Showing papers on "Total synthesis published in 2007"

A simple and modular strategy for small molecule synthesis: iterative Suzuki-Miyaura coupling of B-protected haloboronic acid building blocks.

Abstract: We herein describe a simple and highly modular strategy for small molecule synthesis involving the iterative cross-coupling of B-protected bifunctional haloboronic acids. Enabling this approach, we have newly discovered that the pyramidalization of boronic acids via complexation with the trivalent ligand N-methyliminodiacetic acid inhibits their reactivity towards cross-coupling. This ligand is remarkably stable to anhydrous Suzuki−Miyaura conditions yet readily cleaved using mild aqueous base (1 M aqueous NaOH/THF, 10 min, 23 °C or saturated aqueous NaHCO3/MeOH, 23 °C, 6 h). Although the reactivity of aryl, heteroaryl, alkenyl, and alkyl boronic acids can vary dramatically, this methodology is effective for protecting and deprotecting all four classes of nucleophiles. Harnessing this potential, we achieved the first total synthesis of the natural product ratanhine using the Suzuki−Miyaura reaction iteratively to bring together a collection of easily synthesized, readily purified, and highly robust buildi...

Palladium-catalyzed enantioselective domino Heck-cyanation sequence: development and application to the total synthesis of esermethole and physostigmine

Abstract: An efficient synthesis of functionalized 3-alkyl-3-cyanomethyl-2-oxindole by a palladium-catalyzed domino Heck-cyanation reaction has been developed. Reaction of ortho-iodoanilide with potassium ferro(II)cyanide in the presence of palladium acetate and sodium carbonate afforded oxindole derivs., e.g., I, in good to excellent yields. An enantioselective domino Heck-cyanation process has been developed for the first time using (S)-DIFLUORPHOS as a chiral supporting ligand, and an enantioselectivity of up to 79% ee in the enantiomerically enriched oxindole was obtained under optimized conditions. A concise total synthesis of (+-)-esermethole (II, R = Me) and (+-)-physostigmine (II, R = CONHMe), powerful inhibitors of acetyl- and butyryl-cholinesterase, is also described. [on SciFinder (R)]

Oxidative Amidation of Phenols through the Use of Hypervalent Iodine Reagents: Development and Applications

Abstract: This contribution reviews a family of reactions devised in our laboratory that effect the oxidative conversion of phenols into 4-amido-dienones. A salient feature of this chemistry is the use of hypervalent iodine reagents, especially diacetoxyiodobenzene (DIB), as uniquely capable oxidants in the context of the new transformation. The advent of this methodology has created new opportunities in alkaloid synthesis. Our efforts toward FR-901483, TAN-1251C, cylindricine C, and other nitrogenous natural products illustrate some applications in that domain. 1 Introduction 2 Background 3 First-Generation Oxidative Amidation of Phenols: The Oxazoline Route 4 Initial Applications: Total Synthesis of FR-901483 and of TAN-1251C 5 Second-Generation Oxidative Amidation of Phenols: Sulfonamide Technology 6 Applications: Total Synthesis of (-)-Cylindricine C 7 Third-Generation Solution: The Bimolecular Reaction 8 Conclusion

Total synthesis of (+/-)-communesin F.

Abstract: Total synthesis of the complex polycyclic indole alkaloid (±)-communesin F has been accomplished in 23 reaction steps in about 3% overall yield. The key steps relied on a highly efficient methodology for assembling the pentacyclic substructure 2 with the C7 quaternary carbon, the stereoselective preparation of the second C8 quaternary carbon by a two-step one-pot O-allylation and consecutive 3,3-rearrangement (2 to 3), and the stereoselective acid-catalyzed cyclization of 4 to form the azepine ring (5). These highly stereoselective reactions guaranteed the stereochemical results, allowing the construction of the C, E, F, and G ring systems.

