A Short Route toward Chiral, Polyhydroxylated Indolizidines and Quinolizidines

In this paper, a rapid route toward functionalized bicyclic alkaloids is presented. In only three steps, an easily accessible carbohydrate derivative was converted into iodomethyl indolizidine 13, which can equilibrate to the corresponding iodoquinolizidine 15. We provide strong evidence that this equilibration proceeds via an aziridinium ion intermediate. Furthermore, nucleophilic substitution of the iodomethyl indolizidine as well as the aziridinium intermediate gives access to highly functionalized indolizidine and quinolizidine alkaloids.

Determining the Scope of the Organolanthanide-Catalyzed, Sequential Intramolecular Amination/Cyclization Reaction: Formation of Substituted Quinolizidines, Indolizidines, and Pyrrolizidines

The scope of the lanthanide-mediated, intramolecular amination/cyclization reaction was determined for the formation of substituted quinolizidines, indolizidines, and pyrrolizidines. A methyl group was installed at diverse positions in the substrates to determine the sense and magnitude of diastereoselection. High diastereoselectivity (>20:1) was achieved for the formation of some quinolizidines and indolizidines. The sense of relative asymmetric induction was contrary to previously studied systems, and although some questions remain, a rationalization for these results is put forward.

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.

Synthesis and Evaluation of Isourea-Type Glycomimetics Related to the Indolizidine and Trehazolin Glycosidase Inhibitor Families

A practical synthesis of reducing isourea-derived azasugar glycomimetics related to the indolizidine and trehazolin glycosidase inhibitor families with different pK(a) values is disclosed. The polyhydroxylated bicyclic system was built from readily accessible hexofuranose derivatives through a synthetic scheme that involves the preparation of a 5-deoxy-5-carbodiimido adduct by triphenylphosphine-mediated tandem Staudinger--aza-Wittig-type coupling of azide and isothiocyanate precursors, intramolecular cyclization of a transient vic-hydroxycarbodiimide derivative, and nucleophilic addition of the endocyclic nitrogen atom of the generated 2-amino-2-oxazoline intermediate, with a pseudo-C-nucleoside structure, to the masked aldehyde group of the monosaccharide. The last step is pH-dependent so that the final compounds can pivot between the furanose and the 2-oxaindolizidine forms. Nevertheless, the indolizidine tautomer having the R configuration at the aminoacetalic center, fitting the anomeric effect, was the only species detected in solution at neutral or slightly acidic pH when starting from solutions at basic pH. Glycosidase inhibition tests (K(i) values down to 1.9 microM) showed a marked dependence of the selectivity and potency toward alpha- and beta-glucosidases upon the nature of the substituent at the exocyclic isourea nitrogen, shifting from alpha- to beta-selectivity when going from hydrophilic to hydrophobic substituents. Enzyme inhibition is also pH dependent, supporting a dominant role for the uncharged form of the polyhydroxyiminoindolizidine system in the inhibition of beta-glucosidases.

Asymmetric, Stereocontrolled Total Synthesis of Paraherquamide A

The first total synthesis of paraherquamide A, a potent anthelmintic agent isolated from various Penicillium sp. with promising activity against drug-resistant intestinal parasites, is reported. Key steps in this asymmetric, stereocontrolled total synthesis include a new enantioselective synthesis of alpha-alkylated-beta-hydroxyproline derivatives to access the substituted proline nucleus and a highly diastereoselective intramolecular S(N)2' cyclization to generate the core bicyclo[2.2.2]diazaoctane ring system.

An efficient approach to 3,4-disubstituted pyridin-2-ones. Formal synthesis of mappicine ketone

Treatment of 3,4-dihydro-3-tosylpyridin-2-one 5 with sodium hydride and then alkyl halides gave various 3,4-disubstituted pyridin-2-ones 6. Formal synthesis of mappicine ketone (1) was also reported.

One-pot hydrogenation conditions for a sequential process to (+)-monomorine

(+)-Monomorine has been synthesized under mild hydrogenation conditions initiating deprotection followed by intramolecular, sequential reductive amination reactions. The precursors could be prepared concisely using B-alkyl Suzuki cross coupling of a chiral homoallylamine and a vinyl iodide or an iodofuran derivative.

