Total Synthesis of (+)-Amphidinolide A. Assembly of the Fragments

Stereoselective synthesis of the northern hemisphere of the proposed structure of neaumycin B

The stereoselective synthesis of the northern hemisphere (C20-C41) of the purported structure of the extremely potent anticancer natural product neaumycin B has been accomplished. Twelve out of nineteen asymmetry centers have been installed chemically. The key highlights of this synthesis include the Krische iridium catalyzed anti-diastereoselective carbonyl crotylation, the Crimmins aldol reaction, HWE olefination, CBS reduction, vanadium promoted stereoselective epoxidation, Evans methylation and spiroketalization.

Skipped dienes in natural product synthesis

Covering: 2000-2020The 1,4-diene motif, also known as a skipped diene, is widespread across various classes of natural products including alkaloids, fatty acids, terpenoids, and polyketides as part of either the finalized structure or a biosynthetic intermediate. The prevalence of this nonconjugated diene system in nature has resulted in numerous encounters in the total synthesis literature. However, skipped dienes have not been extensively reviewed, which could be attributed to overshadowing by the more recognized 1,3-diene system. In this review, we aim to highlight the relevance of skipped dienes in natural products through the lens of total synthesis. Subjects that will be covered include nomenclature, structural properties, prevalence in natural products, synthetic strategies and the future direction of the field.

Asymmetric Total Synthesis of Amphirionin-2

A convergent route for the asymmetric total synthesis of potent anticancer polyketide natural product amphirionin-2 has been developed. Our initial synthetic trials revealed that the proposed structures of amphirionin-2 need to be revised consistent with a recent report of Fuwa et al., where the actual structure of amphirionin-2 was established. The key features of our synthesis comprised Sharpless asymmetric dihydroxylation, followed by cycloetherification, Wittig olefination, Julia-Kocienski olefination, and Crimmins propionate aldol reaction.

Synthesis, Stability, and Biological Studies of Fluorinated Analogues of Thromboxane A2

Platelet activation results in the generation of thromboxane A₂ (TxA₂), which promotes thrombus formation by further amplifying platelet function, as well as causing vasoconstriction. Due to its role in thrombus formation and cardiovascular disease, its production is the target of antiplatelet drugs such as aspirin. However, the study of TxA₂-stimulated cellular function has been limited by its instability (t _1/2 = 32 s, pH = 7.4). Although more stable analogues such as U46619 and difluorinated 10,10-F₂-TxA₂ have been prepared, we targeted a closer mimic to TxA₂ itself, monofluorinated 10-F-TxA₂, since the number of fluorine atoms can affect function. Key steps in the synthesis of F-TxA₂ included α-fluorination of a lactone bearing a β-alkoxy group, and a novel synthesis of the strained acetal. F-TxA₂ was found to be 10⁵ more stable than TxA₂, and surprisingly was only slightly less stable than F₂-TxA₂. Preliminary biological studies showed that F-TxA₂ has similar potency as TxA₂ toward inducing platelet aggregation but was superior to F₂-TxA₂ in activating integrin α_IIbβ₃.

Iriomoteolides: novel chemical tools to study actin dynamics

Despite its promising biological profile, the cellular targets of iriomoteolide-3a, a novel 15-membered macrolide isolated from Amphidinium sp., have remained unknown. A small library of non-natural iriomoteolide-3a analogues is presented here as a result of a novel, highly convergent, catalysis-based scaffold-diversification campaign, which revealed the suitable sites for chemical editing in the original core. We provide compelling experimental evidence for actin as one of iriomoteolides' primary cellular targets, establishing the ability of these secondary metabolites to inhibit cell migration, induce severe morphological changes in cells and cause a reversible cytoplasmic retraction and reduction of F-actin fibers in a time and dose dependent manner. These results are interpreted in light of the ability of iriomoteolides to stabilize F-actin filaments. Molecular dynamics simulations provide evidence for iriomoteolide-3a binding to the barbed end of G-actin. These results showcase iriomoteolides as novel and easily tunable chemical probes for the in vitro study of actin dynamics in the context of cell motility processes including cell invasion and division.

