C-glycosides to fused polycyclic ethers. A formal synthesis of (+/-)-hemibrevetoxin B

Development of an Approach to the Synthesis of the ABC Ring System of Hemibrevetoxin B

An efficient approach for the synthesis of a model of the ABC ring system of Hemibrevetoxin B is described. Key features include a ring expansion to yield the ring C oxepane, the reduction of a 2-furyl ketone with high levels of 1,3-stereocontrol, and an Achmatowicz oxidative ring expansion to yield the ring A tetrahydropyran. All seven stereogenic centers present in the model compound were controlled with high levels (>98:<2) of diastereoselectivity.

Sequential N-Acylamide Methylenation−Enamide Ring-Closing Metathesis: Construction of Benzo-Fused Nitrogen Heterocycles

The dimethyltitanocene methylenation of N-acylamides derived from ortho-vinylanilines, ortho-allylaniline, and ortho-vinylbenzylamine provides the corresponding enamides, which upon exposure to the second generation Grubbs ruthenium catalyst give access to indoles, 1,4-dihydroquinolines, and 1,2-dihydroisoquinolines, respectively. This sequential protocol also allows the synthesis of dihydrobenzoazepines, although the ring-closing metathesis (RCM) step is complicated by the alkene isomerization processes. From certain substrates, the direct annulation is observed in the titanium-mediated step, which is likely to occur through an olefin metathesis-intramolecular olefination sequence.

Antiproliferation and apoptosis induced by C-glycosides in human leukemia cancer cells

A large series of alkyl C-glycosides was synthesized from D-glucal or D-galactal. These compounds were screened against the human promyelocytic leukemia cell line (HL60), showing significant activity and apoptosis. Up to 13 C-glucopyranosides, but no C-galacto- or C-mannopyranosides, exhibited inhibitory concentrations (IC(50) values) below 20 microM, five of them in the range 4-8 microM. Preliminary structure-activity relationships were established.

Total Synthesis of Gambierol: The Generation of the A–C and F–H Subunits by Using a C-Glycoside Centered Strategy

Gambierol, a representative of the marine ladder toxin family, consists of eight ether rings, 18 stereocenters, and two challenging pyranyl rings having methyl groups that are in a 1,3-diaxial orientation to one another. Herein we describe the generation of gambierol's A-C and F-H ring systems and demonstrate the versatility of the glycosyl anhydride, enol ether-olefin RCM strategy to fused polycyclic ethers. This work has both enabled us to generate sufficient quantities of the gambierol precursors and has enabled us to better understand the chemical transformations that were key to these efforts. Fundamental work included efforts to C-glycosides and C-ketosides, Claisen rearrangements, and enol ether-olefin RCM reactions.

Construction of fused polycyclic ethers by strategies involving ring-closing metathesis

Large fused polycyclic ether natural products of marine origin are some of the most complex and formidable synthetic targets found in Nature, and they continue to fascinate and inspire those engaged in target-directed synthesis and the development of new synthetic methods. Novel strategies for the rapid and stereoselective assembly of fused polyethers have been devised in which ring-closing metathesis reactions are used to accomplish cyclic ether construction. Two-directional and iterative ring construction approaches involving ring-closing metathesis are being employed to assemble polyether sequences found in marine natural products such as the ciguatoxins and gambieric acids.

Regioselective debenzylation of C-glycosyl compounds by boron trichloride

Boron trichloride has been found to promote selective deprotection of 1,2- or 1,3-cis oriented secondary benzyl ethers of per-benzylated C-glycosyl derivatives. The reactivity towards BCl(3) follows the order: C-4>or=C-2>C-6>C-3 for C-glucopyranosyl derivatives and C-3>or=C-4>C-6>C-2 for C-galactopyranosyl derivatives. Preparatively useful selective debenzylation at secondary positions was possible after careful control of reaction conditions.

