Stereopentads Derived from a Sequence of Mukaiyama Aldolization and Free Radical Reduction on α-Methyl-β-alkoxy Aldehydes: A General Strategy for Efficient Polypropionate Synthesis

Total Synthesis and Biological Evaluation of the Anti-Inflammatory (13R,14S,15R)-13-Hydroxy-14-deoxyoxacyclododecindione

The first total synthesis of the natural product (13R,14S,15R)-13-hydroxy-14-deoxyoxacyclododecindione, which was isolated in 2018 as a member of the oxacyclododecindione family, is reported. A synthetic strategy through intramolecular Friedel-Crafts acylation combined with the stereoselective synthesis of a new triol key fragment allowed the preparation of the macrolactone. Due to mismatching physical data of the synthetic product, a revision of the configuration of the natural product isolated in 2018 is required. Light-induced E/Z-isomerism of the macrolactone backbone is described for the first time in the class of oxacyclododecindione-type macrolactones. The hydroxylated macrolactone prepared herein was found to show highly promising IC50 values in biological assays addressing the inhibition of inflammatory responses.

Stereodivergent Catalysis

This review covers diastereo- and enantiodivergent catalyzed reactions in acyclic and cyclic systems using metal complexes or organocatalysts. Among them, nucleophilic addition to carbon-carbon and carbon-nitrogen double bonds, α-functionalization of carbonyl compounds, allylic substitutions, and ring opening of oxiranes and aziridines are considered. The diastereodivergent synthesis of alkenes from alkynes is also included. Finally, stereodivergent intramolecular and intermolecular cycloadditions and other cyclizations are also reported.

Toward a Unified Total Synthesis of the Xiamycin and Oridamycin Families of Indolosesquiterpenes

A unified synthetic strategy toward the oridamycin and xiamycin families of natural products was designed, aiming to access several natural products from a common synthetic intermediate readily prepared from geranyl acetate. Part of this strategy was successfully realized, culminating in the synthesis of oridamycin A and oridamycin B. Key steps include a Mn(III)-mediated oxidative radical cyclization to construct the trans-decalin ring, and a 6π-electrocyclization/aromatization sequence to produce the 2,3-fused carbazole. Oridamycin B was accessed through a late-stage, C-H oxidation that converted the C16 methyl to a hydroxymethyl. A variety of strategies were explored to form a chelated radical intermediate en route to xiamycin A, including enolate SET oxidation, oxo-vanadium oxidation, and atom-transfer cyclization. Unfortunately, none of these strategies provided the desired C16-epimeric trans-decalin. Exploratory studies on photoredox-catalyzed radical cyclizations yielded interesting results, including the formation of a bicyclic lactone arising from oxidative termination of the photoredox-catalyzed radical cyclization, and a double 6-endo cyclization with catalyst loadings as low as 0.01 mol%.

Sequential Catalysis of Phosphine Oxide for Stereoselective Synthesis of Stereopentads

An efficient method for accessing enantiomerically pure stereopentads via a catalytic asymmetric sequential aldol reaction has been developed for the first time. The enantioselective sequential aldol reaction produces a wide range of chiral stereopentad precursors in good yields with excellent enantioselectivities. The key to success is the use of the sequential catalytic system involving a chiral phosphine oxide catalyst and trichlorosilyl triflate.

Stereoselective synthesis of the C13-C22 fragment of callystatin A by a non-aldol approach

An efficient synthetic route to the C13-C22 subunit of callystatin A is reported. The key features include diastereoselective alkylation, using Myers auxiliary, for the preparation of the three carbon synthon 7, stereo- and regioselective oxidative vicinal functionalization of an electron deficient trisubstituted (Z)-olefin using an intramolecular sulfinyl group as the nucleophile, diastereoselective radical debromination of a bromohydrin derivative using Guindon's protocol to prepare the C16-C18 anti-anti stereotriad, Lewis acid promoted crotylation following Keck's protocol to create C19, C20 stereocenters and the use of the Pummerer reaction to reveal an aldehyde for the extension of two carbons by Wittig olefination.

Synthesis of a C1-C11 fragment of Zincophorin using planar chiral, neutral π-allyl iron complexes

A key step in the synthesis of a C1-C11 fragment of the ionophore antibiotic Zincophorin involves the addition of an α-alkoxyalkylcopper(I) reagent to a planar chiral, neutral π-allyl iron complex. The key allylic alkylation reaction is highly regio- and stereoselective with addition taking place at the γ-position anti to the metal centre.

Diastereoselective hydrogen-transfer reactions: an experimental and DFT study

Radical reductions of halogenated precursors bearing a heterocycle exo (α) to the carbon-centered radical proceed with enhanced anti-selectivity, a phenomenon that we termed "exocyclic effect". New experimental data and DFT calculations at the BHandHLYP/TZVP level demonstrate that the origin of the exocyclic effect is linked to the strain energy required for a radical intermediate to reach its reactive conformation at the transition state (ΔE(≠)(strain)). Furthermore, radical reductions of constrained THP systems indicate that high 2,3-anti inductions are reached only when the radical chain occupies an equatorial orientation. Hydride deliveries to different acyclic substrates and calculations also suggest that the higher anti-selectivities obtained with borinate intermediates are not related to the formation of a complex mimicking an exocycle. From a broader standpoint, this study reveals important conformational factors for reactions taking place at a center vicinal to a heterocycle or an α-alkoxy group.

The thiopyran route to polypropionates

The strategic use of thiopyran templates to facilitate polypropionate synthesis was first demonstrated in Woodward's landmark total synthesis of erythromycin A in 1981 where the topology of a fused bicyclic system was exploited. In the ensuing three decades, various alternative strategic applications of thiopyran motifs to achieve key stereoselective transformations have emerged including, inter alia, unique substrates for chemoenzymatic syntheses, surrogates for 3-pentanone in enantioselective aldol reactions, and templates for enantiotopic group selective reactions. This review summarizes these developments.

Stereocontrolled synthesis of C1-C17 fragment of narasin via a free radical-based approach

An efficient synthesis of the C1-C17 western unit of narasin was achieved from (S)-Roche ester. Highlights in our synthesis include the successful exploitation of three stereoselective sequences of Lewis acid mediated reaction followed by free-radical-based hydrogen transfer.