Total Synthesis of (+)-Nakadomarin A

Abstract: The total synthesis of (+)-nakadomarin A is described. A three-component cycloaddition of a hydroxylamine, aldehyde, and cyclopropane to form a highly functionalized tetrahydro-1,2-oxazine serves as the foundation for this synthesis. The resulting oxazine is formed as a single diastereomer with the absolute configuration being dictated by the chirality of the cyclopropane. Other key steps include: desymmetrization of a malonate by reduction, Heck cyclization and pyrrolidine formation, and ring-closing metathesis to form both cycloalkenes. Overall, the synthesis required 23 linear steps from the cyclopropane, which in turn is available (six steps) in optically pure form from commercially available d-mannitol.

Scope and mechanism of direct indole and pyrrole couplings adjacent to carbonyl compounds: total synthesis of acremoauxin A and oxazinin 3.

Abstract: Full details are provided for a recently invented method to couple indoles and pyrroles to carbonyl compounds. The reaction is ideally suited for structurally complex substrates and exhibits high levels of chemoselectivity (functional group tolerability), regioselectivity (coupling occurs exclusively at C-3 of indole or C-2 of pyrrole), stereoselectivity (substrate control), and practicality (amenable to scaleup). In addition, quaternary stereocenters are easily and predictably generated. The reaction has been applied to a number of synthetic problems including total syntheses of members of the hapalindole family of natural products, ketorolac, acremoauxin A, and oxazinin 3. Mechanistically, this coupling protocol appears to operate by a single electron-transfer process requiring generation of an electron-deficient radical adjacent to a carbonyl which is then intercepted by an indole or pyrrole anion.

Total Synthesis of Dimeric Pyrrole−Imidazole Alkaloids: Sceptrin, Ageliferin, Nagelamide E, Oxysceptrin, Nakamuric Acid, and the Axinellamine Carbon Skeleton

Abstract: The dimeric pyrrole imidazole natural products are a growing class of alkaloids with exotic connectivity, unique topologies, high nitrogen content, and exciting bioactivities. This full account traces the evolution of a strategy that culminated in the first total syntheses of several members of this family, including sceptrin, ageliferin, nagelamide E, nakamuric acid (and its methyl ester), and oxysceptrin. Details on the fascinating conversion of sceptrin to ageliferin, which has been used to produce gram quantities of this sensitive natural product, are provided. In addition, the first enantioselective total synthesis of sceptrin and ageliferin are reported by programming the fragmentation of an oxaquadricyclane. A hallmark of our approach to this family of alkaloids is the minimal use of protecting groups despite the presence of 10 nitrogen atoms in the target compounds. Thus, the fundamental chemistry of the 2-aminoimidazole heterocycle was explored without masking its innate reactivity. Insights gained during these explorations led to total syntheses of oxysceptrin and nakamuric acid and a successful construction of the carbon skeleton of axinellamine.

Chiral N-heterocyclic carbenes in natural product synthesis: application of Ru-catalyzed asymmetric ring-opening/cross-metathesis and Cu-catalyzed allylic alkylation to total synthesis of baconipyrone C.

Abstract: allylic alkylation asymmetric ringclosing metathesis copper ruthenium N-heterocyclic carbenes desymmetrization D . G . G I L L I N G H A M , A . H . H O V E Y D A * ( B O S T O N C O L L E G E , CH E S T N U T H I L L , U S A ) Chiral N-Heterocyclic Carbenes in Natural Product Synthesis: Application of Ru-Catalyzed Asymmetric RingOpening/Cross-Metathesis and Cu-Catalyzed Allylic Alkylation to Total Synthesis of Baconipyrone C Angew. Chem. Int. Ed. 2007, 46, 3860-3864.

Concise Total Synthesis of (±)-Marcfortine B

Abstract: The marcfortine alkaloids are complex indolic secondary metabolites isolated from various Penicillium sp. and are potent anthelmintic agents. Herein, we report the first total synthesis of marcfortine B. The key features of our synthesis are the use of the carboxylative TMM-cycloaddition to construct the central spirocyclic cyclopentane core and an intramolecular Michael-addition and radical cyclization to access its strained bicyclo[2.2.2]diazaoctane ring system.

Concise, asymmetric, stereocontrolled total synthesis of stephacidins A, B and notoamide B.

