Asymmetric Total Synthesis of Caribenol A

Asymmetric Total Synthesis of Indole Diterpenes Paspalicine, Paspalinine, and Paspalinine-13-ene

Paspaline-derived indole diterpenes (IDTs) are structurally complex mycotoxins with unique tremorgenic activity. Reported are asymmetric total syntheses of three paspaline-derived IDTs paspalicine, paspalinine and paspalinine-13-ene. Our synthesis features a green Achmatowicz rearrangement/bicycloketalization for the efficient construction of FG rings (75 % yield) and a cascade ring-closing metathesis of dienyne for highly regioselective formation of CD rings (72 % yield). Other highlights include four palladium-mediated reactions (Stille, aza-Wacker, Suzuki, and Heck) to forge the BE rings and the installation of two continuous all-carbon quaternary stereocenters via reductive ring-opening of cyclopropane and α-methylation of the conjugate ester. Our new synthetic strategy is expected to be applicable to the chemical synthesis of other paspaline-derived IDTs and will facilitate the bioactivity studies of these agriculturally and pharmacologically important IDTs.

Synthesis of an isomer of lycoplanine A via cascade cyclization to construct the spiro-N,O-acetal moiety

A 6/10/5/5 tetracyclic isomer of lycoplanine A is synthesized using D–A reaction and cascade cyclization to respectively establish the [9.2.2] pentadecane skeleton and spirocenter, providing sufficient experience to synthesize lycoplanine A.

Rh(i)-Catalyzed enantioselective and scalable [4 + 2] cycloaddition of 1,3-dienes with dialkyl acetylenedicarboxylates

An asymmetric intermolecular [4 + 2] cycloaddition of 1,3-dienes with dialkyl acetylenedicarboxylates, which was catalyzed by a rhodium(i)-chiral phosphoramidite complex, was developed. This protocol provided a highly enantioselective access to prepare carbonyl substituted cyclohexa-1,4-dienes with up to 96% yield and >99% ee. Notably, a cycloaddition on the 10 g scale gave the product in 92% yield and with 99% ee, which showed great potential for the scale-up synthesis of carbonyl substituted cyclohexa-1,4-dienes. In addition, oxidative aromatizations and hydrolysis of the products were also investigated.

A Rh(iii)-catalyzed regioselective intermolecular oxa-Pauson–Khand reaction of alkynes, arylboronic acids and CO to form butenolides

A Rh(iii)-catalyzed regioselective intermolecular oxa-Pauson–Khand reaction of arylboronic acids, alkynes and CO for the synthesis of butenolides has been developed.

TiCl4-n-Bu3N-mediated cascade annulation of ketones with α-ketoesters: a facile synthesis of highly substituted fused γ-alkylidene-butenolides

A facile protocol for the synthesis of highly substituted fused γ-alkylidene butenolides using direct annulation of ketones with α-ketoesters, which proceeds through TiCl4-n-Bu3N mediated aldol addition followed by an intramolecular enol-lactonization/cyclization cascade, is reported. Diverse 6-5, 7-5 and 8-5 fused bicyclic γ-ylidene butenolides and highly substituted monocyclic analogs related to biologically relevant natural products were prepared from readily accessible ketone and α-ketoester building blocks. The highly step-economic cascade nature, good substrate scope, easy access to complex products with good to excellent yields, gram-scalability, demonstration of synthetic utility, and unambiguous structural confirmation through X-ray crystallography analyses and analogy are the salient features of this work.

Total Synthesis of (-)-Indoxamycins A and B

The concise total syntheses of (-)-indoxamycins A and B is reported. The chemistry features a seven-step preparation of a highly congested [5.5.6] tricyclic advanced common intermediate from a readily available R-carvone derivative. Key steps involve a Pauson-Khand reaction for the rapid construction of a basic scaffold bearing a quaternary carbon, a copper-catalyzed Michael addition for the introduction of another adjacent all-carbon quaternary stereocenter, and a tandem retro-oxa-Michael addition/1,2-addition/oxa-Michael addition for the installation of a trisubstituted olefin side chain. This synthetic strategy allows for easy access to both enantiomers of this family of natural products and their analogues from cost-effective starting material through straightforward chemical transformations.

Recent advances on the total synthesis of alkaloids in mainland China

Alkaloids are a large family of natural products that mostly contain basic nitrogen atoms. Because of their intriguing structures and important functions, they have long been popular targets for synthetic organic chemists. China's chemists have made significant progress in the area of alkaloid synthesis over past decades. In this article, selected total syntheses of alkaloids from research groups in mainland China during the period 2011–16 are highlighted.

