Selective Synthesis of N-Acylnortropane Derivatives in Palladium-Catalysed Aminocarbonylation.

The aminocarbonylation of various alkenyl and (hetero)aryl iodides was carried out using tropane-based amines of biological importance, such as 8-azabicyclo[3.2.1]octan-3-one (nortropinone) and 3α-hydroxy-8-azabicyclo[3.2.1]octane (nortropine) as N-nucleophile. Using iodoalkenes, the two nucleophiles were selectively converted to the corresponding amide in the presence of Pd(OAc)2/2 PPh3 catalysts. In the presence of several iodo(hetero)arenes, the application of the bidentate Xantphos was necessary to produce the target compounds selectively. The new carboxamides of varied structure, formed in palladium-catalyzed aminocarbonylation reactions, were isolated and fully characterized. In this way, a novel synthetic method has been developed for the producing of N-acylnortropane derivatives of biological importance.

Concise synthesis of calystegines B2 and B3via intramolecular Nozaki-Hiyama-Kishi reaction.

The key step in the concise syntheses of calystegine B2 and its C-2 epimer calystegine B3 was the construction of cycloheptanone 8via an intramolecular Nozaki-Hiyama-Kishi (NHK) reaction of 9, an aldehyde containing a Z-vinyl iodide. Vinyl iodide 9 was obtained by the Stork olefination of aldehyde 10, derived from carbohydrate starting materials. Calystegines B2 (3) and B3 (4) were synthesized from d-xylose and l-arabinose derivatives respectively in 11 steps in excellent overall yields (27% and 19%).

A one-step automated synthesis of the dopamine transporter ligand [(18)F]FECNT from the chlorinated precursor.

The use of [(18)F]labelled nortropane derivative 2ß-carbomethoxy-3ß-(4-chlorophenyl)-8-(2-fluoroethyl)-nortropane (FECNT) as a dopamine transporter ligand for PET imaging is dependent on efficient radiosynthesis method. Herein, the automated synthesis of [(18)F]FECNT from its chlorinated precursor in commercially available SynChrom [(18)F] R&D module has been developed. The synthesis unit was readily configured for the one-step synthesis from corresponding chlorinated precursor. The radiolabeling process involved a classical [(18)F]fluoride nucleophilic substitution performed at 110 °C for 12 min and finally HPLC and SPE purification. Crude [(18)F]FECNT was obtained with a radiolabeling yield of 59 ± 12% (n = 5). The average uncorrected amount of [(18)F]FECNT in the final formulated dose was 2.0 ± 0.5 GBq (32 ± 7% overall decay-corrected yields) obtained with radiochemical purity over 99% and specific activity of 55 GBq/µmol. The total duration of the procedure was 80-90 min. An automated radiosynthesis of [(18)F]FECNT with high radiochemical purity may provide a simple and robust method of radiopharmaceutical preparation for routine clinical applications.

Design and Synthesis of Labystegines, Hybrid Iminosugars from LAB and Calystegine, as Inhibitors of Intestinal α-Glucosidases: Binding Conformation and Interaction for ntSI.

This paper identifies the required configuration and orientation of α-glucosidase inhibitors, miglitol, α-1-C-butyl-DNJ, and α-1-C-butyl-LAB for binding to ntSI (isomaltase). Molecular dynamics (MD) calculations suggested that the flexibility around the keyhole of ntSI is lower than that of ctSI (sucrase). Furthermore, a molecular-docking study revealed that a specific binding orientation with a CH-π interaction (Trp370 and Phe648) is a requirement for achieving a strong affinity with ntSI. On the basis of these results, a new class of nortropane-type iminosugars, labystegines, hybrid iminosugars of LAB and calystegine, have been designed and synthesized efficiently from sugar-derived cyclic nitrones with intramolecular 1,3-dipolar cycloaddition or samarium iodide catalyzed reductive coupling reaction as the key step. Biological evaluation showed that our newly designed 3(S)-hydroxy labystegine (6a) inherited the selectivity against intestinal α-glucosidases from LAB, and its inhibition potency was 10 times better than that of miglitol. Labystegine, therefore, represents a promising new class of nortropane-type iminosugar for improving postprandial hyperglycemia.