Asymmetric synthesis of an axially chiral antimitotic biaryl via an atropo-enantioselective Suzuki cross-coupling

A catalytic asymmetric synthesis of the axially chiral bridged biaryl (-)-2, a structural analogue of natural (-)-rhazinilam possessing original antimitotic properties, is described. The key step is an intermolecular asymmetric Suzuki coupling, furnishing the nonbridged biaryl (-)-6, precursor of (-)-2, with up to 40% ee using binaphthyl ligand 7a. Various known or new binaphthyl and ferrocenyl phosphines as well as phosphetanes were screened as ligands in this reaction, the conditions of which were optimized. The comparison with another Suzuki coupling system showed that 7a is the most versatile ligand described to date for this type of transformation. This work gives the first application of the asymmetric Suzuki coupling to a biologically relevant target.

Antimycobacterial activity of 1-substituted indolizines

Indolizine derivatives were tested as antimycobacterials against Mycobacterium tuberculosis H(37)Rv (ATCC 27294)in BACTEC 12B medium using the Microplate Alamar Blue Assay. The most active compounds examined carry an alpha-hydroxybenzyl substituent in the indolizine 1-position. MICs against Mycobacterium tuberculosis for these compounds were 6.25 microg/mL

A flexible approach toward trisubstituted piperidines and indolizidines: synthesis of 6-epi-indolizidine 223A

2,5,6-Trisubstituted piperidines are readily prepared by a combination of an aza-Achmatowicz oxidation of a furyl-substituted benzenesulfonamide followed by a conjugate addition to the resulting 2H-pyridone and subsequent addition of various nucleophiles to a transient N-sulfonyliminium ion. The stereochemistry of the conjugate addition product is the result of axial attack from the face opposite the diaxial substituents at C(2) and C(6). This can be attributed to steric hindrance between the pseudoaxially oriented 2,6-substituents and the equatorially approaching nucleophile, thereby leading to the exclusive formation of the kinetically favored axial 1,4-adduct. Indolizidine alkaloid 223A was isolated from a skin extract of a Panamanian population of the dendrobatid Dendrobates pumilio Schmidt (Dendrobatidae). Synthesis of the originally proposed structure of this alkaloid was achieved in 13 steps in 13.1% overall yield by using an aza-Achmatowicz oxidative rearrangement and a diastereoselective 1,4-conjugate addition as the key steps. The structure of the natural 223A alkaloid (5b) differs from that of the epi-isomer 5a synthesized in this study in the configuration at the 6-position of the indolizidine ring.

Asymmetric total synthesis of (-)-alkaloid 223A and its 6-epimer

The enantiopure gamma-amino alcohols 7 and 18 are prepared by using the diastereoselective Michael addition of lithium N-benzyl (R)-alpha-methylbenzylamide to alpha,beta-unsaturated esters as a key step. The Michael addition of 7 or 18 to an alkynone 8 followed by an intramolecular cyclization afford the cyclic enamine 10 or 20, which are subjected to the diastereoselective hydrogenation, and the subsequent transformations provide 6-epi-alkaloid 223A and alkaloid 223A, respectively.

Novel 6,7-diphenyl-2,3,8,8a-tetrahydro-1H-indolizin-5-one analogues as cytotoxic agents

A series of 6,7-diphenyl-2,3,8,8a-tetrahydro-1H-indolizin-5-one analogues were synthesized and evaluated for cytotoxic activity against eight human cancer cell lines. Compounds 18, 21, 28, 29, 30 and 31 showed cytotoxic activity with GI(50) values in the range of 2.1-8.1 microM concentration. Among these, compounds 21 and 28 exhibited good pharmacokinetic properties. These compounds were further evaluated for their in vivo efficacy in modified hollow fibre assay (HFA).

Non-imidazole heterocyclic histamine H3 receptor antagonists

Continued exploration of the SAR around the lead imidazopyridine histamine H(3) antagonist 1 has led to the discovery of several related series of heterocyclic histamine H(3) antagonists. The synthesis and SAR of indolizine, indole and pyrazolopyridine based compounds are now described.

