Adventures and Detours in the Synthesis of Hydropentalenes

Functionalized hydropentalenes (i.e., bicyclo[3.3.0]octanones) constitute important building blocks for natural products and for ligands for asymmetric catalysis. The assembly and tailored functionalization of this convex roof-shaped scaffold is challenging and has motivated a variety of synthetic approaches including our own contributions, which will be presented in this account.

1 Introduction

2 Biosynthesis of Hydropentalenes

3 Hydropentalenes through the Pauson–Khand Reaction

4 Hydropentalenes through Transannular Oxidative Cyclization of Cycloocta-1,4-diene

5 Functionalization of Bicyclo[3.3.0]octan-1,4-dione to Dodecahydrocyclopenta[a]indenes

6 Functionalization of Bicyclo[3.3.0]octan-1,4-diones to Crown Ether Hybrids

7 Functionalization of Bicyclo[3.3.0]octan-1,4-dione to Cylindramide

8 Tandem Ring-Opening Metathesis/Ring-Closing Metathesis/Cross-Metathesis of Bicyclo[2.2.1]heptanes

9 Functionalization of Bicyclo[3.3.0]octan-1,4-dione to Geodin A

10 Hydropentalenes through Enantioselective Desymmetrization of Weiss Diketones

11 Functionalization of Weiss Diketones by Carbonyl Ene Reactions

12 Functionalization of the Weiss Diketone to Cylindramide and Geodin A Core Units

13 Biological Properties of Bicyclo[3.3.0]octanes

14 Hydropentalenes through Vinylcyclopropane Cyclopentene Rearrangement

15 Functionalization of Bicyclo[3.3.0]octanes toward Chiral Dienes

16 Miscellaneous Syntheses of Hydropentalenes

17 Conclusion and Outlook

Nickel-Catalyzed Anionic Cross-Coupling Reaction of Lithium Sulfonimidoyl Alkylidene Carbenoids With Organolithiums

The mechanistic platform for a novel nickel0 -catalyzed anionic cross-coupling reaction (ACCR) of lithium sulfonimidoyl alkylidene carbenoids (metalloalkenyl sulfoximines) with organometallic reagents is reported herein, affording substituted alkenylmetals and lithium sulfinamides. The Ni0 -catalyzed ACCR of three different types of metalloalkenyl sulfoximines, including acyclic, axially chiral and exocyclic derivatives, with sp2 organolithiums and sp2 and sp3 Grignard reagents has been studied. The ACCR of metalloalkenyl sulfoximines with PhLi in the presence of the Ni0 -catalyst and precatalyst Ni(PPh3 )2 Cl2 afforded alkenyllithiums, under inversion of configuration at the C atom and complete retention at the S atom. In a combination of experimental and DFT studies, we propose a catalytic cycle of the Ni0 -catalyzed ACCR of lithioalkenyl sulfoximines. Computational studies reveal two distinctive pathways of the ACCR, depending on whether a phosphine or 1,5-cyclooctadiene (COD) is the ligand of the Ni atom. They rectify the underlying importance of forming the key Ni0 -vinylidene intermediate through an indispensable electron-rich Ni0 -center coordinated by phosphine ligands. Fundamentally, we present a mechanistic study in controlling the diastereoselectivity of the alkenyllithium formation via the key lithium sulfinamide coordinated Ni0 -vinylidene complex, which consequently avoids an unselective formation of an alkylidene carbene Ni-complex and ultimately racemic alkenyllithium.

Improved Synthesis and Isolation of Bedaquiline

Bedaquiline (BDQ) is the most critical pharmaceutical in the world for treating multidrug-resistant Mycobacterium tuberculosis. Despite it being highly effective, BDQ asymmetric synthesis remains a challenge. Herein, the influence of chiral bases, namely, bis(1-phenylethyl)amine, bisoxazoline, and sparteine on the diastereoselective lithiation reaction to obtain BDQ was investigated. The highest diastereoselective ratio (dr) emerged as 90:10 from the (+)-bis[(R)-1-phenylethyl] lithium amide. This is a significant improvement from the 50:50 dr achieved from the commercial synthesis. Thereafter, the desired (90:10 RS, SR) diastereomeric mixture was easily isolated via a gravity column and subjected to chiral supercritical fluid chromatography (SFC) to access the desired enantiomer (1R, 2S)-BDQ. The advantages of this procedure are enhanced diastereoselection as well as a greener, faster way to achieve excellent enantioseparation (up to 1.0 g scale).