Abstract: Concise asymmetric total syntheses of the fungal metabolites (-)-stephacidin A, (+)-stephacidin B, and (+)-notoamide B are described. Key features of these total syntheses include (1) a facile synthesis of (R)-allyl proline methyl ester, (2) a revised route toward the pyranoindole ring system, (3) a novel cross-metathesis strategy for the introduction of important functional groups, and (4) an SN2' cyclization to form the [2.2.2] bridged bicyclic ring system. Furthermore, our synthesis has taken advantage of microwave heating to shorten reaction times as well as increase yields for the preparation of vital intermediates.

Total Synthesis of Iejimalide A-D and Assessment of the Remarkable Actin-Depolymerizing Capacity of These Polyene Macrolides

Abstract: A concise and convergent total synthesis of the highly cytotoxic marine natural products iejimalide A−D (1−4) is reported, which relies on an effective ring-closing metathesis (RCM) reaction of a cyclization precursor containing no less than 10 double bonds. Because of the exceptional sensitivity of this polyunsaturated intermediate and its immediate precursors toward acid, base, and even gentle warming, the assembly process hinged upon the judicious choice of protecting groups and the careful optimization of all individual transformations. As a consequence, particularly mild protocols for Stille as well as Suzuki reactions of elaborate coupling partners have been developed that hold considerable promise for applications in other complex settings. Moreover, a series of non-natural “iejimalide-like” compounds has been prepared, differing from the natural lead in the polar head groups linked to the macrolide's N-terminus. With the aid of these compounds it was possible to uncover the hitherto unknown effect...

Enzymatic total synthesis of enterocin polyketides

Abstract: Polyketides are clinically important natural products that often require elaborate organic syntheses owing to their complex chemical structures. Here we report the multienzyme total synthesis of the Streptomyces maritimus enterocin and wailupemycin bacteriostatic agents in a single reaction vessel from simple benzoate and malonate substrates. To our knowledge, our results represent the first in vitro assembly of a complete type II polyketide synthase enzymatic pathway to natural products.

Palladium-catalyzed enantioselective cyclization of silyloxy-1,6-enynes.

Abstract: The enantioselective intramolecular addition of silyl enol ethers and silyl ketene aminals onto alkyne is described. The reaction employs DTBMSegphos−Pd(II) or Binaphane−Pd(II) complexes as catalysts for the formation of methylene cyclopentane adducts from 1,6-enyne precursors. The utility of this reaction is illustrated by its application to the total synthesis of the cytotoxic cyclolaurane-type sesquiterpene, (−)-laurebiphenyl.

Golden opportunities in natural product synthesis: first total synthesis of (-)-isocyclocapitelline and (-)-isochrysotricine by gold-catalyzed allene cycloisomerization.

Abstract: The first enantioselective total syntheses of the β-carboline alkaloids (–)-isochrysotricine (1) and (–)-isocyclocapitelline (2) are reported which confirm the absolute configuration of these natural products. Key steps are the copper-mediated SN2′-substitution of propargyl oxiranes13/14 and the gold-catalyzed cycloisomerization of α-hydroxyallene 15, resulting in a highly efficient center-to-axis-to-center chirality transfer.

Total Synthesis of (+)-Frondosin A. Application of the Ru-Catalyzed [5+2] Cycloaddition

Abstract: The first total synthesis of (+)-frondosin A was accomplished in 19 longest linear and 21 total steps from commercially available materials. The key features of the synthesis include a Ru-catalyzed [5+2] cycloaddition, a Claisen rearrangement, and a ring expansion to construct the core of the frondosin A in a diastereoselective and regioselective fashion. This is the first application of a Ru-catalyzed [5+2] cycloaddition in the total synthesis of a natural product. Through this synthesis, the absolute configuration of (+)-frondosin A was established.

