Efforts toward the total synthesis of (±)-toxicodenane A utilizing an oxidopyrylium-based [5+2] cycloaddition of a silicon-tethered BOC-pyranone

Recent advances in [5+2] cycloadditions

Abstract:

The existence of a seven-membered cyclic core in several natural products and biomolecules vitalized the research on its synthesis. [5+2] cycloaddition has become a promising strategy for the construction of seven-membered ring systems by the formation of carbon-carbon bonds in a single step, with strong regioselectivity and stereoselectivity. This review mainly focuses on recent developments in the area of [5+2] cycloaddition since 2019. Total synthesis of natural products involving [5+2] cycloaddition as key step leading to heptacyclic core is also discussed. Synthesis of fused and bridged ring systems via the reactions involving inter and intramolecular [5+2] cycloadditions like oxidopyrylium-mediated [5+2] cycloadditions, [5+2] cycloadditions of vinyl cyclopropanes (VCPs), vinyl phenols, etc is explained in the review with the latest examples. This review provides a useful guide for researchers exploring this powerful strategy to create more elegant heptacycles in their future research.

Asymmetric Total Synthesis of Toxicodenane A by Samarium-Iodide-Induced Barbier-Type Cyclization and Its Cell-Protective Effect against Lipotoxicity

The asymmetric total synthesis of toxicodenane A, a sesquiterpenoid expected to be promising for diabetic nephropathy, was achieved. In the synthesis, a samarium iodide (SmI2)-induced Barbier-type cyclization and a regio- and stereoselective allylic oxidation followed by a dehydration cyclization were employed as key steps. Furthermore, the first asymmetric syntheses of both enantiomers were accomplished using the previously mentioned synthetic strategy. Finally, the synthetic compounds significantly inhibited lipotoxicity-mediated inflammatory and fibrotic responses in mouse renal proximal tubular cells.

Synthetic Studies on the Synthesis of Toxicodenane A and 8,11-epi-Toxicodenane A

The diverse synthesis of oxatricyclotridecanes and oxatricyclododecanes, which are the core structures of toxicodenane A and its skeletal analogues, via a unified manner is presented. The stereochemistry at the bridgehead position of the oxa-bridged bicycle could be efficiently controlled through a diastereoselective anti- and syn-Grignard allylation reaction by appropriately tuning the reaction conditions such as the solvent, the counterion of the Grignard reagent, the substrate, or a combination of these. The ring size could be precisely elaborated via a Lewis acid-mediated intramolecular transacetalation and Prins cyclization cascade reaction by varying the steric hindrance of olefin moiety. Namely, substrates bearing a terminally unsubstituted olefinic functionality afforded oxatricyclotridecanes with an overwhelming preference, while those bearing a dimethyl-substituted olefinic group produced exclusively oxatricyclododecanes. The wide utility and generality of the above key transformations are highlighted by the applications in the unified synthesis of (±)-toxicodenance A, (+)-toxicodenane A, (+)-8,11-epi-toxicodenane A, and other oxatricyclic cores with different stereochemistries and ring sizes.

Enantioselective Total Synthesis and Absolute Configuration Assignment of (+)-Toxicodenane A

We present the first enantioselective total synthesis and absolute configuration assignment of (+)-toxicodenane A via a nine-step sequence from the readily available material. The synthesis features a desymmetric enantioselective reduction of 2,2-disubstituted 1,3-cyclohexanedione for the synthesis of a chiral 2,2-disubstituted 3-hydroxy cyclohexanone building block, a highly diastereoselective Grignard reaction for the incorporation of an allyl group, and a Lewis acid-mediated intramolecular transacetalation and Prins cascade reaction for the construction of oxa-bridged bicyclic rings.

Investigations into a Stoichiometrically Equivalent Intermolecular Oxidopyrylium [5 + 2] Cycloaddition Reaction Leveraging 3-Hydroxy-4-pyrone-Based Oxidopyrylium Dimers

Oxidopyrylium [5 + 2] cycloaddition reactions are powerful strategies for constructing complex bicyclic architectures. However, intermolecular cycloadditions of oxidopyrylium ylides are limited due to competing dimerization processes; consequently, high equivalents of dipolarophiles are often used to help intercept the ylide prior to dimerization. Recent studies by our lab have revealed that oxidopyrylium dimers derived from 3-hydroxy-4-pyrones are capable of reverting back to ylides in situ and as a result can be used as clean oxidopyrylium ylide sources. The following manuscript investigates intermolecular cycloaddition reactions between 3-hydroxy-4-pyrone-derived oxidopyrylium dimers and stoichiometrically equivalent ratios of alkyne dipolarophiles under thermal conditions. With certain reactive alkynes, pure cycloadducts can be obtained following a simple evaporation of the solvent, which is a benefit of the completely atom-economical reaction conditions. However, when less reactive alkynes are used the yields suffer due to a competing dimer rearrangement. Finally, when reactive-yet-volatile alkynes are used, such as methyl propiolate, competing 2:1 ylide/alkyne cycloadducts are observed. Intriguingly, these complex cycloadducts, which can be obtained in good yields from the pure cycloadducts, form with high regio- and stereoselectivities; however, both the regio-and stereoselectivities differ remarkably based on the source of the oxidopyrylium ylide.

Synthesis of Polycyclic Ether-Benzopyrans and In Vitro Inhibitory Activity against Leishmania tarentolae

Construction of a focused library of polycyclic ether-benzopyrans was undertaken in order to discover new therapeutic compounds that affect Leishmania growth and infectivity. This is especially of interest since there are few drug therapies for leishmaniasis that do not have serious drawbacks such high cost, side effects, and emerging drug resistance. The construction of these polycyclic ether-benzopyrans utilized an acetoxypyranone-alkene [5+2] cycloaddition and the Suzuki-Miyaura cross-coupling. The multi-gram quantity of the requisite aryl bromide was obtained followed by effective Pd-catalyzed coupling with boronic acid derivatives. Compounds were tested in vitro using the parasitic protozoan, Leishmania tarentolae. Effects of concentration, time, and exposure to light were evaluated. In addition, the effects on secreted acid phosphatase activity and nitric oxide production were investigated, since both have been implicated in parasite infectivity. The data presented herein are indicative of disruption of the Leishmania tarentolae and thus provide impetus for the development and testing of a more extensive library.