Synthesis and evaluation of galacto-noeurostegine and its 2-deoxy analogue as glycosidase inhibitors.

An epimer of the known glycosidase inhibitor noeurostegine, galacto-noeurostegine, was synthesised in 21 steps from levoglucosan and found to be a potent, competitive and highly selective galactosidase inhibitor of Aspergillus oryzae ß-galactosidase. Galacto-noeurostegine was not found to be an inhibitor of green coffee bean α-galactosidase, yeast α-glucosidase and E. coli ß-galactosidase, whereas potent but non-competitive inhibition against sweet almond ß-glucosidase was established. The 2-deoxy-galacto-noeurostegine analogue was also prepared and found to be a less potent inhibitor of the same enzymes.

Synthesis of novel halo and tosyloxy nortropane derivatives as efficient precursors for the one-step synthesis of the dopamine transporter PET ligand [(18)F]FECNT.

The fluorine-18 labeled nortropane derivative 2ß-carbomethoxy-3ß-(4-chlorophenyl)-8-(2-fluoroethyl)-nortropane (FECNT) is a dopamine transporter (DAT) ligand. Currently, it is considered as reference for positron emission tomography imaging. Herein, the synthesis of novel precursors (N-tosyloxy-, chloro-, and bromo- analogues) for one-step radiosynthesis of [(18)F]FECNT is reported. Using the N-mesyloxy- precursor in a one-step radiosynthesis, the crude [(18)F]FECNT was obtained with the radiolabeling yield of 45 ± 10%, confirming the practical efficiency of this approach in the design of novel precursors for labeling.

Executing and rationalizing the synthesis of a difluorinated analogue of a ring-expanded calystegine B2.

A difluorinated analogue of a ring-expanded calystegine B(2) and some N-protected species were prepared via microwave-mediated transannular ring-opening of an epoxyketone. The diastereofacial selectivity of the epoxidation reaction, which delivers the key intermediate, and the regioselectivity of the transannular reactions were analyzed by density functional theory (DFT) methods. The epoxidation stereoselectivity arises from simple steric control, whereas the ring-closure reactions are subject to thermodynamic control.

Total synthesis of ent-calystegine B4 via nitro-Michael/aldol reaction.

Optically active ent-calystegine B4 was prepared in 13 steps from commercially available chiral L-dimethyl tartrate. The synthesis was achieved by the Michael addition and the aldol reaction of nitromethane to form cycloheptanone in a stereoselective manner. Reduction of the nitro group in the presence of Boc(2)O accomplished an efficient conversion to amino cycloheptanone, which readily afforded the desired ent-calystegine B4.

Synthesis of N-alkylated noeurostegines and evaluation of their potential as treatment for Gaucher's disease.

The potent and selective inhibitor of ß-glucosidases, noeurostegine, was evaluated as an inhibitor of glucocerebrosidase (GCase) to give an IC(50) value of 0.4 µM, being 250- and 150-fold better than N-butyl and N-nonyl noeurostegine, respectively. The parent noeurostegine and its N-butyl and N-nonyl alkylated congeners were also tested as pharmacological chaperones against a N370S GCase mutant. Of these, only noeurostegine, was found to increase enzyme activity, which in potency was comparable to that previously reported for isofagomine.

Discovery of a nortropanol derivative as a potent and orally active GPR119 agonist for type 2 diabetes.

The lead optimization studies of a series of GPR119 agonists incorporating a nortropanol scaffold are described. Extensive structure-activity relationship (SAR) studies of the lead compound 20f led to the identification of compound 36j as a potent, single digit nanomolar GPR119 agonist with high agonist activity. Compound 36j was orally active in lowering blood glucose levels in a mouse oral glucose tolerance test and increased plasma insulin levels in a rat hyperglycemic model. It showed good to excellent pharmacokinetic properties in rats and monkeys and no untoward activities in counter-screen assays. Compound 36j demonstrated an attractive in vitro and in vivo profile for further development.

