Studies toward diazonamide A: initial synthetic forays directed toward the originally proposed structure

A brief introduction into the chemistry of diazonamide A (1) is followed by first-generation sequences to access the originally proposed structure for this unusual marine natural product. These explorations identified a route capable of delivering a model compound possessing the complete heteroaromatic core of the natural product, highlighting in the process several unanticipated synthetic challenges which led both to new methodology as well as an improved synthetic plan that was successfully applied to fully functionalized intermediates.     

Synthetic studies directed toward the proposed structure for heteroscyphic acid A

A route to the carbon skeleton of the proposed structure for heteroscyphic acid A was developed utilizing a Mn(III)/Cu(II) mediated oxidative free-radical cyclization and nucleophilic addition to (3-methylpentadienyl)iron(1+) cation.     

Studies toward soraphen A: an aldol-metathesis avenue to the macrocyclic framework

[reaction: see text] We describe a convergent approach to soraphen A, 1, that involves coupling of two fragments by an aldol condensation-olefin metathesis sequence. This route permits rapid access to congeners of 1.     

Total synthesis of the originally proposed and revised structures of scleritodermin A

The first total synthesis of the originally proposed and revised structure of scleritodermin A, along with an isomer, were achieved by the use of an alpha-azido carboxyl group serving as the key alpha-ketoamide precursor, thus leading to a revision of the structure originally proposed for natural scleritodermin A.     

Synthetic studies toward tricyclic cembranoids: a modular approach for the construction of the tricyclic framework of eunicin

The synthesis of a sugar derived allene and its intramolecular silver mediated etherification followed by ring closing metathesis has been explored for building the tricyclic framework of eunicin.     

Efficient construction of the core framework of lysidicin A via three Claisen rearrangements including a cascade reaction

A model compound (6) with the core skeleton of lysidicin A was synthesized as a racemate. The key step (8→7) includes three Claisen rearrangements of the triether with phloroglucinols; two of which rearranged in a cascade manner and the other was a simple rearrangement. This one-pot reaction enabled the introduction of three phloroglucinol units at the correct positions and makes the synthetic approach significantly efficient.     

Application of ruthenium-induced macrocyclization for the construction of macrocyclic depsipeptides

The synthesis of a biaryl ether containing macrocyclic depsipeptide 1 was achieved in 6% overall yield. The desired macrocycle was constructed by cyclization of a phenol into eta(6)-ruthenium complex. The ruthenium metal was subsequently photolytically deprotected to obtain the macrocycle 1.     

Studies directed toward the construction of the polypropionate fragment of superstolide A

[reaction: see text] Highly stereoselective approaches directed toward the synthesis of the C18-C27 fragment of superstolide A are disclosed taking ketone 6 and aldehyde 7 as the only sources of chirality.     

Total synthesis and structural elucidation of azaspiracid-1. Construction of key building blocks for originally proposed structure

Syntheses of the three key building blocks (65, 98, and 100) required for the total synthesis of the proposed structure of azaspiracid-1 (1a) are described. Key steps include a TMSOTf-induced ring-closing cascade to form the ABC rings of tetracycle 65, a neodymium-catalyzed internal aminal formation for the construction of intermediate 98, and a Nozaki-Hiyama-Kishi coupling to assemble the required carbon chain of fragment 100. The synthesized fragments, obtained stereoselectively in both their enantiomeric forms, were expected to allow for the construction of all four stereoisomers proposed as possible structures of azaspiracid-1 (1a-d), thus allowing the determination of both the relative and absolute stereochemistry of the natural product.     

Studies toward a synthesis of C3-epimauritine D: construction of the macrocycle

We successfully synthesized a 14-membered cyclic enamide of C3-epimauritine D from the cis-2,3-pyrrolidinediol derivative. Treatment of the pyrrolidinediol with nitrobenzonitrile in an S_NAr reaction efficiently installed the desired aryl-alkyl ether linkage on the N-heterocyclic system. Macrocyclization was successfully achieved by the use of TFFH as the peptide coupling reagent in the presence of HOAt.     

Chemistry and biology of diazonamide A: first total synthesis and confirmation of the true structure

With the addition of a tenth ring, the exchange of an oxygen atom for a nitrogen in the heart of the molecule, and a different terminal residue, the revised architecture for diazonamide A (1) provided an even more challenging molecular puzzle for chemical synthesis than its predecessor. In this article, we detail the first successful total synthesis of diazonamide A, an endeavor which not only verified its proper connectivities and established the stereochemistry of its previously unassignable C-37 chiral center, but which also was attended by the development of several new synthetic strategies and tactics.     

