Oxidative polycyclization with rhenium(VII) oxides: application of the stereoselectivity rules in the total synthesis of rollidecins C and D

The direct oxidative diene cyclization and related reactions in natural product synthesis

The direct oxidative cyclization of 1,5-dienes is a valuable synthetic method for the (dia)stereoselective preparation of substituted tetrahydrofurans. Closely related reactions start from 5,6-dihydroxy or 5-hydroxyalkenes to generate similar products in a mechanistically analogous manner. After a brief overview on the history of this group of transformations and a survey on mechanistic and stereochemical aspects, this review article provides a summary on applications in natural product synthesis. Moreover, current limitations and future directions in this area of chemistry are discussed.

Transition metal-promoted biomimetic steps in total syntheses

Important biomimetic steps in natural product synthesis have been promoted by transition metals, as exemplified by this beautiful ruthenium-catalyzed rearrangement of an endoperoxide into elysiapyrone A. Such reactions are supposed to occur during the biosynthesis, yet under different catalysis conditions.

Assignment of the C5' relative stereochemistry in (+)-lepadin F and (+)-lepadin G and absolute configuration of (+)-lepadin G

Concise assignments of the C5' stereochemistry in (+)-lepadin F and (+)-lepadin G and the absolute configuration of (+)-lepadin G via the first total syntheses of (+)-5'-epi-lepadin F, (+)-lepadin G, and (+)-5'-epi-lepadin G are described. This work represents an illustrative example in which a diastereomeric pair can possess sufficient spectroscopic difference for clear assignment despite differing only at a highly insulated acyclic stereocenter.

Recent progress on the total synthesis of acetogenins from Annonaceae

An overview of recent progress on the total synthesis of acetogenins from Annonaceae during the past 12 years is provided. These include mono-tetrahydrofurans, adjacent bis-tetrahydrofurans, nonadjacent bis-tetrahydrofurans, tri-tetrahydrofurans, adjacent tetrahydrofuran-tetrahydropyrans, nonadjacent tetrahydrofuran-tetrahydropyrans, mono-tetrahydropyrans, and acetogenins containing only gamma-lactone. This review emphasizes only the first total synthesis of molecules of contemporary interest and syntheses that have helped to correct structures. In addition, some significant results on the novel synthesis and structure-activity relationship studies of annonaceous acetogenins are also introduced.

Total synthesis of (+)-lepadin F

An enantioselective total synthesis of (+)-lepadin F is described. The synthetic sequence features an intermolecular aza-[3 + 3] annulation, homologation of a vinylogous amide via Eschenmoser's episulfide contraction, and a highly stereoselective hydrogenation essential for achieving the 1,3-anti relative stereochemistry at C2 and C8a.

The Mechanism of the Permanganate-Promoted Oxidative Cyclization of 1,5-Dienes - a DFT Study

The mechanism of the permanganate oxidation of 1,5-dienes was investigated by density functional theory calculations (B3LYP/6-311+G*). The experimentally observed (unexpected) products, 2,5-bis(hydroxymethyl) tetrahydrofurans, are formed in a concerted reaction, which explains the observed stereoselectivity of the reaction. Water plays an important role in the reaction mechanism by significantly lowering the activation energies.

Facile Entry to Substituted Decahydroquinoline Alkaloids. Total Synthesis of Lepadins A−E and H

Condensation of a L-alanine derived delta-bromo-beta-silyloxy-propylamine with 1,3-cyclohexadione followed by alkylative cyclization produces a bicyclic enone. Diastereoselective Pt/C-catalyzed hydrogenation of this enone in HOAc provides a 5-oxo-cis-fused decahydroquinoline. Wittig olefination of this decahydroquinoline and subsequent epimerization of the resulting 5-formyl intermediate gives rise to a 5-beta-formyl decahydroquinoline exclusively. In a parallel procedure, Peterson reaction of this decahydroquinoline and subsequent hydrogenation of the generated 5-exo-olefin provides a decahydroquinoline with a 5-alpha-substituent predominantly. For these two diastereoselective processes, using the intermediates without N-protection as the substrates is essential because the corresponding N-Boc intermediates give poor diastereoselectivity. The intermediate with beta-form side chain is further converted into lepadins A-C via carbon chain elongation, while the intermediate with alpha-form side chain is transformed into lepadins D, E, and H and corresponding 5'-epimers via connection with two sulfones generated from two Sharpless epoxidation products. By comparison of the rotations and NMR data, the stereochemistry of lepadins D, E, and H is assigned as 2S,3R,4aS,5S,8aR,5'R.

A Bidirectional Approach to the Synthesis of a Complete Library of Adjacent-Bis-THF Annonaceous Acetogenins

Thirty-six stereoisomers of bifunctional adjacent bis-THF (tetrahydrofuran) lactones have been synthesized, which can afford a complete library of the adjacent bis-THF Annonaceous acetogenins. The bis-THF lactones were synthesized, starting from the enantioselectively pure 8,9:12,13-(E,E and Z,E)-16-benzyloxy-5-hydroxy-hexadeca-1,4-olide, in a highly distereoselective manner using oxidative reactions, including rhenium(VII) oxides-mediated oxidative cyclization, Shi's asymmetric epoxidation, and Sharpless asymmetric dihydroxylation reactions. Using the nonsymmetrical bis-THF lactones, syntheses of two nonnatural acetogenins were achieved.

A metal–oxo mediated approach to the synthesis of 21,22-diepi-membrarollin

A synthesis of 21,22-diepi-membrarollin (14) is described, which employed sequential metal-oxo mediated oxidative cyclisations to introduce six of the seven stereogenic centres.