A concise stereoselective synthesis of (-)-erycibelline.

(-)-Erycibelline, the dihydroxynortropane alkaloid isolated from Erycibe elliptilimba Merr. et Chun., was synthesized using a cyclic nitrone as advanced intermediate, wherein the key step was the SmI(2)-induced intramolecular reductive coupling of cyclic nitrone with aldehyde which resulted in good yield and stereoselectivity.

Synthesis of uronic-noeurostegine--a potent bacterial ß-glucuronidase inhibitor.

Inhibition of ß-glucuronidases has recently been shown to be useful in alleviating drug toxicity for common colon cancer chemotherapeutic CPT-11 (also called Irinotecan). We have prepared a new compound of the nortropane-type, uronic-Noeurostegine, and demonstrated that this is a competitive and potent E. coli ß-glucuronidase inhibitor, while inhibition of the mammalian ß-glucuronidase from bovine liver was found to be less significant. Although not intended, two other compounds having N-ethyl and N-(4-hydroxybutyl) substituents were also prepared in this study due to the sluggish debenzylation in the final step. The N-substituents are believed to come from reaction with the solvents used being ethanol and THF, respectively. These compounds also inhibited the two ß-glucuronidases albeit to a lesser extent compared to the parent compound. Noeurostegine and the three uronic-noeurostegines were additionally evaluated as inhibitors against a wide panel of glycosidases with the former showing potent inhibition of rat intestinal lactase and trehalase, whereas the latter was found to be inactive.

Synthesis and glycosidase inhibitory activity of noeurostegine-a new and potent inhibitor of beta-glucoside hydrolases.

A new, stable hemi-aminal nor-tropane christened noeurostegine was synthesised in 22 steps from levoglucosan and tested for inhibitory activity against glycoside hydrolases. Sweet almond and Thermotoga maritimabeta-glucosidases, coffee bean alpha-galactosidase, and Asp. oryzaebeta-galactosidase were inhibited in the low micromolar region but significant tightening of binding to K(i) 50 nM for almond beta-glucosidase was found to occur after pre-incubation. Yeast alpha-glucosidase and E. colibeta-galactosidase were not inhibited at 1 mM.

A versatile access to calystegine analogues as potential glycosidases inhibitors.

An efficient metathetic strategy and nitrone chemistry have been suitably tethered to construct 8-azabicyclo[3.2.1]octanes as versatile precursors for the synthesis of several calystegine analogues. This synthetic strategy relies on the ability of mannose-derived nitrone to undergo a highly stereoselective nucleophilic addition of various Grignard reagents to access syn orientation of alkenes, which then smoothly undergo ring-closing metathesis (RCM) to provide this framework. These RCM products 18 and 20 have been successfully used as advance precursors to synthesize many calystegine analogues (27, 36, 38, 40, 43, and 44) either by syn-dihydroxylation or by hydrogenation and followed by global deprotection. Interestingly, both compounds 36 and 40 exhibited significant noncompetitive inhibition against alpha-mannosidase and N-acetyl-beta-D-glucosaminidase.

Synthesis and monoamine transporter affinity of 3alpha-arylmethoxy-3beta-arylnortropanes.

A series of 3-arylnortrop-2-enes and 3alpha-arylmethoxy-3beta-arylnortropanes were synthesized and evaluated for binding affinity at monoamine transporters. The 3-(3,4-dichlorophenyl)nortrop-2-ene (6e) exhibited high affinity for the SERT (K(i)=0.3 nM). The 3alpha-arylmethoxy-3beta-arylnortropanes were generally SERT selective with the 3alpha-(3.4-dichlorophenylmethoxy)-3betaphenylnortrop-2-ene (7c) possessing subnanomolar potency (K(i)=0.061 nM). However, 3alpha-(3,4-dichlorophenylmethoxy)-3beta-phenylnortrop-2-ene (7b) exhibited high affinity at all three transporters [(DAT K(i)=22 nM), (SERT K(i)=6 nM) and (NET K(i)=101 nM)].

