Development of a Concise and Diversity-Oriented Approach for the Synthesis of Plecomacrolides via the Diene−Ene RCM

Modular Total Synthesis of iso-Archazolids and Archazologs

Full details on the design, development, and successful implementation of suitable synthetic strategies directed toward the total synthesis of iso-archazolids and archazologs are reported. Both a biomimetic and a multistep total synthesis of iso-archazolid B, the most potent and least abundant archazolid, are described. The bioinspired conversion from archazolid B was realized by a high-yielding 1,8-Diazabicyclo[5.4.0]undec-7-ene catalyzed one-step double-bond shift. A highly stereoselective total synthesis was accomplished in 25 steps, involving a sequence of highly stereoselective aldol reactions, an efficient aldol condensation to forge two elaborate fragments, and a challenging ring-closing metathesis macrocyclization with an unusual Stewart-Grubbs catalyst. These strategies proved to be generally useful and could be successfully implemented for the preparation of three novel iso-archazolids as well as five novel archazologs, lacking the thiazole side chain. A wide variety of further archazolids and archazologs may now be targeted for exploration of the promising anticancer potential of these polyketide macrolides.

Cis Selective RCM Study to the 14-Membered Cyclic Subunit of Bielschowskysin

A concise, (Z)-selective ring-closing metathesis (RCM) route to the 14-membered carbocycle of bielschowskysin is detailed using naturally occurring chiral starting materials. Unproductive RCM substrates were attributed to alkyne chelation of the ruthenium catalyst and steric disadvantages within the cembranoid precursors, which was eventually circumvented by using cyclic diol benzylidene protection involving a C8-quaternary carbinol center.

Synthesis and Structural Implication of the JKLMN-Ring Fragment of Caribbean Ciguatoxin C-CTX-1

Synthesis of the JKLMN-ring fragment of Caribbean ciguatoxin C-CTX-1, the causative toxin of ciguatera fish poisoning in the Caribbean Sea and the Northeast Atlantic areas, is described in detail. Key to the synthesis are a [2,3]-sigmatropic rearrangement to construct a seven-membered α-hydroxy exo-enol ether, stereoselective construction of an angular tetrasubstituted stereogenic center on the seven-membered M-ring by a hydrogen atom transfer-based reductive olefin coupling, Suzuki-Miyaura coupling of the KLMN-ring enol phosphate with a highly congested M-ring, and silica gel-mediated epoxide ring opening to form the J-ring. Comparison of the nuclear magnetic resonance spectroscopic data for the synthesized fragment with those for the natural product provided support for the formerly assigned structure of the N-ring in the right-hand terminal of C-CTX-1.

Development of pyrene-based fluorescent ether lipid as inhibitor of SK3 ion channels

We report the synthesis of three bioactive pyrene-based fluorescent analogues of Ohmline which is the most efficient and selective inhibitor of SK3 ion channel. The interaction of these Ohmline-pyrene (OP1-3) with liposomes of different composition reveals that only OP2 and OP3 are readily integrated into liposomes. Fluorescence measurements indicate that, depending on their concentration, OP2 and OP3 exist either as monomer or as a mixture of monomer and excimers within the liposome bilayer. Among the three Ohmline Pyrene compounds (OP1-3) only OP2 is able to reduce SK3 currents and is the first efficient fluorescent modulator of SK3 channel as revealed by patch clamp measurements (- 71.3 ± 13.3% at 10 μM) and by its inhibition of SK3-dependent cancer cell migration at (-32.5% ± 4.8% at 1 μM). We also report the first fluorescence study on living breast cancer cells (MDA-MB-231) showing that OP2 is rapidly integrated in bio-membranes followed by cell internalization.

Stereoselective three-component cascade synthesis of α-substituted 2,4-dienamides from gem-difluorochloro ethanes

Herein, we describe a new transition metal-free Claisen rearrangement for the synthesis of α-substituted 2,4-dienamides. The one-pot, stereoselective three-component cascade reaction between a series of propargyl alcohols, amines, and gem-difluorochloro ethane derivatives afforded various polysubstituted 2,4-dienamides in good yields. This synthetic method for 1,1-captodative dienes, α-substituted 2,4-dienamides, can be utilized for preparing pharmaceutical analogues containing an indolin-2-one or lactone moiety.

Studies toward the Total Synthesis of Caribbean Ciguatoxin C-CTX-1: Synthesis of the LMN-Ring Fragment through Reductive Olefin Cross-Coupling

Synthesis of the LMN-ring fragment of Caribbean ciguatoxin C-CTX-1, the principal causative toxin for ciguatera fish poisoning around the Caribbean Sea areas, is described. The key feature of the synthesis is the stereoselective introduction of an angular methyl group on the sterically encumbered seven-membered M-ring by the application of a hydrogen atom transfer-based reductive olefin coupling.