SAR studies on hydropentalene derivatives--Important core units of biologically active tetramic acid macrolactams and ptychanolides

Structurally diverse bicyclo[3.3.0]octanes were prepared and tested for their biological activity. Both the antiproliferative activity and the results of phenotypic characterization varied with the substitution patterns. Two derivatives displayed high inhibitory (IC50 ≤3μM) activity against the L-929 cell line, but differed in their mode of action. A cluster analysis with impedance profiling data showed the two compounds in relationship to microtubule interfering compounds. In PtK2 cells treated with both derivatives a perturbing effect on the microtubular network was observed, whereas the actin cytoskeleton in incubated PtK2 cells was disturbed only by one compound. The effects on tubulin and actin polymerization could be confirmed by in vitro polymerization experiments.

Asymmetric Synthesis of 3-Oxa-15-deoxy-16-(m-tolyl)-17,18,19,20-tetranorisocarbacyclin and Its Neuroprotective Analogue 15-Deoxy-16-(m-tolyl)-17,18,19,20-tetranorisocarbacyclin Based on the Conjugate Addition–Azoalkene–Asymmetric Olefination Strategy

A fully stereocontrolled synthesis of 3-oxa-15-deoxy-16-(m-tolyl)-17,18,19,20-tetranorisocarbacyclin (3-oxa-15-deoxy-TIC, 7 b) and a formal one of 15-deoxy-16-(m-tolyl)-17,18,19,20-tetranorisocarbacyclin (15-deoxy-TIC, 7 a) are described. 15-Deoxy-TIC is specific for the neuronal prostacyclin receptor (IP2) and exhibits neuroprotective activities, and the new 3-oxa-15-deoxy-TIC is expected to be metabolically more stable than 15-deoxy-TIC. The syntheses of 7 a and 7 b are based on the convergent conjugate addition-azoalkene-asymmetric olefination strategy. Key building blocks are the readily available bicyclic azoalkene 14 and the alkenylcopper derivative 15. The stereoselective conjugate addition of 15 to 14 gave hydrazone 13, which was stereoselectively converted to the bicyclic ketone 11. The key steps for the construction of the alpha side chain of 7 a and 7 b and the regioselective introduction of the endocyclic Delta6,6a double bond are: 1) a highly selective asymmetric olefination of ketone 11 with the chiral Horner-Wadsworth-Emmons reagent 28 and 2) a regioselective deconjugation of the alpha,beta-unsaturated ester (E)-10 with the chiral lithium amide 29, which gave the beta,gamma-unsaturated ester anti-9 with high selectivity. The homoallylic alcohol 8 served at a late stage as the joint intermediate in the syntheses of 7 a and 7 b. While an etherification of 8 furnished, after hydrolysis and deprotection, 3-oxa-15-deoxy-TIC, its alkylation afforded alcohol 37, the known precursor for the synthesis of 15-deoxy-TIC.

Novel synthetic strategies for the preparation of prostacyclin and prostaglandin analogues ? off the beaten track

The recent increase in activity in the fields of neuroscience and life sciences has been mirrored by the design and synthesis of novel chemically and metabolically stable prostaglandin and prostacyclin analogues. Consequently, convenient and practical access to these important classes of compounds is greatly coveted. Various strategies for the preparation of prostacyclin, prostaglandin and isoprostane analogues are discussed, with particular focus on novel approaches developed in our own laboratories.

Development of a Common Fully Stereocontrolled Access to the Medicinally Important and Promising Prostacyclin Analogues Iloprost, 3-Oxa-Iloprost and Cicaprost

We describe new fully stereocontrolled syntheses of the prostacyclin analogues iloprost (2), the most active component of the drugs Ilomedin and Ventavis, and 3-oxa-iloprost (3), a derivative that is expected to have a significantly higher metabolic stability than 2 perhaps allowing an oral application. The syntheses are based on the same strategy and chiral bicyclic building block as used in the synthesis of cicaprost (4), the third most potent analogue that exhibits, besides prostacyclin-like activities, antimetastatic activities. Reaction of the enantiopure C6-C13 bicyclic aldehyde 17 with Cl(3)CCOOH/Cl(3)CCOONa afforded trichlorocarbinol 24 which was converted via mesylate 25 to the C6-C14 bicyclic alkyne 9. The palladium-catalysed hydrostannylation of alkyne 9 gave with high regio- and stereoselectivity the alkenylstannane 26, Sn/Li exchange of which afforded the E-configured alkenyllithium derivative 8. Coupling of the C6-C14 building block 8 with the enantiopure C15-C20 building block, the N-methoxyamide 7, gave the C6-C20 bicyclic ketone 6 in high yield without epimerisation at C16. The configuration at C15 of iloprost (2) and 3-oxa-iloprost (3) was established through a highly diastereoselective reduction of ketone 6 with catecholborane and the chiral oxazaborolidine 28 which furnished alcohol (15S)-29. The highly stereoselective conversions of alcohol (15S)-29 to iloprost (2) and 3-oxa-iloprost (3), which include as key stereoselective steps an olefination with a chiral phosphonoacetate and a copper-mediated allylic alkylation, have already been described.