Bioinspired Asymmetric Synthesis of Hispidanin A

The first enantiospecific synthesis of hispidanin A (4), a dimeric diterpenoid from the rhizomes of Isodon hispida, was achieved with a longest linear sequence of 12 steps in 6.5 % overall yield. A key component is the use of the abundant and naturally occurring diterpenoids (+)-sclareolide and (+)-sclareol as starting materials, which enables the gram-scale preparation of the key intermediates totarane (1) and s-trans-12E,14-labdadien-20,8β-olide (2). Subsequently a thermal or an erbium-catalyzed intermolecular Diels-Alder reaction of totarane (1) with labdadienolide (2) provide convergent and rapid access to the natural product hispidanin A (4). The synthetic studies have offered significant impetus for the efficient construction of these architecturally complex natural products.

Domino intramolecular Diels-Alder reactions to construct a 6/6/5/5 fused tetracyclic framework

Domino intramolecular Diels-Alder (IMDA) reactions towards the 6/6/5/5 fused tetracyclic natural products were developed in satisfactory yield and high stereoselectivity. Four rings, six contiguous stereocenters and four C-C bonds were formed in a single operation. 4-epi-Hydromitchellene B was also synthesised efficiently via this strategy.

Organocatalytic asymmetric strategies to carbocyclic structures by γ-alkylation-annulation sequences

Attractive carbocyclic structures are accessed via a highly regio- and enantioselective aminocatalytic γ-addition of cyclic enals to vinyl phosphonates followed by a one-pot intramolecular Horner-Wadsworth-Emmons reaction. It is also demonstrated that nitro olefins can act as electrophiles in a similar reaction concept, providing carbocycles in equally high stereoselectivity.

Recent applications of intramolecular Diels–Alder reaction in total synthesis of natural products

Diels–Alder (D–A) reaction is undoubtedly the most powerful [4 + 2] cycloaddition reaction in organic synthesis.

Total synthesis of aplykurodinone-1

The concise total synthesis of aplykurodinone-1 with an unusual cis-fused hydrindane moiety has been accomplished without the need for any protecting group chemistry using a unique SmI2 mediated reductive cascade cyclization reaction and a direct cuprate mediated 1,4-addition. This work represents the first example of the use of a SmI2-mediated intramolecular cascade cyclization reaction between "halide, alkene and aldehyde" groups.

Total synthesis of sandresolide B and amphilectolide

The total synthesis of the diterpenoids sandresolide B and amphilectolide from a common furan building block is presented. Key steps include palladium-mediated carbonylation, lanthanide catalyzed ring closure, Myers alkylation, intramolecular Friedel-Crafts acylation, photooxygenation, and a Kornblum-DeLaMare rearrangement.

Bioinspired total synthesis of bolivianine: a Diels-Alder/intramolecular hetero-Diels-Alder cascade approach

We report the first total synthesis of bolivanine in a 14-step pathway involving the synthesis of onoseriolide. Our synthesis features a palladium-catalyzed intramolecular cyclopropanation involving an allylic metal carbene and a Diels-Alder/intramolecular hetero-Diels-Alder cascade, allowing the single-step assembly of a tricyclic system with proper configuration. The synthetic efforts validate our modified biogenetic hypothesis and allow us to confirm the absolute configuration of bolivianine.

Asymmetric total synthesis of caribenol A via an intramolecular Diels-Alder reaction

A total synthesis of the caribenol A (1), a novel natural product with an intriguing tetracyclic framework, has been achieved. The synthesis features an intramolecular Diels-Alder (IMDA) reaction for the facile construction of the tricyclic [5-7-6] skeleton of caribenol A (1) and a biomimetic oxidation reaction for the formation of the 2-hydroxyfuran-2(5H)-one motif of caribenol A (1) as key steps. This synthetic approach also reveals that the sp(2) carbon at C(2) in substrate 8 is a critical factor for the formation of the tricyclic [5-7-6] skeleton in 7.

Highly enantioselective copper-catalyzed electrophilic trifluoromethylation of β-ketoesters

Enantioselective Cu-catalyzed trifluoromethylation of β-ketoesters using commercially available trifluoromethylating reagents is reported. A number of α-CF(3) β-ketoesters are obtained with up to 99% ee. The trifluoromethylated products were then transformed diastereoselectively to α-CF(3) β-hydroxyesters with two adjacent quaternary stereocenters via a Grignard reaction.