Syntheses of the Carotane-type Terpenoids (+)-Schisanwilsonene A and (+)-Tormesol via a Two-Stage Approach

Synthesis of Daucane Natural Products Enabled by a Gold(I)-Catalyzed Tandem Cycloisomerization/(4 + 3) Cycloaddition

A divergent synthesis of three members of the daucane family of natural products is reported, enabled by a gold(I)-catalyzed cycloisomerization/formal (4 + 3) cycloaddition as the key step. The synthesis of penigrisacid A features a vanadium-catalyzed tandem epoxidation/SN2' cyclization, whereas a Suárez radical cyclization enables the synthesis of aspterric acid. This work has also led to the reassignment of the structure of penigrisacid A as well as a short formal synthesis of schisanwilsonene A.

Unified enantiospecific synthesis of drimane meroterpenoids enabled by enzyme catalysis and transition metal catalysis

Merging the advantages of biocatalysis and chemocatalysis in retrosynthetic analysis can significantly improve the efficiency and selectivity of natural product synthesis. Here, we describe a unified approach for the synthesis of drimane meroterpenoids by combining heterologous biosynthesis, enzymatic hydroxylation, and transition metal catalysis. In phase one, drimenol was produced by engineering a biosynthetic pathway in Escherichia coli. Cytochrome P450BM3 from Bacillus megaterium was engineered to catalyze the C-3 hydroxylation of drimenol. By means of nickel-catalyzed reductive coupling, six drimane meroterpenoids (+)-hongoquercins A and B, (+)-ent-chromazonarol, 8-epi-puupehenol, (-)-pelorol, and (-)-mycoleptodiscin A were synthesized in a concise and enantiospecific manner. This strategy offers facile access to the congeners of the drimane meroterpenoid family and lays the foundation for activity optimization.

A catalytic enantioselective stereodivergent aldol reaction

The aldol reaction is among the most powerful and strategically important carbon-carbon bond-forming transformations in organic chemistry. The importance of the aldol reaction in constructing chiral building blocks for complex small-molecule synthesis has spurred continuous efforts toward the development of direct catalytic variants. The realization of a general catalytic aldol reaction with control over both the relative and absolute configurations of the newly formed stereogenic centers has been a longstanding goal in the field. Here, we report a decarboxylative aldol reaction that provides access to all four possible stereoisomers of the aldol product in one step from identical reactants. The mild reaction can be carried out on a large scale in an open flask, and generates CO₂ as the only by-product. The method tolerates a broad substrate scope and generates chiral β-hydroxy thioester products with substantial downstream utility.

Dual Proton/Silver-Catalyzed Serial (5 + 2)-Cycloaddition and Nazarov Cyclization of (E)-2-Arylidene-3-hydroxyindanones with Conjugated Eneynes: Synthesis of Indanone-Fused Benzo[cd]azulenes

A one-pot step-economic tandem process involving (5 + 2)-cycloaddition and Nazarov cyclization reactions has been reported for the facile synthesis of indanone-fused benzo[cd]azulenes from (E)-2-arylidene-3-hydroxyindanones and conjugated eneynes. This highly regio- and stereoselective bisannulation reaction is enabled by dual silver and Brønsted acid catalysis and opens up a new avenue for the construction of important bicyclo[5.3.0]decane skeletons.

Enantioselective conjugate addition of malonates to α,β-unsaturated aldehydes catalysed by 4-oxalocrotonate tautomerase

The enantioselective conjugate addition of malonates to α,β-unsaturated aldehydes catalysed by 4-oxalocrotonate tautomerase is described. High conversions, high enantioselectivities, and good isolation yields were achieved for a range of substrates. We further completed a four-step synthesis of the antidepressant (+)-femoxetine by utilizing this reaction and an enzymatic reductive amination reaction.

Bio-inspired formal total synthesis of (±)-bisabosqual A

A new approach was developed to construct a hexahydrobenzofurobenzopyran ring system by an oxa-[3+3], Diels–Alder reaction and oxidative aromatization. As a synthetic application, the bio-inspired formal synthesis of bisabosqual A was achieved.

Virenscarotins A-M, thirteen undescribed carotane sesquiterpenes from the fungus Trichoderma virens

A document investigation on the fungus Trichoderma virens led to the isolation of thirteen undescribed carotane sesquiterpenes and homologous. All structures were elucidated on the basis of NMR and HRESIMS data, and their absolute configurations were assigned by ECD calculation. Especially, virenscarotins A and B were first ramifications forged by aldol condensation of 4-hydroxy-3-isopentenyl-benzaldehyde with two hydroxyl groups in ring A of traditional carotane sesquiterpenes. Ring rearrangement/expansion and oxidative cleavage of normal carotane sesquiterpenes lead to the six-membered ring A of compound virenscarotin C and the ring A cleavage of compound virenscarotin D. All compounds were evaluated for cytotoxic, anti-inflammatory, and seed germination inhibitory activities.

