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PubMed2003

Stereoselective synthesis of tetrasubstituted 2,3-dihydrofurans by one-step cyclization of beta-ketosulfides of benzothiazole and aldehydes in ionic liquids.

A stereoselective synthesis of tetrasubstituted 2,3-dihydrofurans was carried out in n-butylpyridinium tetrafluoroborate ([bpy(+)][BF(4)(-)]) as solvent. The reaction proceeds smoothly in one step starting from simple materials such as aldehydes and beta-ketosulfides of benzothiazole. A comparison between several ionic liquids (ILs) is presented, and the role of the benzothiazolyl moiety is discussed. Workup proved to be very easy and recycling of IL possible.

Journal Article↗
PubMed2003

Synthesis of isocoumarins and alpha-pyrones via electrophilic cyclization.

A variety of substituted isocoumarins and alpha-pyrones are readily prepared in excellent yields under very mild reaction conditions by the reaction of o-(1-alkynyl)benzoates and (Z)-2-alken-4-ynoates with ICl, I(2), PhSeCl, p-O(2)NC(6)H(4)SCl, and HI. This methodology accommodates various alkynyl esters and has been successfully extended to the synthesis of polycyclic aromatic and biaryl compounds.

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PubMed2003

Pd(II)-catalyzed coupling cyclization of 2,3-allenoic acids with allylic halides. An efficient methodology for the synthesis of beta-allylic butenolides.

An effective method for the synthesis of beta-allyl polysubstituted butenolides from the easily available allylic halides and 2,3-allenoic acids is described. By using this method optically active butenolides can be obtained. According to the results presented in this paper, the reaction may proceed via three consecutive steps: cyclic oxypalladation of the allene, insertion of the C=C bond in allylic halides, and beta-dehalopalladation.

Journal Article↗
PubMed2003

Concise enantioselective synthesis of 3,5-dialkyl-substituted indolizidine alkaloids via sequential cross-metathesis-double-reductive cyclization.

An efficient stereoselective synthesis of two 3,5-dialkyl-substituted indolizidine alkaloids is reported. The convergent syntheses are based on a novel sequence of a cross-metathesis (CM) reaction of an alpha,beta-unsaturated ketone and a chiral homoallylic amine followed by a domino reaction involving hydrogenation, N-deprotection, and two diastereoselective reductive aminations. Our concept presents one of a few examples of a highly selective CM reaction in the synthesis of a natural product.

Alkaloids↗
PubMed2003

Strategic use of pinacol-terminated Prins cyclizations in target-oriented total synthesis.

An important objective of contemporary synthesis endeavors is the development of new transformations that rapidly evolve molecular complexity in a stereocontrolled fashion. One approach toward this goal is to combine two or more distinct reactions into a single transformation, producing a process often referred to as a sequential, tandem, cascade, or domino reaction. In this Perspective, we discuss the development of one such tandem reaction, the acid-promoted (or catalyzed) Prins-pinacol rearrangement, with particular emphasis on its implementation as the key strategic element in the total synthesis of heterocyclic and carbocyclic natural products.

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PubMed2004

Base-catalyzed endo-mode cyclization of allenes: easy preparation of five- to nine-membered oxacycles.

A reliable and efficient procedure for constructing five- to eight-membered oxacycles has been developed. Allenes with both a phosphoryl group and a suitable delta-hydroxyalkyl side chain at the C1-position underwent an endo mode ring-closing reaction to give five- to seven-membered oxacycles. Changing the phosphoryl group to a phosphono functionality facilitated the preparation of eight-membered congeners. Introduction of a cis double bond to the alkyl side chain of the starting allenes made possible the easy formation of medium-sized oxacycles, such as the dihydrooxocin and tetrahydrooxonin frameworks, regardless of the electron-withdrawing group (POPh2, PO(OEt)2, SOPh, and SO2Ph) at the C1-position.

Journal Article↗
PubMed2004

Palladium(0)-catalyzed cyclization of electron-deficient enynes and enediynes.

In the presence of a Pd(0) precatalyst, Pd2(bq)2(nbe)2 or Pd2(dba)3, 1,6-enyne esters were heated in refluxing benzene to give cyclodimers as single regioisomers. On the other hand, the combination of the Pd(0) precatalyst and triphenyl phosphite gave rise to various cycloisomerization products depending on the substitution pattern of the enyne esters. Six-membered ring cycloisomerization products were predominantly obtained from enyne esters bearing methallyl or 2-phenyl-2-propenyl moieties, while other enyne esters afforded normal five-membered ring cycloisomerization products. Intramolecular [2 + 2 + 2] cyclocotrimerization of enediyne esters also proceeded in the presence of the Pd(0) precatalyst and triphenylphosphine to give fused cyclohexadienes.

Journal Article↗
PubMed2005

Silatropic carbonyl ene cyclizations in the synthesis of pseudosugars and hydroxylated piperidines.

[reaction: see text] We describe two applications of silicon-tethered thermal allyl transfer reactions of alpha-silyloxyaldehydes; formally, these processes can be regarded as silatropic carbonyl ene reactions in which the silicon tether is transferred to the aldehyde oxygen concurrent with carbonyl allylation. In the first application, isoserinal substrates, which bear side-chain nitrogen functionality, are elaborated to dihydroxypiperidines. In the second application, a product of cyclohexadienyl transfer is taken on to carbocylic analogues of, for example, mannose. In both series, the silatropic ene reactions are effected thermally, with no added Lewis acid, and are both stereospecific and highly stereoselective.

Journal Article↗
PubMed2005

Preparation of carbocycles via base-catalyzed endo-mode cyclization of allenes.