Synthesis, radiolabeling and preliminary in vivo evaluation of [18F]FE-PE2I, a new probe for the dopamine transporter.

A new dopamine transporter (DAT) ligand, (E)-N-(3-iodoprop-2-enyl)-2beta-carbofluoroethoxy-3beta-(4'-methyl-phenyl) nortropane (FE-PE2I, 6), derived from PE2I (1), was prepared and found to be a potent inhibitor of rodent DAT in vitro. Compound 6 was radiolabelled with fluorine-18 (t(1/2)=109.8 min) for PET studies in monkeys. In vivo PET measurements showed a regional distribution in brain that corresponds to the known distribution of DAT. This binding was specific, reversible and the kinetics of [(18)F]6 binding in brain were faster than for its lead compound, [(11)C]1. The possible presence of a hydroxymethyl-radiometabolite formed by oxidation in the 3beta-benzylic position of [(18)F]6 warrants further detailed evaluation of the metabolism of [(18)F]6. [(18)F]6 is a potential radioligand for imaging DATs in the human brain with PET.

Identification of 3-substituted N-benzhydryl-nortropane analogs as nociceptin receptor ligands for the management of cough and anxiety.

A series of nortropane analogs based on previously reported compound 1 have been synthesized and shown to bind to the nociceptin receptor with high affinity. The synthesis and structure-activity relationships around the C-3 nortropane substitution are described. From the SAR study and hPXR screening effort, compound 15 was identified to possess potent oral antitussive and anxiolytic-like activities in the guinea pig models.

Synthesis and structure-activity relationship of 3beta-(4-alkylthio, -methylsulfinyl, and -methylsulfonylphenyl)tropane and 3beta-(4-alkylthiophenyl)nortropane derivatives for monoamine transporters.

Early studies led to the identification of 3beta-(4-methoxyphenyl)tropane-2beta-carboxylic acid methyl ester (5) with high affinity at the DAT (IC(50)=6.5nM) and 5-HTT (K(i)=4.3nM), while having much less affinity at the NET (K(i)=1110nM). In the present study, we replaced the 4'-methoxy group of the 3beta-phenyl ring with a bioisosteric 4'-methylthio group to give 7a. We also synthesized a number of 3beta-(4-alkylthiophenyl)tropanes 7b-e, 3beta-(4-methylsulfinylphenyl) and 3beta-(4-methylsulfonylphenyl)tropane analogues 7f-h as well as the 3beta-(4-alkylthiophenyl)nortropane derivatives 8-11 to further characterize the structure-activity relationship of this type of compound for binding at monoamine transporters. With exception of the 4'-methylsulfonyl analogue 7h, all the tested compounds possessed high binding affinities at the 5-HTT. The K(i) values ranged from 0.19nM to 49nM. The 3beta-(4-methylthiophenyl)tropane 7a and its N-(3-fluoropropyl) analogue 9a and N-allyl analogue 10a are the most selective compounds for the 5-HTT over the NET (NET/5-HTT=314-364) in the series. However, none of the compounds showed selectivity similar to 5 for both the DAT and 5-HTT relative to the NET. This study provided useful SAR information for rational design of potent and selective monoamine transporter inhibitors.

Synthesis of nortropane alkaloid calystegine B2 from methyl α-d-xylopyranoside.

A new synthetic route for formation of a central cycloheptanone intermediate leading to the nortropane alkaloid calystegine B2 is described. The approach installs the desired ketone functionality directly in a ring-closing metathesis step. The target compound was prepared over 10 steps from commercially available methyl α-d-xylopyranoside.

Structure-activity relationships of 3-substituted N-benzhydryl-nortropane analogs as nociceptin receptor ligands for the treatment of cough.

A series of 3-axial-aminomethyl-N-benzhydryl-nortropane analogs have been synthesized and identified to bind to the nociceptin receptor with high affinity. Many of these analogs showed high binding selectivity over classic opioid receptors such as mu receptor. The synthesis and structure-activity relationships around the C-3 nortropane substitution are described. Selected compounds with potent oral antitussive activity in the guinea pig model are disclosed.