Total synthesis of ( -)-archazolid B

Abstract: A highly convergent synthesis of archazolid B, a potent and highly selective V-ATPase inhibitor, is described A relay ring-closing metathesis reaction was used to form the 24-membered macrocyclic lactone, whereas the sensitive cis-triene moiety of the archazolids was assembled with a modified Stille coupling

Total Syntheses of the Actin‐Binding Macrolides Latrunculin A, B, C, M, S and 16‐epi‐Latrunculin B

Abstract: The latrunculins are highly selective actin-binding marine natural products and as such play an important role as probe molecules for chemical biology. A short, concise and largely catalysis-based approach to this family of bioactive macrolides is presented. Specifically, the macrocyclic skeletons of the targets were forged by ring-closing alkyne metathesis (RCAM) or enyne–yne metathesis of suitable diyne or enyne–yne precursors, respectively. This transformation was best achieved with the aid of [(tBu)(Me2C6H3)N]3Mo (37) as precatalyst activated in situ with CH2Cl2, as previously described. This catalyst system is strictly chemoselective for the triple bond and does not affect the olefinic sites of the substrates. Moreover, the molybdenum-based catalyst turned out to be broader in scope than the Schrock alkylidyne complex [(tBuO)3WCCMe3] (38), which afforded cycloalkyne 35 in good yield but failed in closely related cases. The required metathesis precursors were assembled in a highly convergent fashion from three building blocks derived from acetoacetate, cysteine, and (+)-citronellene. The key fragment coupling can either be performed via a titanium aldol reaction or, preferentially, by a sequence involving a Horner–Wadsworth–Emmons olefination followed by a protonation/cyclization/diastereoselective hydration cascade. Iron-catalyzed CC-bond formations were used to prepare the basic building blocks in an efficient manner. This synthesis blueprint gave access to latrunculin B (2), its naturally occurring 16-epimer 3, as well as the even more potent actin binder latrunculin A (1) in excellent overall yields. Because of the sensitivity of the 1,3-diene motif of the latter, however, the judicious choice of protecting groups and the proper phasing of their cleavage was decisive for the success of the total synthesis. Since latrunculin A and B had previously been converted into latrunculin S, C and M, respectively, formal total syntheses of these congeners have also been achieved. Finally, a previously unknown acid-catalyzed degradation pathway of these bioactive natural products is described. The cysteine-derived ketone 18, the tetrahydropyranyl segment 31 serving as the common synthesis platform for the preparation of all naturally occurring latrunculins, as well as the somewhat strained cycloalkyne 35 formed by the RCAM reaction en route to 2 were characterized by X-ray crystallography.

De novo approach to 2-deoxy-beta-glycosides: asymmetric syntheses of digoxose and digitoxin.

Abstract: A highly enantioselective and straightforward route to trisaccharide natural products digoxose and digitoxin has been developed. Key to this approach is the iterative application of the palladium-catalyzed glycosylation reaction, reductive 1,3-transposition, diastereoselective dihydroxylation, and regioselective protection. The first total synthesis of natural product digoxose was accomplished in 19 total steps from achiral 2-acylfuran, and digitoxin was fashioned in 15 steps starting from digitoxigenin 2 and pyranone 8β. This flexible synthetic strategy also allows for the preparation of mono- and disaccharide analogues of digoxose and digitoxin.

Total Synthesis of (-)-Pseudolaric Acid B

Abstract: We report a full account of our work toward the total synthesis of pseudolaric acid B (1a), a diterpene acid isolated from the bark of Pseudolarix kaempferi Gordon (pinaceae). Compound 1a is an antifungal and antifertility agent. Furthermore, its capacity for inhibiting tubulin polymerization makes it a potential lead for cancer therapy. Herein, we describe the use of a Ru- or Rh-catalyzed [5 + 2] intramolecular cycloaddition reaction of an alkyne and a vinylcyclopropane for the construction of the polyhydroazulene core of the molecule. Our first unsuccessful strategy for the introduction of the quaternary center based on an epoxide opening with cyanide led to the discovery of a new TBAF-mediated isomerization of a 1,4-diene to a 1,3-diene and a vinylogous eliminative opening of an epoxide to form a dienol. Our second strategy, based on the cyclization of an alkoxycarbonyl radical upon a diene system, succeeded in forming the quaternary center. Detailed studies showed the dependence of this underutilized approach for the synthesis of lactones on substrate structure and reaction conditions. In the late stage of the synthesis, the unique capacity of cerium organometallic reagents to add to a sensitive, sterically hindered ketone was demonstrated. The easy formation of an oxo-bridged derivative was the major hurdle to the completion of the synthesis and showcased the intriguing reactivity of the complex core of the pseudolaric acids.