Facile synthesis of α,β-acetylenic ketones and 2,5-disubstituted furans: consecutive activation of triple and double bond with ZnBr2 toward the synthesis of furan ring

α,β-Acetylenic ketones were synthesized from the reaction of acid chlorides and acetylenic compounds in the presence of ZnBr₂ and DIEA in acetonitrile. From the acetylenic ketones having nearby methylene unit, 2,5-disubstituted furan derivatives could be synthesized under the same reaction conditions.     

Synthesis of polyhydroxylated bicyclic tetrahydrofurans and tetrahydropyrans via a stereoselective domino cyclization/reduction reaction

A novel reaction cascade involving a Lewis acid-induced migration of an isopropylidene protecting group followed by the formation of a pyranose or furanose ring and subsequent reduction of the hemiacetal is described. Depending on the reaction conditions, as well as, the stereochemistry of the substrate, polyhydroxylated tetrahydrofurans or tetrahydropyrans can be obtained in reasonable yields. The synthons used in this transformation were prepared via a highly stereoselective one-pot tandem reaction, consisting of a 1,4-Michael addition of vinylmagnesium bromide to d-glucose-derived cyclohexenone followed by aldol reaction with 2,3-O-isopropylidene-d-glyceraldehyde.     

Synthesis of sterically congested bicyclic tetrahydrofurans via Pd-catalyzed cyclization

Prins-type cyclization followed by palladium-catalyzed cyclization provided sterically congested bi- or tricyclic tetrahydrofurans, which are very stereoselective as well. cis-2,5-Disubstituted tetrahydrofurans obtained from Prins-type cyclization have an allenyl group and a functional group such as alcohol, carboxylic acid, and aryl halide. The tetrahydrofurans bearing an allene group underwent palladium-catalyzed cyclization to give sterically congested bi- or tricyclic tetrahydrofurans.     

A new approach to the synthesis of 1-oxaspiro[4.n]alkanes and tetrahydrofurans by the 1,5-CH insertion reaction of magnesium carbenoids

1-Alkoxy-1-[2-chloro-2-(p-tolylsulfinyl)ethyl]cycloalkanes were prepared from various cyclic ketones in good overall yields. Treatment of these cycloalkanes bearing a sulfinyl group with i-PrMgCl resulted in the formation of 1-oxaspiro[4.n]alkanes in high to quantitative yields via the 1,5-CH insertion reaction of generated magnesium carbenoid intermediates. When this procedure was commenced with acyclic ketones, multi-substituted tetrahydrofurans were obtained in up to a 96% yield. This procedure provides a new and good way for the synthesis of 1-oxaspiro[4.n]alkanes and tetrahydrofurans with the formation of a carbon–carbon bond between a carbenoid carbon and a non-activated carbon in high yields. The oxygen atom in the magnesium carbenoid intermediates was proved to act very important roles in the 1,5-CH insertion reaction.     

Nucleophilic 5-endo-trig cyclization of 2-(trifluoromethyl)allylic metal enolates and enamides: Synthesis of tetrahydrofurans and pyrrolidines bearing exo-difluoromethylene units

Ketones and imines bearing a 2-(trifluoromethyl)allylic moiety successfully underwent nucleophilic 5-endo-trig cyclization via their metal enolates and enamides. O- or N-Cyclization proceeded exclusively in each case to afford the corresponding five-membered heterocycles with both exo-difluoromethylene and exo-alkylidene units. On treatment with potassium hexamethyldisilazide (KHMDS) or lithium diisopropylamide (LDA), 2-(trifluoromethyl)allylic ketones or imines provided the corresponding tetrahydrofurans or pyrrolidines bearing a Z-alkylidene group with perfect or substantial stereoselectivity, respectively.     

Highly functionalized trans-2,5-disubstituted tetrahydrofurans from ribofuranoside templates: precursors to linked polycyclic ethers

Iodoetherification of C5 allylated ribo-furanosides leads to trans-2,5-disubstituted tetrahydrofurans with high selectivity. The application of this reaction to the synthesis of complex polycyclic ethers is illustrated in the preparation of a truncated, tricyclic analog of monensin A.     

I2-promoted tandem cyclization to synthesize polysubstituted pyrano[3,2-c]chromene-2,5-diones

An efficient and convenient method for the synthesis of various substituted pyrano[3,2-c]chromene-2,5-diones was developed via the I₂-promoted tandem cyclization of commercially available aryl methyl ketones and 4-hydroxycoumarins. Preliminary mechanism studies indicated that the reaction involved a consecutive iodination/Kornblum oxidation/annulation process. HI produced in the I₂-DMSO system acted as an important promoter, accelerating the annulation protocol.     