A new and reliable procedure for constructing five- to seven-membered carbocycles via an endo-mode ring-closing reaction of 1-phenylsulfonylallenes with a substituent that has a terminal active methine moiety at the C1-position has been developed. Trisubstituted 1-phenylsulfonylallenes underwent a similar endo-mode ring-closing reaction to produce the corresponding five- to seven-membered carbocycles, while the formation of six- and seven-membered carbocycles from the corresponding tetrasubstituted allene was not realized. In addition, the introduction of an aromatic ring to the alkyl side chain of the starting allenes made possible the construction not only of normal-sized carbocycles but also an eight-membered framework.

Journal Article↗
PubMed2005

Stereoselective cyclization reactions of IBX-generated alkoxyamidyl radicals.

In this paper, a method for the generation of alkoxyamidyl radicals is presented. These N-centered radicals can efficiently be formed starting from the corresponding acylated alkoxyamines using IBX as an oxidant. Stereoselective 5-exo and 6-exo reactions with these N-heteroatom-centered radicals leading to isoxazolidines and [1,2]oxazinanes are discussed. The N-O bond in the heterocycles can readily be cleaved with SmI(2) to provide N-acylated 1,3-amino alcohols.

Journal Article↗
PubMed2006

Catalytic reduction and intramolecular cyclization of haloalkynes in the presence of nickel(I) salen electrogenerated at carbon cathodes in dimethylformamide.

[reaction: see text] Pentylidenecyclopentane can be conveniently prepared in up to 86% yield via the catalytic reduction of 1-iodo- or 1-bromo-5-decyne by [[2,2'-[1,2-ethanediylbis(nitrilomethylidyne)]bis[phenolato]]-N,N',O,O']nickelate(I) electrogenerated at a carbon cathode in dimethylformamide containing tetramethylammonium tetrafluoroborate. This electrosynthesis can be accomplished at potentials for which the haloalkynes are electroinactive, and it can be completed within 30 min at room temperature. Attempts to synthesize pentylidenecyclobutane and pentylidenecyclohexane from 1-halo-4-nonynes and 11-halo-5-undecynes, respectively, under similar conditions afford the carbocycles in very low yields (2% and 6%, respectively). Other products derived from the various haloalkynes are dimers, alkynes, and 1-alkenynes. Dimers (alkadiynes) arising from 1-halo-4-nonynes and 11-halo-5-undecynes are formed in yields ranging from 80% to 89%, whereas icosa-5,15-diyne (the dimer obtained from a 1-halo-5-decyne) is found in significantly lower yield (</=13%). Alkynes and 1-alkenynes are produced in yields of 3-10% and 2-3%, respectively. A mechanistic scheme, involving alkyn-1-yl radicals arising from nickel(I) salen catalyzed cleavage of the carbon-halogen bond of each haloalkyne, is proposed to account for the formation of all products.

Journal Article↗
PubMed2006

An unexpectedly mild thermal Alder-ene-type cyclization of enallenes.

A mild, thermal Alder-ene reaction of enallenes has been developed. The allenic double bond acts as the "ene" and generates a carbon-carbon bond to an unactivated olefinic "enophile" in DMF at 120 degrees C to give [n.3.0] bicyclic systems (n = 3-5) in good yields. Except for a minor [2 + 2] cycloaddition byproduct, the reaction proceeded with complete atom economy, as there is no requirement of a catalyst or additional reactants, and no waste products are formed in the process.

Journal Article↗
PubMed2006

N,N'-carbonyldiimidazole-mediated cyclization of amino alcohols to substituted azetidines and other N-heterocycles.

Amino alcohols are important synthons for N-heterocycles. We have developed an efficient method to activate hydroxyl groups, which avoids the use of toxic reagents and tolerates a wide variety of functional groups. Our strategy has been applied to the synthesis of functionalized p-methoxyphenyl-protected azetidines, pyrrolidines, and piperidines. The required amino alcohols were synthesized according to an optimized proline-catalyzed Mannich protocol. An azetidine analogue of ezetimibe was synthesized to demonstrate the potential for the synthesis of drug-like molecules.

Amino Alcohols↗
PubMed1996

Simple Designs for the Construction of Complex Trans-Fused Polyether Toxin Frameworks. A Convergent Strategy Based on Hydroxy Ketone Cyclization of C-Linked Oxacycles.

A single, unified convergent strategy for the stereocontrolled synthesis of trans-fused polyethers was developed. It was demonstrated that the epimerization and reductive intramolecular coupling of hydroxy ketones in reactions with silane-Lewis acids (SI-LA) to generate ethers in C-linked oxacycles is affected by its conformational preference in a predictable manner. The obtained results make evident that the influence of hydrogen bonding between a hemiketal hydroxyl and a 1,3-diaxial C-O bond is regular and predictable and that convergent synthesis of trans-fused polyethers may confidently be conducted on driving ring closure to oxane rings under thermodynamic conditions

Journal Article↗
PubMed1996

Synthesis of Six-Membered Compounds by Environmentally Friendly Cyclization Using Indirect Electrolysis.

[Ni(cyclam)](ClO(4))(2)-catalyzed indirect electroreduction of olefinic bromides produced six-membered compounds in low to high yields. The synthetic intermediate 49 of Ipecac and Corynanthe alkaloids was obtained in 88% yield in a highly stereoselective manner. Lactam 66, the synthetic precursor of tacamonine, was prepared in 49% yield as a mixture of two diastereoisomers. The electrolysis of the bromoacetates gave the debrominated compounds in good yields.

Journal Article↗