Sensory activity, chemical structure, and synthesis of Maillard generated bitter-tasting 1-oxo-2,3-dihydro-1H-indolizinium-6-olates

Thermal treatment of aqueous solutions of xylose, rhamnose, and l-alanine led to a rapid development of a bitter taste of the reaction mixture. To characterize the key compounds causing this bitter taste, the recently developed taste dilution analysis (TDA), which is based on the determination of the taste threshold of reaction products in serial dilutions of HPLC fractions, was performed to locate the most intense taste compounds in the complex mixture of Maillard reaction products. By application of this TDA, 26 fractions were obtained, among which seven fractions were evaluated with a high taste impact. LC/MS and NMR spectroscopy as well as synthetic experiments revealed the 1-oxo-2,3-dihydro-1H-indolizinium-6-olates 1-5 as the key compounds contributing the most to the intense bitter taste of the Maillard mixture. Calculation of the taste impact of these compounds based on a dose/activity relationship indicated that these five compounds already accounted for 56.8% of the overall bitterness of the Maillard mixture, thus demonstrating this class of 1-oxo-2,3-dihydro-1H-indolizinium-6-olates as the key bitter compounds. First synthetic studies on the relationship between the chemical structure and the human psychobiological activity of 1-oxo-2,3-dihydro-1H-indolizinium-6-olates revealed that substitution of the furan rings of 1 by 5-methylfuryl moieties (compounds 3-5) or by 5-(hydroxymethyl)furyl groups (compound 6) led to a significant increase of the bitter threshold. In contrast, the substitution of the oxygen atoms in the furan rings of 1 by sulfur atoms induced a significant decrease of the detection threshold of the 1-oxo-2,3-dihydro-1H-indolizinium-6-olate; for example, the thiophene derivative 7 showed the extraordinarily low bitter detection threshold of 6.3 x 10(-5) mmol/kg (water).

New synthetic approach to enantiopure bicyclic sulfonium salts: swainsonine analogues

The enantioselective synthesis of bicyclic sulfonium salts 8 or 9, thioanalogues of swainsonine derivatives, is described. The synthetic strategy is based on a stereo- and regiospecific transannular cyclization reaction of nine-membered cyclic sulfides, mediated by Me(3)SiI or carried out under acidic catalysis.

Studies on the biosynthesis of paraherquamide A and VM99955. A theoretical study of intramolecular Diels-Alder cycloaddition

Intramolecular Diels-Alder reactions of 2-azadiene models have been studied quantum chemically at the B3LYP/6-31G level in order to elucidate the stereochemical features of the cyclization step involved in the biosynthesis of paraherquamide A and VM99955. These cycloadditions take place through concerted transition states associated with [4 + 2] processes. Analysis of the energies along the competitive paths reveals that while the cycloadditions of the oxindoles present a large anti selectivity, the indoles show a low syn selectivity for the formation of the C20 stereogenic center that is larger for the reduced tertiary amide form. The presence of the C14 methyl of the beta-methylproline ring produces a low hindrance along the reaction coordinate for the syn approach of the isoprene framework, in agreement with the low facial selectivity found experimentally. An analysis of the electrophilicity and activation parameters for experimental models of the inter- and intramolecular Diels-Alder reactions reveals several significant factors controlling these biosynthetic cyclizations. The results are in reasonable agreement with the available experimental data.

Paraherquamides, brevianamides, and asperparalines: laboratory synthesis and biosynthesis. An interim report

Studies from our laboratories on the paraherquamide, brevianamide, and asperparaline families of natural products are reviewed. It has been proposed that the unique core ring system that is common to this family of compounds arises by a biological intramolecular Diels-Alder cycloaddition reaction. Key biosynthetic studies are described, along with classical synthetic approaches as well as those inspired by Nature for the synthesis of these interesting molecules.

Total synthesis of (+/-)-rhazinal, an alkaloidal spindle toxin from Kopsia teoi

The title alkaloid 1 and its B-nor-congener 3, both of which display potent and unusual anti-mitotic properties, have been synthesized in racemic form and characterised by single-crystal X-ray analysis.

