Total synthesis of (.+-.)-poitediol and (.+-.)-dactylol

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.

Bicyclo [6.3.0] Undecane Sesquiterpenoids: Structures, Biological Activities, and Syntheses

Sesquiterpenoids constitute a marvelously varied group of natural products that feature a vast array of molecular architectures. Among them, the unusual bicyclo [6.3.0] undecane sesquiterpenoids are one of the most representative. To date, only approximately 42 naturally occurring compounds with this unique scaffold, which can be classified into seven different groups, have been reported. As the first-found member of each type, dactylol, asteriscanolide, dumortenol, toxicodenane C, and capillosanane S are characteristic of the four methyl groups on the five-eight-membered ring system. Only 11-hydroxyjasionone and sinuketal decorate the core with an isopropyl group. These natural products exhibit a wide range of bioactivities, including antifouling, anti-inflammatory, immune suppression, cytotoxic, antimutagenic, antiplasmodial, and antiviral activities. It was noted that some total syntheses of precapnellane-sesquiterpenoids (dactylol, poitediol, precapnelladiene), asteriscanolide, and 11-hydroxyjasionone have been achieved, because their cyclooctanoid core represents an important target for the development of synthetic strategies to prepare eight-membered ring-containing compounds. This review focuses on these natural sesquiterpenoids and their biological activities and synthesis, and aims to provide a foundation for further research of these interesting compounds.

Applications of Friedel-Crafts reactions in total synthesis of natural products

Over the years, Friedel-Crafts (FC) reactions have been acknowledged as the most useful and powerful synthetic tools for the construction of a special kind of carbon-carbon bond involving an aromatic moiety. Its stoichiometric and, more recently, its catalytic procedures have extensively been studied. This reaction in recent years has frequently been used as a key step (steps) in the total synthesis of natural products and targeted complex bioactive molecules. In this review, we try to underscore the applications of intermolecular and intramolecular FC reactions in the total syntheses of natural products and complex molecules, exhibiting diverse biological properties.

Efficient synthesis of bicyclo[6.3.0]undecadienones by Nazarov cyclization/regioselective elimination

Easily accessible cyclooctadienyl vinyl ketones can generate fused bicyclic 8-5 (bicyclo[6.3.0]undecadienone) ring systems via the Nazarov cyclization when treated with Lewis acid. The resultant products contain three new stereocenters as well as olefins suitably situated to allow further structural elaboration. This concise sequence allows for the rapid preparation of intermediates that should be applicable to a variety of natural product targets containing this bicyclic core.

Ring-closing metathesis of allylsilanes/electrophilic desilylation to prepare exo-methylidenecycloalkanes. Short syntheses of teucladiol and poitediol

A general strategy for the synthesis of exo-methylidenecycloalkanes, which are salient features of many terpenoid natural products, is presented. Ring-closing alkene metathesis of allylsilanes provides intermediates that can be protodesilylated with alkene transposition to afford the exocyclic alkene; alternatively, the reactivity of the cyclic allylsilane intermediate can be harnessed to introduce allylic functionality. These two modes of reactivity are showcased in short syntheses of the sesquiterpene natural products teucladiol and poitediol, respectively.

Lithium Amide Assisted Asymmetric Mannich-Type Reactions of Menthyl Acetate with PMP-Aldimines

A lithium enolate of menthyl acetate added to PMP-imines, in the presence of an equimolar amount of lithium diisopropylamide, affords the Mannich-type addition products in high stereoselectivity. [reaction--see text]

Design and synthesis of a novel class of sugar-peptide hybrids: C-linked glyco beta-amino acids through a stereoselective "acetate" Mannich reaction as the key strategic element

A new type of sugar-amino acid hybrid, which is comprised of a sugar unit (gluco-, galacto-, or mannopyranose) linked through a C-glycosidic linkage to the beta-position of an alpha-unsubstituted beta-amino acid unit, is presented. It is hypothesized that these new compounds, or the oligomeric peptides derived therefrom, might possess the structural features of beta-amino acid oligomers and the chemical and enzymatic resistance of C-glycosides to hydrolysis. The synthetic strategy is based on a new Mannich-type reaction between a chiral acetate enolate equivalent and alpha-amido sulfones derived from the corresponding sugar-C-glycoside aldehydes. While the sugar-C-glycoside aldehyde partner is prepared from well-established transformations on known sugar precursors, the lithium enolate derived from (1R)-endo-2-acetylisoborneol 3 is employed as the key element. This Mannich approach proceeds with essentially perfect diasteromeric control leading to the new beta-amino carbonyl adducts in good yields. Further, cleavage of the camphor auxiliary is smoothly performed by oxidative treatment with ammonium cerium nitrate (CAN). Complementarily, direct peptide-type coupling of the beta-amino carbonyl Mannich adducts with an alpha- or beta-amino acid residue and subsequent CAN-promoted detachment of the auxiliary yields dipeptide fragments bearing a sugar-containing aliphatic side chain and is a process that can be iterated. A preliminary conformational study based on the combination of experimental NMR data and molecular mechanics and molecular dynamics (MD) of one particular adduct is also provided.