Design, synthesis, and biological evaluation of new phosphodiesterase type 4 inhibitors

The design, synthesis, and biological evaluation of new phosphodiesterase type 4 inhibitors, which possess new templates instead of a cyclohexane ring, are described. The mode of interaction with the enzyme is discussed based on the structure-activity relationship (SAR) data obtained for the synthesized inhibitors. Furthermore, the roles of three pharmacophores, a catechol moiety, a nitrile moiety, and acidic moieties, are discussed using in silico docking studies. More detailed biological evaluations of selected compounds are also presented.

Asymmetric synthesis of the highly potent anti-metastatic prostacyclin analogue cicaprost and its isomer isocicaprost

An asymmetric synthesis of the anti-metastatic prostacyclin analogue cicaprost and a formal one of its isomer isocicaprost by a new route are described. A key step of these syntheses is the coupling of a chiral bicyclic C6-C14 ethynyl building block with a chiral C15-C21 omega-side chain amide building block with formation of the C14-C15 bond of the target molecules. A highly stereoselective reduction of the thereby obtained C6-C21 intermediate carrying a carbonyl group at C15 of the side chain was accomplished by the chiral oxazaborolidine method. The chiral phosphono acetate method was used for the highly stereoselective attachment of the alpha-side chain to the bicyclic C6-C21 intermediate carrying a carbonyl group at C6. Asymmetric syntheses of the bicyclic C6-C14 ethynyl building blocks were carried out starting from achiral bicyclic C6-C12 ketones by using the chiral lithium amide method. In the course of these syntheses, a new method for the introduction of an ethynyl group at the alpha-position of the carbonyl group of a ketone with formation of the corresponding homopropargylic alcohol was devised. Its key steps are an aldol reaction of the corresponding silyl enol ether with chloral and the elimination of a trichlorocarbinol derivative with formation of the ethynyl group. In addition, a new aldehyde to terminal alkyne transformation has been realized. Its key steps are the conversion of an aldehyde to the corresponding 1-alkenyl dimethylaminosulfoxonium salt and the elimination of the latter with a strong base. Two basically different routes have been followed for the synthesis of the enantiomerically pure C15-C21 omega-side chain amide building block. The first is based on the chiral oxazolidinone method and features a highly stereoselective alkylation of (4R)-N-acetyl-4-benzyloxazolidin-2-one, and the second encompasses a malonate synthesis of the racemic amide and its efficient preparative scale resolution by HPLC on a chiral stationary phase containing column.

A new strategy for the enantioselective synthesis of carba-prostacyclin analogues based on organocopper conjugate addition to a bicyclic azoene and its application to the synthesis of 13,14-dinor-inter-p-phenylene carbacyclin

An enantioselective synthesis of E/Z-13,14-dinor-inter-p-phenylene carbacyclin (E/Z-2d) by a new strategy has been realized that holds the prospect of serving as a general route for carba-prostacyclin analogues. The key intermediate in this synthesis is the bicyclic azoene Ts-9, and the key step is the regio- and stereoselective conjugate addition of the chiral arylcopper compound Cu-8d/P-n-Bu3 to the azoene with formation of hydrazone 7d. Enantioselective synthesis of azoene Ts-9 of 95% ee from ketone 4 was accomplished in four and five steps, respectively. Thus, enantioselective deprotonation of bicyclic ketone 4 with chiral base Li-10 and trapping of lithium enolate 11 with ClSiMe3 gave enol ether 12, which was chlorinated with N-chlorosuccinimide (NCS) to afford chloro ketone 13. Alternatively, chloro ketone 13 was also prepared upon chlorination of 11 with NCS. Chloro ketone 13 was converted to chloro hydrazone 14, which upon treatment with a mild base furnished azoene Ts-9. Arylcopper compound 8d of 98% ee was obtained in two steps from alcohol 16, which was prepared by enantioselective reduction of ketone 17 with (-)-diisopinocampheylchloroborane. Carbacyclin derivative E/Z-2d was found to be essentially inactive as an inhibitor of ADP induced human platelet aggregation, having an IC50 of >10 micromol/L.