Preparation of new nitrogen-bridged heterocycles. 53. Syntheses of 3-(benzylthio)thieno[3,4-b]indolizine derivatives and their intramolecular arene-arene interactions

Various ethyl 1-arylcarbonyl-3-[(un)substituted methylthio]thieno[3,4-b]indolizine-9-carboxylates were synthesized in good yields by a novel methodology in which the S-alkylation of 5-arylcarbonyl-4-ethoxycarbonylmethyl-3-(1-pyridinio)thiophene-2-thiolates with alkyl or benzyl halides, the 1,5-dipolar cyclization of the resulting pyridinium salts in the presence of a base, and the aromatization were performed. In the X-ray analyses of some 3-(benzylthio)thieno[3,4-b]indolizine-9-carboxylates, a gauche and two anti conformers in relation to the exocyclic sulfide linkage were found. Interestingly, all of the 3-(benzylthio)thieno[3,4-b]indolizine derivatives showed significant high-field shifts (delta up to 0.3 ppm) for the 5- and 6-proton signals compared with those of the 3-methylthio derivatives in the (1)H-NMR spectra and exhibited a definite absorption band near 425 nm in their UV spectra, indicating an intramolecular arene-arene interaction between the thieno[3,4-b]indolizine and the phenyl ring.

Preparation of New Nitrogen-Bridged Heterocycles. 54. Increased Arene-Arene Interactions of 3-(Bicyclic and Tricyclic Arylmethylthio)thieno [3,4-b] indolizine Derivatives

Some thieno[3,4-b]indolizine derivatives having a 1-naphthylmethylthio, 2-methyl-1-naphthylmethylthio, 2-naphthylmethylthio, or 9-anthrylmethylthio group at the 3-position were prepared and their intramolecular arene-arene interactions were investigated. In comparison with 3-(methylthio)thieno[3,4-b]indolizines which have no such interactions, the (1)H-NMR spectra of title compounds showed large high-field shifts (delta 0.06-0.89 ppm) for the protons of the pyridine ring in the thieno[3,4-b]indolizine, and these values were considerably larger than those (delta <0.3 ppm) in 3-(benzylthio)thieno[3,4-b]indolizines. The UV spectra also exhibited a characteristic absorption band near 425 nm attributable to the arene-arene interaction. In the X-ray analyses of some compounds, however, the presence of both the gauche and the anti conformers at the sulfide spacer were confirmed.

Double cycloisomerization as a novel and expeditious route to tricyclic heteroaromatic compounds: short and highly diastereoselective synthesis of (+/-)-tetraponerine T6

[reaction: see text] Cu-Assisted double cycloisomerization of bis-alkynylpyrimidines afforded the 5-6-5 tricyclic heteroaromatic skeleton. This transformation was used as a key step in the highly diastereoselective total synthesis of (+/-)-tetraponerine T6.

First total synthesis of (+/-)-strychnofoline via a highly selective ring-expansion reaction

An efficient synthesis of the antitumor alkaloid (+/-)-strychnofoline is documented. Key to the development of the highly convergent strategy delineated is the coupling of a cyclic imine with spiro[cyclopropan-1,3'-oxindole], which takes place in a highly diastereoselective manner. The ability to conduct annulation reactions of spirocyclopropyloxindoles with functionalized cyclic imines provides new avenues for the preparation of this important class of biologically active structures.

Indolizidine and quinolizidine alkaloids

This review covers the isolation, structure determination, synthesis and biological activity of indolizidine and quinolizidine alkaloids from microbial, plant and animal sources. Included in the review are slaframine; hydroxylated indolizidines and their analogues; alkaloids from ants and amphibians; metabolites of the genera Prosopis, Streptomyces and Nuphar and the Lythraceae; phenanthroindolizidines and related alkaloids; lupin alkaloids; and alkaloids from sponges. tunicates and coccinellid beetles. The literature from July 2000 to June 2001 is reviewed, and 172 references are cited.

Targeting thrombin and factor VIIa: design, synthesis, and inhibitory activity of functionally relevant indolizidinones

Guided by molecular modeling, docking experiments, and available X-ray crystal structure data on the serine protease Factor VIIa and thrombin, a series of indolizidinone derivatives was designed and synthesized having diverse functionality at the P1, P2, and P3 sites.