Synthesis of angular triquinanes from 1-Alkynylbicyclo[3.2. 0]hept-2-en-7-ones. A tandem alkoxy-cope ring Expansion/Transannular ring closure reaction

Addition of ethenyllithium reagents to the carbonyl group of dialkyl squarate-derived 1-alkynylbicyclo[3.2.0]hept-2-ene-7-ones (15), followed by a TBAF workup, results in a low-temperature anion-accelerated alkoxy-Cope rearrangement which proceeds by way of a strained cyclic allene intermediates (e.g.,17). This leads to the formation of angularly fused triquinanes (e.g., 20) in which each of the rings is functionally differentiated. Bicyclo[6.3. 0]undecadienones (e.g., 36) are the major products when the reactions are quenched with aqueous bicarbonate rather than TBAF. Under analogous conditions 2-alkylidene-1-alkynylbicyclo[3.2. 0]heptan-7-ones also give bicyclo[6.3.0]undecadienones by a mechanism that was established to involve a 1,5-hydrgen shift in a strained allene intermediate. The synthetic scope and mechanism of these and related transformations are discussed.

Intramolecular 4 + 3 cycloadditions. Aspects of stereocontrol in the synthesis of cyclooctanoids. A synthesis of (+)-dactylol

[reaction: see text] The judicious placement of stereocenters on precursors for 4 + 3 cycloaddition reactions can lead to high levels of stereocontrol in the 4 + 3 cycloaddition process of cyclopentenyl cations and tethered butadienes. This concept was successfully tested in the context of a synthesis of (+)-dactylol.

Cyclobutenone-Based Syntheses of Polyquinanes and Bicyclo[6.3.0]undecanes by Tandem Anionic Oxy-Cope Reactions. Total Synthesis of (+/-)-Precapnelladiene

The addition of ethenyllithium derivatives to the carbonyl of dialkyl squarate-derived bicycloheptenones, e.g., 1a and 6a, initiates a low-temperature anion-accelerated oxy-Cope rearrangement to provide polyquinanes by a transannular aldol reaction of the intermediate bicyclo[6.3.0]undecadienone 4. Additional functionality is introduced by alkylation of the enolate 3 resulting from the oxy-Cope rearrangement. Phosphorylation or triflation of enolate 3 provides an entry into the bicyclo[6.3.0]undecane ring system. An application of this new methodology is demonstrated by the total synthesis of the sesquiterpene natural product (+/-)-precapnelladiene from diisopropyl squarate (10 steps, 12%).

A Concise Total Synthesis of Dactylol via Ring Closing Metathesis

A straightforward total synthesis of the cyclooctenoid sesquiterpene dactylol (1) and of 3a-epi-dactylol (13) has been achieved in six synthetic operations. The unusual rearranged bicyclo[6.3.0]undecane isoprenoid skeleton of these target molecules has been formed via an initial three-component coupling triggered by 1,4-addition of a methylcopper reagent (MeLi, CuI, Bu(3)P) to cyclopentenone, followed by trapping of the enolate formed with 2,2-dimethyl-4-pentenal as the electrophile. The aldol 8 thus obtained was elaborated into the trans-disubstituted cyclopentanone derivative 10 which reacted with a methallylcerium reagent to afford a mixture of the tertiary alcohols 11a and 12a. Separation and O-silylation of these diastereoisomers, ring-closing metathesis (RCM) of the resulting dienes 11b and 12b to form the cyclooctene ring using Schrocks molybdenum carbene 5 as a precatalyst, and a final deprotection afforded the title compound and its epimer in excellent yields. This approach clearly surpasses previous ones in terms of efficiency, flexibility, accessibility of the substrates, number of steps, atom economy, and overall yield.