Bioinspired Synthesis of (-)-PF-1018

Enantioselective Intramolecular ortho Photocycloaddition Reactions of 2-Acetonaphthones

2-Acetonaphthones, which bear an alkenyl group tethered to its C1 carbon atom via an oxygen atom, were found to undergo an enantioselective intramolecular ortho photocycloaddition reaction. A chiral oxazaborolidine Lewis acid leads to a bathochromic absorption shift of the substrate and enables an efficient enantioface differentiation. Visible light irradiation (λ=450 nm) triggers the reaction which is tolerant of various groups at almost any position except carbon atom C8 (16 examples, 53-99 % yield, 80-97 % ee). Consecutive reactions were explored including a sensitized rearrangement to tetrahydrobiphenylenes, which occurred with full retention of configuration. Evidence was collected that the catalytic photocycloaddition occurs via triplet intermediates, and the binding mode of the acetonaphthone to the chiral Lewis acid was elucidated by DFT calculations.

Enantioselective synthesis of the aglycone of burnettramic acid A

Enantioselective total synthesis of the aglycone of burnettramic acid A, an antifungal pyrrolizidinedione with a terminally mannosylated long acyl chain produced by Aspergillus fungi, has been achieved from a known carboxylic acid by a 14-step sequence. The key steps include 2 types of asymmetric alkylation, coupling of an acetylide intermediate with (S)-epichlorohydrin to provide an acetylenic epoxide in 1 pot, and the Birch reduction to effect desulfonylation, semi-reduction of triple bond, and debenzylation in a concurrent manner. Good agreement of the synthetic aglycone with naturally occurring one in 1H and 13C nuclear magnetic resonance (NMR) spectra, coupled with previously reported unambiguous stereochemical assignment of the sugar moiety, has confirmed the structure of burnettramic acid A.

A computational investigation towards substitution effects on 8π electrocyclisation of conjugated 1,3,5,7-octatetraenes

A computational investigation using M06-2X/6-31+G(d) method is reported for the substitution effects on 8π electrocyclisation of conjugated octatetraene. This systematic study describes the mono- and di-substitution effect across the 1,3,5,7-octatetraene skeleton. A general preference of the outward substitution over the inward, at C1 position of the monosubstituted system is observed. However, mesomerically electron donating group (-NH2 and -OH) display an opposite effect with respect to secondary orbital interaction (SOI) between the lone pair on the substituent and the orbital. A comparative evaluation on the computed activation energies for the 1-, 2-, 3-, and 4-monosubstituted system showed an insignificant impact on the rate of the reaction, in contrast to the electrocyclic ring closure of the unsubstituted compound. Computations of disubstituted system are more pronounced, where a remarkable acceleration is observed for 2-NO2-7-NO2 substituted octatetraene at 4.9 kcal mol-1, and a noticeable deceleration for 4-CH3-5-CH3 substituted octatetraene at 25.4 kcal mol-1 from the parent molecule, 17.0 kcal mol-1. A visible accelerated effects are commonly exhibited by the substitution on the terminal double bonds (C1, C2, C7, and C8), that are 1,2-, 1,7-, 1,8-, and 2,7-patterns, in regard to the greater orbital interaction for the new σ-bond formation. Despite the unfavourable steric clashes of the substituents in the 1,8-system, an apparent reduction in the energy barrier up to 7.4 kcal mol-1 is computed for 1-NH2-8-NO2 system from 17.0 kcal mol-1. This is due to the synergistic effect of the electron donor and electron acceptor, enhancing the stability of the transition structure. The electrocyclic ring closure involving vicinal substitution patterns, such as 1,2-, 2,3-, 3,4-, and 4,5-systems are critically dominated by steric crowding between the adjacent functional groups. In certain cases of the 1,2-substituted system, a noticeable accelerated effects are found for 1-NH2-2-NH2-substituted compound (9.7 kcal mol-1) due to an increased in electronic density on the substituted terminal double bond (C1-C2), hence favouring the formation of the new σ-bond.

Total Synthesis and Structural Revision of the 6,11-Epoxyisodaucane Natural Sesquiterpene Using an Anionic 8π Electrocyclic Reaction

A new synthetic strategy that forms a seven-membered carbocycle using an anionic 8π electrocyclic reaction facilitated the first total synthesis of the 6,11-epoxyisodaucane natural sesquiterpene in 9.0% yield over 10 steps in the longest linear sequence. The misassigned proposed stereochemistry was corrected by the synthesis of both the proposed structure and its C6 epimer. In addition, the 5-7-fused ring system was concisely constructed by tandem decyanation/five-membered-ring formation from an epoxynitrile.

Gram-Scale Enantioselective Synthesis of (+)-Lucidumone

The first enantioselective total synthesis of (+)-lucidumone is described through a 13-step synthetic pathway (longest linear sequence). The key steps involve the formation of a bridged bicyclic lactone by an enantioselective inverse-electron-demand Diels-Alder cycloaddition, C-O bond formation to assemble two fragments, and a one-pot retro-[4 + 2]/[4 + 2] cycloaddition cascade. The synthesis is scalable, and more than one gram of natural product was synthesized in one batch.