Total Synthesis of (-)-Lasonolide A

The lasonolides are novel polyketides that have displayed remarkable biological activity in vitro against a variety of cancer cell lines. Herein we describe our first-generation approach to the formal synthesis of lasonolide A. The key findings from these studies ultimately allowed us to go on and complete a total synthesis of lasonolide A. The convergent approach unites two highly complex fragments utilizing a Ru-catalyzed alkene-alkyne coupling. This type of coupling typically generates branched products; however, through a detailed investigation, we are now able to demonstrate that subtle structural changes to the substrates can alter the selectivity to favor the formation of the linear product. The synthesis of the fragments features a number of atom-economical transformations which are highlighted by the discovery of an engineered enzyme to perform a dynamic kinetic reduction of a β-ketoester to establish the absolute stereochemistry of the southern tetrahydropyran ring with high levels of enantioselectivity.

Ruthenium-catalyzed alkene-alkyne coupling of disubstituted olefins: application to the stereoselective synthesis of trisubstituted enecarbamates

The Ru-catalyzed alkene-alkyne coupling reaction has been demonstrated to be an enabling methodology for the synthesis of complex molecules. However, to date, it has been limited to monosubstituted olefins. Herein we report the first general utilization of disubstituted olefins in the Ru-catalyzed alkene-alkyne coupling reaction by employing carbamate directing groups. The products are stereodefined trisusbstituted enecarbamates. The elaboration of these structures toward the asymmetric synthesis of complex aminocyclopentitols and 1,2-amino alcohols is discussed.

Formation of substituted tetrahydropyrans through oxetane ring opening: application to the synthesis of C1-C17 fragment of salinomycin

The stereoselective synthesis of C1-C17 fragment of salinomycin is achieved. The strategy employs a desymmetrization approach and utilizes an intramolecular oxetane opening reaction with O-nucleophile to result in the tetrahydropyran skeleton as the key step.

Exploiting hidden symmetry in natural products: total syntheses of amphidinolides C and F

The total synthesis of amphidinolide C and a second-generation synthesis of amphidinolide F have been accomplished through the use of a common intermediate to access both the C1-C8 and the C18-C25 sections. The development of a Ag-catalyzed cyclization of a propargyl benzoate diol is described to access both trans-tetrahydrofuran rings. The evolution of a Felkin-controlled, 2-lithio-1,3-dienyl addition strategy to incorporate C9-C11 diene as well as C8 stereocenter is detailed. Key controlling aspects in the sulfone alkylation/oxidative desulfurization to join the major subunits, including the exploration of the optimum masking group for the C18 carbonyl motif, are discussed. A Trost asymmetric alkynylation and a stereoselective cuprate addition to an alkynoate have been developed for the rapid construction of the C26-C34 subunit. A Tamura/Vedejs olefination to introduce the C26 side arm of amphidnolides C and F is employed. The late-stage incorporation of the C15, C18 diketone motif proved critical to the successful competition of the total syntheses.

Studies for the total synthesis of amphidinolide P

A convergent, enantiocontrolled total synthesis of the 15-membered macrolide, amphidinolide P, is described. The synthesis utilizes three nonracemic components for an efficient assembly of the macrolactone in 12 steps via the longest linear pathway. Key developments include studies of the Hosomi-Sakurai reaction for the formation of the C6-C7 bond, a "ligandless" palladium-mediated Stille cross-coupling of the vinylic stannane 4 and the alkenyl bromide 5 to produce a highly functionalized dienol, and a thermally induced, intramolecular lactonization via the late-stage formation of an intermediate α-acylketene.

Amphidinolide B: total synthesis, structural investigation, and biological evaluation

The total syntheses of amphidinolide B1 and the proposed structure of amphidinolide B2 have been accomplished. Key aspects of this work include the development of a practical, non-transition-metal-mediated method for the construction of the C13-C15 diene, the identification of α-chelation and dipole minimization models for diastereoselective methyl ketone aldol reactions, the discovery of a spontaneous Horner-Wadsworth-Emmons macrocyclization strategy, and the development of a novel late stage method for construction of an allylic epoxide moiety. The originally proposed structure for amphidinolide B2 and diastereomers thereof display potent antitumor activities with IC50 values ranging from 3.3 to 94.5 nM against human solid and blood tumor cells. Of the different stereoisomers, the proposed structure of amphidinolide B2 is over 12-fold more potent than the C8,9-epimer and C18-epimer in human DU145 prostate cancer cells. These data suggest that the epoxide stereochemistry is a significant factor for anticancer activity.