Stereoelectronic control in the solvolysis of spiroepoxidic 1-norbornyl triflates: unexpected reactivity in 3-bromomethyl derivatives

Interesting norbornane-fused tetrahydrofurans, with an additional synthetically-valuable vicinal dioxy-substitution in the norbornane skeleton, are enantiospecifically obtained in high yield from epimeric camphor-derived 3-endo-bromomethyl-substituted spiroepoxidic 1-norbornyl triflates. The process takes place via a domino reaction stereoelectronically controlled by the bromine atom. The described process has synthetic value, since it opens the way for a future enantiospecific preparation of 2,3-disubstituted tetrahydrofurans from camphor.     

Multi‐Gram Scale Synthesis of Chiral 3‐Methyl‐2,5‐trans‐tetrahydrofurans

In this article, we report the rapid and facile synthesis of chiral 3‐methyl‐2,5‐trans‐tetrahydrofurans. This reaction utilizes cheap and easily available starting materials. A domino hydrolysis and intramolecular Michael‐type ring closure reaction was the key step. As a result, synthesis of the desired 3‐methyl‐2,5‐trans‐tetrahydrofurans could be achieved in gram‐scale over seven linear steps with high chemical yield and high diastereoselectivity.     

Stereoselective iodine-induced cyclisation of alkene acetals. Application to the synthesis of 3-deoxy-exo-glycals and substituted tetrahydrofurans

2,5-Substituted tetrahydrofurans have been stereoselectively prepared by a iodine-induced cyclisation of alkene acetals, and the iodo derivatives obtained were transformed into 3-deoxy-exo-glycals and in polyhydroxy substituted tetrahydrofurans.     

Alkylation of pyrimido[5′,4′:5,6]pyrido[3,2-b]indol-4-one derivatives

Alkylation of 11-benzyl-3,11-dihydro-4H-pyrimido[5′,4′:5,6]pyrido[3,2-b]indol-4-one with methyl iodide and methyl bromoacetate in DMF gave 3-alkylpyrimidopyridoindolones as the corresponding salts. The reaction in acetone in the presence of K₂CO₃ yielded 3,6-disubstitution products. Alkylation with DMF dimethyl acetal gave a mixture of the 3- and 6-alkylpyrimidopyridoindol-4-one bases. The structure of 4-oxo-4,6-dihydro-3H-pyrimido-[5′,4′:5,6]pyrido[3,2-b]indol-11-ium chloride (3b) was proved by X-ray diffraction analysis.     

Enantioselective one-pot synthesis of β-sulfonyl ketones and trisubstituted tetrahydrothiophenes via β-elimination/cycloaddition sequence

A mild and efficient enantioselective one-pot synthesis of β-sulfonyl ketones and trisubstituted tetrahydrothiophenes via introducing 1,4-dithiane-2,5-diol to the simple kinetic resolution of β-sulfonyl ketones has been described herein. The one-pot reaction sequence including kinetic resolution and cascade sulfa-Michael/Aldol reaction proceeded successively to afford corresponding sulfonyl ketones and tetrahydrothiophenes with high enantioselectivities (85%–98% ee and 84%–95% ee, respectively).     

Synthesis of imidazolidin-4-one and 1H-imidazo[2,1-a]isoindole-2,5(3H,9bH)-dione derivatives of primaquine: scope and limitations

The synthesis of imidazolidin-4-one derivatives of primaquine as potential antimalarial agents is described. The target compounds were synthesized in three steps: (i) condensation of (±)-primaquine with N^α-protected amino acids, (ii) removal of the N^α-protecting group, and (iii) reaction of the N-acylprimaquine with a carbonyl compound: acetone, three cyclic ketones and veratraldehyde. Using 2-formylbenzoic acid in the third step afforded 1H-imidazo[2,1-a]isoindole-2,5(3H,9bH)-diones. All products were isolated in good to excellent yields. Whereas imidazolidin-4-ones were formed as mixtures of all possible diastereomers in equal amounts, 1H-imidazo[2,1-a]isoindole-2,5(3H,9bH)-diones were produced in a stereoselective fashion. The compounds hydrolyse very slowly (t_1/2 5-30 d) in pH 7.4 buffer to release primaquine. These primaquine derivatives are being submitted to biological assays, and preliminary results of their antimalarial activity are quite encouraging.     

Preparation of sterically hindered geometric isomers of 7-cis-β-ionyl and β-ionylidene derivatives in the vitamin A series

The preparation of the relatively unknown 7-cis isomers of β-ionyl and β-ionylidene derivatives via one way sensitized geometric isomerization is described. These highly sterically crowded isomers, once prepared do not readily thermally revert back to the trans. But the trienes, at temperatures ≥ 100° undergo irreversible cyclization to cyclohexadienes. 7-cis Isomers of β-ionylideneacetone and acetaldehyde as well as higher tetraenes and pentaenes in this series could not be prepared by sensitized isomerization. Partial reduction of 7-cis-β-ionylideneacetonitrile isomers led to the preparation of 7-cis- and 7,9-dicis-β-ionylideneacetaldehyde; and methyl Grignard reaction with the same nitrile gave the triene-methyl ketones.     