Total Syntheses of Nannocystins A and A0, Two Elongation Factor 1 Inhibitors

Asymmetric total syntheses of nannocystins A and A0 were achieved in a convergent route starting from simple materials. Nannocystin family natural products bear potent anticancer activity as elongation factor 1 inhibitors. In this synthesis, the challenging tertiary amide bond was constructed by peptide coupling between an acyl chloride and a secondary amine. A late-stage ring-closing metathesis reaction successfully rendered the macrocycle. This efficient synthetic strategy should be applicable to other nannocystins and analogues and therefore should benefit future structure-activity relationship studies.

Control of olefin geometry in macrocyclic ring-closing metathesis using a removable silyl group

Introducing a silyl group at one of the internal olefin positions in diolefinic substrates results in E-selective olefin formation in macrocyclic ring-forming metathesis. The application of this method to a range of macrocyclic (E)-alkenylsiloxanes is described. Protodesilylation of alkenylsiloxane products yields novel Z-configured macrocycles.

Motualevic acids and analogs: synthesis and antimicrobial structure-activity relationships

Synthesis of the marine natural products motualevic acids A, E, and analogs in which modifications have been made to the omega-brominated lipid (E)-14,14-dibromotetra-deca-2,13-dienoic acid or amino acid unit are reported, together with antimicrobial activities against Staphylococcus aureus, methicillin-resistant S. aureus, Enterococcus faecium, and vancomycin-resistant Enterococcus.

Natural products as an inspiration in the diversity-oriented synthesis of bioactive compound libraries

The purpose of diversity-oriented synthesis is to drive the discovery of small molecules with previously unknown biological functions. Natural products necessarily populate biologically relevant chemical space, since they bind both their biosynthetic enzymes and their target macromolecules. Natural product families are, therefore, libraries of pre-validated, functionally diverse structures in which individual compounds selectively modulate unrelated macromolecular targets. This review describes examples of diversity-oriented syntheses which have, to some extent, been inspired by the structures of natural products. Particular emphasis is placed on innovations that allow the synthesis of compound libraries that, like natural products, are skeletally diverse. Mimicking the broad structural features of natural products may allow the discovery of compounds that modulate the functions of macromolecules for which ligands are not known. The ability of innovations in diversity-oriented synthesis to deliver such compounds is critically assessed.

Total Synthesis of Myxovirescin A1

A convergent total synthesis of the antibiotic macrolide myxovirescin A1 (1) is described that is largely based on reagent- and catalyst-controlled transformations. This includes a highly regioselective Negishi reaction of dibromo-alkene 48 with an alkynylzinc reagent, and a palladium catalyzed alkyl-Suzuki coupling of the resulting enyne derivative 12 with the 9-BBN-adduct derived from alkene 61. The latter was obtained via an asymmetric hydrogenation of the chlorinated beta-ketoester 49 and an anti-selective oxyallylation of the functionalized aldehyde 53 as the key steps. The preparation of the bis-borylated allyl-donor 57 used in the oxyallylation step, however, required careful optimization and led to important insights into the nature of the classical hydroborating agent "di(isopinocampheyl)borane (Ipc2BH)". It was unambiguously shown by X-ray crystallography that in the solid state this compound is dimeric, but it is prone to undergo an essentially quantitative mono-deborylation when dissolved in CH2Cl2 or benzene; its composition in ethereal solvents is even more complex as evident from 11B NMR data. Product 71 derived from 12 and 61 was elaborated into the enyne-yne derivative 75, which served as the substrate for an exquisitely selective ring closing alkyne metathesis reaction (RCAM) catalyzed by the molybdenum tris-amido complex 20 activated in situ with CH2Cl2. The resulting cyclic enyne 76 was subjected to a ruthenium catalyzed trans-hydrosilylation/proto-desilylation tandem. Although [Cp*Ru(MeCN)3]PF6 had previously been recommended as catalyst of choice for trans-hydrosilylation reactions of internal alkynes, this complex failed to afford the desired product, whereas its sterically less hindered congener [CpRu(MeCN)3]PF6 permitted the reaction to be performed in appreciable yield, but at the expense of a lower stereoselectivity. AgF-mediated proto-desilylation of the isomeric silanes 79 and 80 followed by cleavage of the remaining acetal protecting groups afforded myxovirescin A1 and its hitherto unknown 14Z-isomer 81, respectively.

Asymmetric catalysis of the transannular Diels-Alder reaction

Transannular chemical reactions are unparalleled in their ability to generate high degrees of stereochemical and architectural complexity in a single transformation. However, the successful application of this approach in synthesis depends on the ability to predict and control the outcome of the transannular reaction. Use of a chiral catalyst in this context represents an attractive, yet unused, strategy. This report describes a catalytic, asymmetric transannnular Diels-Alder (TADA) reaction that affords polycyclic products in high enantiomeric excess. This catalyst system can also alter the inherent diastereoselectivity of cyclizations with substrates containing chiral centers. Additionally, the catalytic enantioselective TADA has been used as the key step in a total synthesis of the sesquiterpene 11,12-diacetoxydrimane; this route may provide a general approach to the polycyclic carbon framework shared by many terpene natural products.