Pd and Mo Catalyzed Asymmetric Allylic Alkylation

The ability to control the alkylation of organic substrates becomes ever more powerful by using metal catalysts. Among the major benefits of metal catalysis is the possibility to perform such processes asymmetrically using only catalytic amounts of the chiral inducing agent which is a ligand to the metal of the catalyst. A unique aspect of asymmetric metal catalyzed processes is the fact that many mechanisms exist for stereoinduction. Furthermore, using the same catalyst system, many types of bonds including but not limited to C-C, C-N, C-O, C-S, C-P, and C-H can be formed asymmetrically. An overview of this process using palladium and molybdenum based metals being developed in my laboratories and how they influence strategy in synthesizing bioactive molecular targets is presented.

Ring strain-promoted allylic transposition of cyclic silyl ethers

Relief of the ring strain of medium-sized rings promotes a regioselective allylic transposition of a C-O bond when catalyzed by rhenium oxide. Through the allylic transposition, eight-membered cyclic silyl ethers undergo ring contraction to the corresponding six-membered siloxacycles.

Asymmetric total synthesis of danicalipin A and evaluation of biological activity

Asymmetric total synthesis of danicalipin A was achieved. The synthesis was characterized by diastereoselective introduction of chlorine atoms. Biological activities with synthetic danicalipin A, its enantiomer, and racemate were also evaluated toward brine shrimp. Both enantiomers of danicalipin A showed almost the same activity.

Diastereoselective synthesis of 2,3,4,5,6-pentafluoroheptanes

A stereocontrolled synthesis of alkanes containing five contiguous fluorine atoms is presented. The compounds were prepared by sequential fluorination of diastereoisomeric alcohol-diepoxides. The chemistry involved epoxide ring-opening with HF.NEt(3) and deshydroxyfluorination reactions of free alcohols with Deoxo-Fluor. The fluorination reactions were all highly stereospecific, with all five fluorines being incorporated in three sequential steps. Three different diastereoisomers of the 2,3,4,5,6-pentafluoroheptyl motif were prepared as heptane-1,7-diol derivatives, a structural format amenable for incorporation of the vicinal pentafluoro scaffold into larger molecular architectures.

Total synthesis and stereochemical assignment of (-)-ushikulide A

We report the determination of the full stereostructure of (-)-ushikulide A (1), a spiroketal containing macrolide by total synthesis. Ushikulide A (1) was isolated from a culture broth of Streptomyces sp. IUK-102 and exhibits potent immunosuppressant activity (IC(50) = 70 nM). To embark upon an ushikulide A synthesis, a tentative assignment was made based on analogy to cytovaricin (2), a related macrolide isolated from a culture of Streptomyces diastatochromogenes whose full structure was previously established via synthesis and X-ray crystallography. This report delineates studies on several key steps, namely a direct aldol reaction catalyzed by the dinuclear zinc ProPhenol complex, a metal catalyzed spiroketalization, as well as application of an unprecedented asymmetric alkynylation of a simple saturated aldehyde with methyl propiolate to prepare the nucleophilic partner for a Marshall-Tamaru propargylation. These studies culminated in the first total synthesis and stereochemical assignment of (-)-ushikulide A and significantly extended the scope of the above-mentioned methodologies.

Total synthesis and biological evaluation of amphidinolide V and analogues

The awesome power of metathesis is illustrated by a concise synthesis of the extremely scarce marine natural product amphidinolide V, which hinges on a sequence of ring-closing alkyne metathesis followed by intermolecular enyne metathesis with ethylene (see scheme). As a complete set of conceivable stereoisomers was prepared, the constitution and absolute configuration of this macrolide could be established and first insights into structure-activity relationships governing its cytotoxicity were obtained.A sequence of ring-closing alkyne metathesis followed by an intermolecular enyne metathesis of the resulting cycloalkyne with ethene was used to forge the macrocyclic skeleton and to set the vicinal exo-methylene branches characteristic for the cytotoxic marine natural product amphidinolide V (1). Comparison of the synthetic material with an authentic sample of this extremely scarce metabolite isolated from a dinoflagellate of the Amphidinium sp. eliminated any doubts about its structure and allowed the absolute configuration of amphidinolide V to be determined as 8R,9S,10S,13R. Moreover, the flexibility inherent to the underlying synthesis blueprint also opened access to a comprehensive set of diastereomers of 1 as well as to synthetic analogues differing from the natural lead in the lipophilic chains appended to the macrocyclic core. This set of designed analogues gave first insights into structure-activity relationships, which revealed that the stereostructure of the macrolactone is a highly critical parameter, whereas the examined alterations of the side chain did not diminish the cytotoxicity of the compounds to any notable extent.