Sequential Cyclization−Elimination Route to Carbohydrate-Based Oxepines

A five-step preparation of carbohydrate-based oxepines from hept-1-enitols is presented. The hept-1-enitols were subjected to silyl protection and hydroboration/oxidation to give the protected heptan-1-itols. Swern oxidation of the homologated alcohols followed by sequential acetal formation/cyclization provided methyl 2-deoxyseptanosides that underwent elimination reactions to give the carbohydrate-based oxepines. The new sequence is an alternative to the ring-closing metathesis for the synthesis of carbohydrate-based oxepines from protected pyranose sugars. The product oxepines can serve as glycosyl donors in the synthesis of novel septanose carbohydrates. In addition, C-methylenealdehydo arabinofuranoside 16 was formed from 2-deoxyseptanoside 10 as the only product during protic acid mediated elimination reactions. This novel ring contraction complements other reported preparations of C-methylenaldehydo furanosides and underscores the acid-mediated reactivity introduced by competing eliminations between the C-1/C-2 and C-2/C-3 bonds.

Convergent Total Synthesis of Gymnocin-A and Evaluation of Synthetic Analogues

The first total synthesis of gymnocin-A (1), a cytotoxic polycyclic ether isolated from a notorious red tide dinoflagellate, Karenia mikimotoi, has been accomplished. The synthesis relies heavily on the Suzuki-Miyaura cross-coupling-based methodology to assemble the tetradecacyclic polyether skeleton. Convergent union of the GHI (5) and KLMN (6) rings, both of which were prepared from a common intermediate 7, and the subsequent ring closure of the J ring delivered the GHIJKLMN ring. The crucial coupling between the ABCD and FGHIJKLMN ring fragments (3 and 4, respectively) and stereoselective installation of the C17 hydroxyl group, followed by cyclization of the E ring gave rise to the tetradecacyclic polyether skeleton 2. Finally, incorporation of the 2-methyl-2-butenal side chain completed the total synthesis of gymnocin-A. The convergent nature of the synthesis, which employs three fragments of comparable complexity, is well-suited for preparation of various structural analogues of gymnocin-A to explore the structure-activity relationship. The results of preliminary structure-activity relationship studies of several synthetic analogues are also provided.

Substitution and Remote Protecting Group Influence on the Oxidation/Addition of α-Substituted 1,2-Anhydroglycosides: A Novel Entry into C-Ketosides

[reaction: see text] C-Ketosides are valuable intermediates in chemical synthesis and as glycoside mimics. This manuscript describes the efficient generation of these substrates from alpha-alkyl-substituted glycals and an oxidative, C-C bond-forming sequence where the choice of C(3) protecting group was critical.

Ruthenium-Catalyzed Olefin Metathesis Double-Bond Isomerization Sequence

A novel ruthenium-catalyzed tandem ring-closing metathesis (RCM) double-bond isomerization reaction is described in this paper. The utility of this method for the efficient syntheses of five-, six-, and seven-membered cyclic enol ethers is demonstrated. It relies on the conversion of a metathesis-active ruthenium carbene species to an isomerization-active ruthenium-hydride species in situ. This conversion is achieved by using various additives. Scope and limitations of the different protocols are discussed, and some mechanistic considerations based on (31)P and (1)H NMR spectroscopic studies are presented.

A convergent total synthesis of hemibrevetoxin B

A convergent biomimetic synthesis of hemibrevetoxin B from d-glucal and d-arabinose utilizes an electrophile-promoted cascade anti-Baldwin cyclization of an epoxy alcohol. The epoxy alcohol arises from a palladium-catalyzed coupling of a highly functionalized organozinc compound and an alkenyl iodide, which serve as two chiral building blocks of similar size and complexity. This first successful implementation of a cascade epoxy alcohol cyclization for the synthesis of marine polycyclic ether toxins proceeds in 39 steps and 4% overall yield.