Bioinspired Asymmetric Total Synthesis of Emeriones A–C**

We report asymmetric bioinspired total syntheses of the fungal metabolites emeriones A–C via stereoselective oxidations of two bicyclo[4.2.0]octadiene diastereomers. The central bicyclic scaffolds are prepared in an 8π/6π electrocyclization cascade of a stereodefined pentaene, which contains the fully assembled side chains of the emeriones. The anti‐aldol side chain is made using a Paterson‐aldol addition, and the epoxide of the dioxabicyclo[3.1.0]hexane side chain via ring‐closure onto an oxidized acetal. Our work has enabled the structural revision of emerione C, and resulted in the synthesis of a “missing” family member, which we call emerione D. DFT calculations identified two methyl groups that govern torquoselectivity in the 8π/6π cascade.

Two Steps to Bicyclo[4.2.0]octadienes from Cyclooctatetraene: Total Synthesis of Kingianic Acid A

Synthetic approaches to bicyclo[4.2.0]octadiene natural products frequently employ the synthesis of linear tetraenes to initiate a biosynthetic 8π/6π-electrocyclization cascade. This work forges a functionalized bicyclo[4.2.0]octadiene in two steps from cyclooctatetraene. The versatility of this method is demonstrated through natural product synthesis, including the first total synthesis of kingianic acid A and formal syntheses of kingianins A, D, and F and cryptobeilic acid D ethyl ester. The unexpected formation of an E,E,Z,E-tetraene byproduct is rationalized through density functional theory modeling.

Recent Applications of the Diels–Alder Reaction in the Synthesis of Natural Products (2017–2020)

The Diels–Alder reaction has long been established as an extremely useful procedure in the toolbox of natural product chemists. It tolerates a wide spectrum of building blocks of different complexity and degrees of derivatization, and enables the formation of six-membered rings with well-defined stereochemistry. In recent years, many total syntheses of natural products have been reported that rely, at some point, on the use of a [4+2]-cycloaddition step. Among classic approaches, several modifications of the Diels–Alder reaction, such as hetero-Diels–Alder reactions, dehydro-Diels–Alder reactions and domino-Diels–Alder reactions, have been employed to extend the scope of this process in the synthesis of natural products. Our short review covers applications of the Diels–Alder reaction in natural product syntheses between 2017 and 2020, as well as selected methodologies which are inspired by, or that can be used to access natural products.

1 Introduction

2 Syntheses from 2017

3 Syntheses from 2018

4 Syntheses from 2019

5 Syntheses from 2020

6 Conclusion

Facile generation of bridged medium-sized polycyclic systems by rhodium-catalysed intramolecular (3+2) dipolar cycloadditions

Bridged medium-sized bicyclo[m.n.2] ring systems are common in natural products and potent pharmaceuticals, and pose a great synthetic challenge. Chemistry for making bicyclo[m.n.2] ring systems remains underdeveloped. Currently, there are no general reactions available for the single-step synthesis of various bridged bicyclo[m.n.2] ring systems from acyclic precursors. Here, we report an unusual type II intramolecular (3+2) dipolar cycloaddition strategy for the syntheses of various bridged bicyclo[m.n.2] ring systems. This rhodium-catalysed cascade reaction provides a relatively general strategy for the direct and efficient regioselective and diastereoselective synthesis of highly functionalized and synthetically challenging bridged medium-sized polycyclic systems. Asymmetric total synthesis of nakafuran-8 was accomplished using this method as a key step. Quantum mechanical calculations demonstrate the mechanism of this transformation and the origins of its multiple selectivities. This reaction will inspire the design of the strategies to make complex bioactive molecules with bridged medium-sized polycyclic systems.

The bridged medium-sized ring bicyclo[m.n.2] family of natural products are commonly found but difficult to synthesize efficiently. Here the authors present a cascade reaction to form the carbon skeleton, via a [3+2] cycloaddition of a captured azavinyl carbene intermediate.

Visible Light-Induced Pericyclic Cascade Reaction for the Synthesis of Quinolinone Derivatives with an Oxabicyclo[4.2.0]octene Skeleton

A photoinduced pericyclic cascade reaction has been developed to afford oxabicyclo[4.2.0]octenes. Mechanistic studies show that this reaction undergoes [2 + 2]-photocycloaddition, base-promoted elimination, retro-4π-electrocyclization, [1,5]-H shift, and 4π-electrocyclization procedures. This reaction features wide substrate scope, good functional group tolerance, and excellent diastereoselectivity.

Recent Advances in the Total Synthesis of Natural Products Containing Eight-Membered Carbocycles (2009-2019)

Natural products containing eight-membered carbocycles constitute a class of structurally intriguing and biologically important molecules such as the famous diterpenes taxol and vinigrol. Such natural products are being increasingly investigated because of their fascinating architectural features and potent medicinal properties. However, synthesis of natural products with cyclooctane moieties has proved to be highly challenging. This review highlights the recently completed total syntheses of natural products with eight-membered carbocycles with a focus on strategic considerations. A collection of 27 representative studies from the literature covering the decade from 2009 to 2019 is described in chronological order with relevant studies grouped together, including syntheses of the same natural product by different research groups using different strategies. Finally, a summary and outlook including a discussion of the major features of each strategy used in the syntheses are presented. This review illustrates the diversity and creativity in the elegant synthetic designs of eight-membered carbocycles. We hope this review will provide timely illumination and beneficial guidance for future synthetic efforts for organic chemists who are interested in this area.