammarane-type merotriterpenoids characterized by their unique 2-phenylpyrano moiety annellated to ring A of the dammarane skeleton. They have been reported from only two genera: Dichapetalum (Dichapetalaceae) and Phyllanthus (Phyllanthaceae). About 100 novel dichapetalins and dichapetalin-type compounds, including the acutissimatriterpenes and their antitumour and other bioactivities have been reported. In the present review, we cover the distribution, ethnobotanical and medicinal importance and the diversity of secondary metabolites reported from the two genera Dichapetalum and Phyllanthus from 2013 to date, with appropriate reference to relevant information prior to 2013. We also propose and discuss possible biosynthetic pathways, antitumour activity against a broad range of human and murine cancer cell lines, structure activity relationships, and other biological activities and mechanisms of action. Finally, the review deals with future perspectives which include expansion of the structural diversity and bioactivity scope, possible simplification of the structural complexity of the compounds to enhance their drug-likeness, in silico studies, and continuation of the search for new dichapetalins and dichapetalin-type compounds from the over 200 Dichapetalum and over 1200 Phyllanthus species yet to be investigated. It is envisaged that the present review will stimulate further multidisciplinary and interdisciplinary studies.

Stereoselective Construction of the ABCDE Pentacyclic Motif of Phainanoids via Norrish-Yang Photocyclization Reaction

A Norrish-Yang photocyclization reaction has been applied to regio- and stereoselective construction of the ABCDE pentacyclic motif of natural product phainanoids. The observed substrate conformation control implicates this powerful reaction could be applied to the construction of structurally diverse natural product scaffolds.

Stereoselective Approach to the Core Structure of (+)-Phainanoid A via Strategically Engineered Cascade Polyene Cyclization

Stereoselective synthesis of 3b and its cascade polyene cyclization to 18b have been described. Acyclic polyene 3b was prepared from allyl bromide 4 and 1,3-dithiane 5, and intermediates 4 and 5 were synthesized from the commercially available geraniol (6) and cyclopenten-2-one (8), respectively, using enantioselective reduction of ketone, Johnson-Claisen rearrangement, and the Suzuki reaction as key steps. Au(I)-mediated diastereoselective polyene cyclization of 3b efficiently afforded tetracyclic compound 18b.

Synthesis towards Phainanoid F: Photo-induced 6π-Electrocyclization for Constructing Contiguous All-Carbon Quaternary Centers

In this paper, we report an efficient strategy for synthesizing the DEFGH rings of phainanoid F. The key to the construction of the 13,30-cyclodammarane skeleton of the molecule was a photo-induced 6π-electrocyclization and a homoallylic elimination. Notably, this is a rare example of using electrocyclization reaction to simultaneously construct two vicinal quaternary carbons in total synthesis. The strategy outlined here forms the basis of our total synthesis of Phainanoid F, and it could also serve as a generally applicable approach for synthesizing other natural products containing similar 13,30-cyclodammarane skeletons.

Stereodivergent Construction of [5,5]-Oxaspirolactones of Phainanoids

A stereodivergent synthesis of [5,5]-oxaspirolactones of phainanoids is presented herein. Through precisely tuning the inherent substitution differences on cyclopropanol, a palladium-catalyzed cascade carbonylative lactonization enables the stereodivergent synthesis of [5,5]-oxaspirolactones of phainanoids.

Asymmetric Total Synthesis of (+)-Phainanoid A and Biological Evaluation of the Natural Product and Its Synthetic Analogues

Here, we report our detailed efforts toward the synthesis of phainanoids, a novel class of dammarane-type triterpenoids with potent immunosuppressive activities and unique structural features. Systematic model studies have been carried out, and efficient approaches have been established to construct the benzofuranone-based 4,5-spirocycle, the D/E/F tricyclic core, the [4.3.1] propellane, and the 5,5-oxaspirolactone moieties. The asymmetric synthesis of (+)-phainanoid A has been achieved through kinetic resolution of the tricyclic core followed by diastereoselective installation of the A/B/C and G/H rings and fragment coupling with the enantioenriched I/J rings. In addition, novel estrone-derived phainanoid analogues have been prepared. The immunosuppressive and cell survival assays revealed that (+)-phainanoid A and some of its synthetic analogues can specifically inhibit stimulation-induced lymphocyte proliferation but not cell survival at their effective concentrations. Preliminary structure-activity relationship information has been obtained, which could inspire future design of immunosuppressive phainanoid analogues.

Bidirectional Total Synthesis of Phainanoid A via Strategic Use of Ketones

Here we report the total synthesis of phainanoid A, a unique dammarane-type triterpenoid (DTT), using an unusual bidirectional synthetic strategy. It features two transition-metal-mediated highly diastereoselective transformations to access the two challenging strained ring systems that branch toward opposite directions from the tricyclic core. This work also highlights the strategic use of ketones (or enol triflates) as versatile handles for rapid growth of molecular complexity in all key transformations, which paves the way for efficient preparations of complex and biologically significant DTTs.

Photoredox-catalyzed hydroxymethylation of β-ketoesters: application to the synthesis of [3.3.3] propellane lactones

Photoredox-catalysed hydroxymethylation of β-ketoesters substituted by an allyl subunit on the α-position afforded directly the corresponding bicyclic lactones possessing both a hydroxy group and an unsaturation. A subsequent regioselective iodoetherification led to the formation of original [3.3.3] propellane structures. Substitution of the iodine atom by various nucleophiles afforded highly functionnalized structures including triazolomethyl derivatives.