New syntheses of condensed heterocycles from isoxazole derivatives. V. Pyrrolo[3,4-b ]pyridin-4-ones

Hydrogenolysis with Raney-Nickel or iron powder in acetic acid of 2,5-diphenyl-4-nitro-3-(3,5-R,R-4-isoxazolyl)pyrrolyl ketones, prepared by the Grignard reaction of 2,5-diphenylpyrrole and 3,5-R,R-4-isoxazolecarboxilic acid chlorides followed by nitration, afforded directly the desired 6H-pyrrolo[3,4-b ]pyridin-4-ones.     

Reaction of 2-Aryl-5-R-5,6-dihydro-7H-[1,2,4]-triazolo[5,1-<Emphasis Type="Italic">b</Emphasis>][1,3]thiazin-7-ones with Aryl Bromomethyl Ketones and Benzyl Bromide

The products of the reaction of 2-aryl-5-R-5,6-dihydro-7H-[1,2,4]triazolo[5,1-b][1,3]thiazin-7-ones with aryl bromomethyl ketones are 2-aryl-7H-[1,2,4]triazolo[5,1-b][1,3]thiazin-7-ones and aryl methyl ketones or 2,5-diaryl[1,3]thiazolo[3,2-b][1,2,4]triazoles and 3-phenyl-2-propenoic acid, depending on the structure of R. The reaction of 2-aryl-5-R-5,6-dihydro-7H-[1,2,4]triazolo[5,1-b][1,3]thiazin-7-ones with benzyl bromide yields 5-aryl-3-benzylthio-4H-1,2,4-triazoles and 3-aryl-2-propenoyl bromide. __________ Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 6, pp. 905–909, June, 2005.     

Stereocontrolled construction of tetrasubstituted tetrahydrofurans: synthesis of 2,5-anhydro d-glucitol

A highly stereoselective construction of 2,3,4,5-tetrasubstituted tetrahydrofurans has been accomplished by an unusual intramolecular 5-endo-tet cyclization of 2,3-epoxy alcohols involving hydroxyl nucleophile. The method has been utilized for the synthesis of 2,5-anhydro d-glucitol through two different approaches starting from the chiral molecule, l(+)-diethyl tartarate or from the non-chiral compound, allyl bromide or cis-but-2-ene-1,4-diol. This synthetic method is a useful example of 5-endo-tet cyclization of 2,3-epoxy alcohols.     

Synthesis of enantiomerically pure perhydrofuro[3,4-b]pyrans and perhydrofuro[3,4-b]furans

Olefinic diols, prepared from (R)-(+)-2,2-dimethyl-1,3-dioxolane-4-carboxaldehyde via olefination and hydrolysis, were converted into enantiomerically pure hydroxy substituted tetrahydrofuran derivatives by cyclization using N-phenylselenophthalimide and BF₃. The PhSe group in the C-4 position of these tetrahydrofurans was then substituted by an allyl group using allyltributylstannane in the presence of AIBN. The selenium promoted cyclizations of the allyl tetrahydrofurans in which the OH and the allyl groups are trans to each other formed the enantiopure perhydrofuro[3,4-b]pyrans, while the cyclization of the allyl tetrahydrofurans in which the OH and the allyl groups are cis gave rise to the perhydrofuro[3,4-b]furans. These bicyclic products were finally deselenenylated with triphenyltin hydride and AIBN.     

A three-component coupling strategy for tetrahydrofuran synthesis: application of the diisopropyl tartrate modified (E)-gamma-(dimethylphenylsilyl)allylboronate as an alpha,gamma-allyl dianion equivalent

[reaction: see text] A highly convergent three-component coupling strategy for the stereocontrolled synthesis of 2,3,5-trisubstituted tetrahydrofurans is described. After allylboration of the first aldehyde with 1, the chiral, nonracemic allylsilanes 2 are coupled with a second aldehyde or ketone with Lewis acid catalysis to give tetrahydrofurans 3 or 4 with excellent selectivity. The 2,5-stereochemistry is controlled by operating under nonchelate (e.g., 3) or chelate (e.g., 4) conditions.     

Reaction of dianions derived from β-ketoesters with epoxides-utility in the preparation of synthetically useful tetrahydrofurans

Presented here are several examples which demonstrate that ether substituents α- or β- to an epoxide ring can be tolerated in the ring-opening reaction with β-keto ester dianions. Subsequent acid-promoted cyclisation of the γ-hydroxy β-ketoesters then leads to synthetically useful tetrahydrofurans, as demonstrated by application of this approach to the preparation of (±)-methyl nonactate and (±)-methyl 8-epi-nonactate.