Semisynthesis of (-)-Bufospirostenin A Enabled by Photosantonin Rearrangement Reaction

An enantioselective semisynthesis of (-)-bufospirostenin A is described. The key steps in the synthesis involve use of our proposed biomimetic and diastereoselective photosantonin rearrangement reaction for construction of the 5/7 bicyclic motif, and a Co-catalyzed reversible double-bond isomerization reaction for installing the double bond in the seven-membered ring.

Alternative metabolic pathways and strategies to high-titre terpenoid production in Escherichia coli

Covering: up to 2021Terpenoids are a diverse group of chemicals used in a wide range of industries. Microbial terpenoid production has the potential to displace traditional manufacturing of these compounds with renewable processes, but further titre improvements are needed to reach cost competitiveness. This review discusses strategies to increase terpenoid titres in Escherichia coli with a focus on alternative metabolic pathways. Alternative pathways can lead to improved titres by providing higher orthogonality to native metabolism that redirects carbon flux, by avoiding toxic intermediates, by bypassing highly-regulated or bottleneck steps, or by being shorter and thus more efficient and easier to manipulate. The canonical 2-C-methyl-D-erythritol 4-phosphate (MEP) and mevalonate (MVA) pathways are engineered to increase titres, sometimes using homologs from different species to address bottlenecks. Further, alternative terpenoid pathways, including additional entry points into the MEP and MVA pathways, archaeal MVA pathways, and new artificial pathways provide new tools to increase titres. Prenyl diphosphate synthases elongate terpenoid chains, and alternative homologs create orthogonal pathways and increase product diversity. Alternative sources of terpenoid synthases and modifying enzymes can also be better suited for E. coli expression. Mining the growing number of bacterial genomes for new bacterial terpenoid synthases and modifying enzymes identifies enzymes that outperform eukaryotic ones and expand microbial terpenoid production diversity. Terpenoid removal from cells is also crucial in production, and so terpenoid recovery and approaches to handle end-product toxicity increase titres. Combined, these strategies are contributing to current efforts to increase microbial terpenoid production towards commercial feasibility.

Characteristic guaiane sesquiterpenes from Daphne penicillata and ECD/NMR-based assignment of C-1 configuration

40 compounds including the first C17 homo-guaiane sesquiterpene (1) were isolated from Daphne penicillata and an efficient method using ECD/NMR strategy to access the C-1 configuration of characteristic guaiane sesquiterpenes has been developed.

Assembling Complex Structures through Cascade and Cycloaddition Processes via Non-Acceptor Gold or Rhodium Carbenes

The ability of highly energetic metal–carbene intermediates to engage in complex cascade or formal cycloaddition processes is one of the most powerful tools for building intricate molecular architectures in a straightforward manner. Among this type of organometallic intermediates, non-acceptor metal carbenes are particularly challenging to access and, therefore, have experienced slower development. In this regard, our group has exploited the use of electrophilic gold(I) complexes to selectively activate certain classes of substrates for the generation of this type of intermediate. Thus, very different types of molecules, such as enynes or 7-substituted cycloheptatrienes, lead to the formation of carbenes under gold(I) catalysis. Related rhodium(II) carbenes can also be generated from cycloheptatrienes. In this account, we aim to summarize our efforts towards the in situ generation of such highly versatile organometallic species as well as studies on their reactivity through formal cycloadditions or complex cascade reactions.

1 Introduction

2 Generation of Au(I)-Vinylcarbenes via a Cycloisomerization/1,5-Alkoxy Migration Cascade

2.1 Intramolecular Trapping of Au(I) Vinylcarbenes

2.1.1 Applications in Total Synthesis

2.2 Intermolecular Trapping of Au(I) Vinylcarbenes

2.2.1 Total Synthesis of Schisanwilsonene A

2.2.2 Trapping with Furans, 1,3-Dicarbonyl Compounds and Cyclic Alkenes

2.2.3 Mechanism of the Cycloisomerization/1,5-Migration Sequence and the Role of the OR Migrating Group

2.2.4 (4+3) Cycloadditions from Enynes

3 Formal Cycloadditions of Simple Donor Metal Carbenes

3.1 The Metal-Catalyzed Retro-Buchner Reaction

3.2 Formal Cycloadditions with Non-Acceptor Carbenes via Metal-Catalyzed Aromative Decarbenations

3.2.1 (4+1) Cycloadditions of Au(I) Carbenes

3.2.2 (3+2) Cycloadditions of Au(I) Carbenes

3.2.3 (4+3) Cycloadditions of Rh(II) Carbenes

4 Concluding Remarks

Practical Enzymatic Production of Carbocycles

The Pd-catalyzed carbon-carbon bond formation pioneered by Heck in 1969 has dominated medicinal chemistry development for the ensuing fifty years. As the demand for more complex three-dimensional active pharmaceuticals continues to increase, preparative enzyme-mediated assembly, by virtue of its exquisite selectivity and sustainable nature, is poised to provide a practical and affordable alternative for accessing such compounds. In this minireview, we summarize recent state-of-the-art developments in practical enzyme-mediated assembly of carbocycles. When appropriate, background information on the enzymatic transformation is provided and challenges and/or limitations are also highlighted.