Enantioselective total syntheses of slagenins A-C and their antipodes

Full details of the total syntheses of slagenins A-C (1a-c) and their antipodes (2a-c), novel bromopyrrole alkaloids with a unique tetrahydrofuro[2,3-d]imidazolidin-2-one moiety, are described in which their absolute stereochemistry was established. The key step in the syntheses involves the efficient condensation of dihydrofuran-3-one or glyoxal with urea to construct the slagenin bicycle core.

Callipeltoside a: total synthesis, assignment of the absolute and relative configuration, and evaluation of synthetic analogues

The total synthesis of the novel antitumor agent callipeltoside A, as well as several analogues, is accomplished and allows assignment of the stereochemistry not previously established. A convergent strategy is employed wherein the target is dissected into three units-the core macrolactone, the sugar callipeltose, and a cyclopropyl bearing chain. The strategy for the synthesis of the macrolactone derives from employment of diastereoselective aldol reactions that emanate from an 11 carbon piece. The stereochemistry of the latter derives from the chiral pool and two asymmetric reactions-a ketone reduction using CBS-oxazaborolidine and a Pd catalyzed asymmetric allylic alkylation (AAA). The novelty of the latter protocol is its control of regioselectivity as well as absolute configuration. The trisubstituted olefin is generated using an alkene-alkyne coupling to create a trisubustituted olefin with complete control of geometry. The excellent chemo- and regioselectivity highlights the synthetic potential of this new ruthenium catalyzed process. The macrolactonization employs in situ formation of an acylketene generated by the thermolysis of a m-dioxolenone. Two strategies evolved for attachment of the side chain-one based upon olefination and a second upon olefin metathesis. The higher efficiency of the latter makes it the method of choice. A novel one pot olefin metathesis-Takai olefination protocol that should be broadly applicable is developed. The sugar is attached by a glycosylation by employing the O-trichloroacetimidate. This route provided both C-13 epimers of the macrolactone by using either enantiomeric ligand in the Pd AAA reaction. It also provided both trans-chlorocyclopropane diastereomers of callipeltoside A which allows the C-20 and C-21 configuration to be established as S and R, respectively. The convergent nature of the synthesis in which the largest piece, the macrolatone, require only 16 linear steps imparts utility to this strategy for the establishment of the structure-activity relationship. Initial biological testing demonstrates the irrelevance of the chloro substituent and the necessity of the sugar.

Endo-oxacyclizations of polyepoxides: biomimetic synthesis of fused polycyclic ethers

Boron trifluoride-etherate promotes the endo-selective oxacyclization of polyepoxides derived from various acyclic terpenoid polyalkenes, including geraniol, farnesol, and geranylgeraniol, providing an efficient and stereoselective synthesis of substituted oxepanes and fused polyoxepanes. The mechanism of the oxacyclization reaction probably involves intramolecular nucleophilic addition of epoxide oxygen to open another epoxide that is activated as an electrophile by the Lewis acid. These oxacyclizations proceed stereospecifically with inversion of configuration upon opening of each epoxide to provide trans-fused polycyclic products. The oxacyclization cascade is terminated by a tethered nucleophile, which may be the carbonyl oxygen of a ketone, ester, or carbonate, or a trisubstituted alkene. The best oxacyclization yields are generally observed with tert-butyl carbonate as the terminating nucleophile, although in some cases the oxacyclization products include formation of tert-butyl ethers as a minor product. The oxacyclization transformations described herein may mimic ring-forming steps in the biosynthesis of trans-syn-trans-fused polycyclic ether marine natural products.

Enantioselective synthesis for the antipodes of slagenins B and C: establishment of absolute stereochemistry

[reaction: see text] The total synthesis for the (-)-antipode of slagenin B (12) and the (+)-antipode of slagenin C (13) was achieved with the condensation of glyoxal hydrate 10 and urea as the key step. The absolute stereochemistries of naturally isolated slagenins B and C were assigned to be (9R,11R,15R)-1b and (9R,11S,15S)-1c, respectively.