Studies towards the synthesis of neopeltolide: synthesis of a ring-closing metathesis macrocyclization precursor

An advanced ring-closing metathesis precursor for the synthesis of the marine macrolide neopeltolide is prepared in a stereocontrolled manner by the coupling of the C2-C10 and C11-C16 subunits. The metathesis reaction of 4 with Grubbs’ II or Nolan’s indenylidene catalyst led to the unexpected formation of cycloheptene 18.     

Studies toward the total synthesis of diterpene antibiotic guanacastepene A: construction of the hydroazulenic core

[reaction: see text] As a part of studies aimed toward the total synthesis of biologically important natural product guanacastepene A of contemporary interest, a new and concise route to a fully functionally endowed hydroazulenic core is delineated. The strategy involves the building of the requisite stereochemical features on a endo-tricyclo[5.2.1.0(2,6)]decane matrix and excision of the five-membered ring through a retro-Diels-Alder reaction. Generation of the seven-membered ring to access the hydroazulenic framework was achieved employing ring closure metathesis (RCM) reaction as the key step.     

Total synthesis of the originally proposed and revised structures of palmerolide A and isomers thereof

Palmerolide A is a recently disclosed marine natural product possessing striking biological properties, including potent and selective activity against the melanoma cancer cell line UACC-62. The total syntheses of five palmerolide A stereoisomers, including the originally proposed (1) and the revised [ent-(19-epi-20-epi-1)] structures, have been accomplished. The highly convergent and flexible strategy developed for these syntheses involved the construction of key building blocks 2, 19-epi-2, 20-epi-2, ent-2, 3, ent-3, 4, and ent-4, and their assembly and elaboration to the target compounds. For the union of the building blocks, the Stille coupling reaction, Yamaguchi esterification, Horner-Wadsworth-Emmons olefination, and ring-closing metathesis reaction were employed, the latter being crucial for the stereoselective formation of the macrocycle of the palmerolide structure. The Horner-Wadsworth-Emmons olefination and the Yamaguchi lactonization were also investigated and found successful as a means to construct the palmerolide macrocycle. The syntheses were completed by attachment of the enamide moiety through a copper-catalyzed coupling process.     

Studies directed toward the synthesis of terreulactone A: rapid construction of the A, B, C rings

[structure: see text]. An efficient, rapid synthesis of the A, B, C rings of terreulactone A is described. Key steps in the synthesis include a diastereoselective benzylic acid rearrangement to create the desired quaternary center at C2 and a mild bromolactonization to assemble the lactonic ring A.     

Synthesis of the proposed structure of plakevulin A: revised structure of plakevulin A

A total synthesis of the proposed structure of plakevulin A was accomplished. However, the NMR spectral data of the synthetic plakevulin A were not identical of those of the reported compound. We next converted the synthetic plakevulin A into 1-dihydrountenone A. The ¹H and ¹³C NMR spectral data of 1-dihydrountenone A were identical with those of reported plakevulin A except for the peaks derived from levulinic acid. Thus, we repurified sample of the natural product and confirmed that the natural sample contained 1-dihydrountenone A and levulinic acid in the ratio of one to one. We also found that not plakevulin A but 1-dihydountenone A possessed the inhibitory activity against mammalian DNA polymerases α and β.     

Studies on the synthesis of landomycin A. Synthesis of the originally assigned structure of the aglycone, landomycinone, and revision of structure

The originally proposed structure (2) of landomycinone, the aglycone of landomycin A, has been synthesized and shown to be nonidentical to the naturally derived landomycin A aglycone. The synthesis of 2 features the D?tz benzannulation reaction of chromium carbene 5 and alkyne 6, and the intramolecular Michael-type cyclization reaction of the phenolic naphthoquinone 20. It is proposed that natural landomycinone possesses the alternative structure 3, but attempts to access this structure via the Michael-type cyclization of the isomeric phenolic naphthoquinone 38 have been unsuccessful.     

A cascade process toward the synthesis of fused polycyclic dihydropyridines

The synthesis of fused polycyclic dihydropyridines was achieved using a cascade process comprising a Suzuki coupling, a nucleophilic cyclization and a hydrogen migration. Several functional groups are tolerated in this reaction and the methodology could be applied with success to quinoline and isoquinoline derivatives.