A new class of glycosidase inhibitor: synthesis of salacinol and its stereoisomers

Salacinol (4) is one of the active principles in the aqueous extracts of Salacia reticulata that are traditionally used in Sri Lanka and India for the treatment of diabetes. The syntheses of salacinol (4), the enantiomer of salacinol (5), and a diastereomer (7) are described. The synthetic strategy relies on the selective nucleophilic attack of 2,3,5-tri-O-benzyl-1,4-anhydro-4-thio-D- or L-arabinitol at C-1 of 2,4-O-benzylidene D- or L-erythritol-1,3-cyclic sulfate. The work serves to resolve the ambiguity about the exact structure of salacinol and establishes conclusively the structure of the natural product.     

A divergent approach for the synthesis of some polyhydroxy pyrrolidines and piperidines from ribosylamine

A simple and efficient synthesis of 1,4-dideoxy-1,4-imino-d-ribitol, 1,4-dideoxy-1,4-imino-l-lyxitol, N-benzyl derivative of d-ribitol, 3,4,5-trihydroxy-piperidine, l-4-epi-isofagomine and d-3-epi-isofagomine, which are glycosidase inhibitors has been described from the commercially available d-ribose as a starting material.     

Fragmentation of carbohydrate anomeric alkoxy radicals. synthesis of polyhydroxy piperidines and pyrrolidines related to carbohydrates

Imino sugars of the piperidine and pyrrolidine types can be specifically obtained when protected 5-amino-5-deoxyfuranopentoses, 5-amino-5-deoxyfuranohexoses, 6-amino-6-deoxyfuranohexoses, and 6-amino-6-deoxypyranohexoses undergo a tandem alkoxy radical beta-fragmentation-intramolecular cyclization reaction. The reaction is promoted by the system: iodosylbenzene-iodine under mild conditions. The tert-butoxycarbonyl, benzyloxycarbonyl, and diphenylphosphinoyl radicals have been studied as amino-protecting groups. Using this methodology, polyhydroxylated pyrrolidines of the D-erythrofuranoses 34 and 35, D-threofuranose 36, L-xylofuranose 42, and D-arabinofuranose 43 series and polyhydroxylated piperidines of the D-arabinopyranose type 37 and 38 were obtained.     

Isolation synthesis and glycosidase inhibition profile of 3-epi-casuarine

3-epi-Casuarine, the first naturally occurring stereoisomer of casuarine, was isolated from Myrtus communis L. The glycosidase inhibition profile and NMR spectra of 3-epi-casuarine are compared with those of casuarine; the change in configuration at one of the six stereogenic centres causes a dramatic change in the conformation of the bicyclic system. The key step in the 6% overall yield synthesis of 3-epi-casuarine from d-gluconolactone is the efficient cyclization of a completely unprotected pentahydroxyaminomesylate to the pyrrolizidine nucleus. A low yield synthesis of casuarine is also reported.     

Stereoselective access to fluorinated and non-fluorinated quaternary piperidines: synthesis of pipecolic acid and iminosugar derivatives

The preparation of optically pure quaternary piperidines, both fluorinated and non-fluorinated, has been achieved from a chiral imino lactone derived from (R)-phenylglycinol. In the case of the fluorinated derivatives, the addition of (trifluoromethyl)trimethylsilane (TMSCF(3)) followed by iodoamination and migration of the CF(3) group allowed access to four derivatives of α-(trifluoromethyl)pipecolic acid. A theoretical study of the CF(3)-group rearrangement has been carried out to help establish the reaction mechanism of this uncommon transformation. Moreover, a route to trifluoromethyl-substituted iminosugars was also developed through the diastereoselective dihydroxylation of suitable synthetic intermediates. Conversely, alkylation of the starting substrate and subsequent cross-metathesis and aza-Michael reactions led to α-alkyl derivatives of the target compounds.     

Quinone-annonaceous acetogenins: synthesis and complex I inhibition studies of a new class of natural product hybrids

The natural product hybrids quinone-mucocin and quinone- squamocin D were synthesized. In these hybrids, the butenolide unit of the annonaceous acetogenins mucocin and squamocin D is exchanged for the quinone moiety of the natural complex I substrate ubiquinone. For both syntheses, a modular, highly convergent approach was applied. Quinone-mucocin was constructed out of a tetrahydropyran (THP) component 1, a tetrahydrofuran (THF) unit 2, and a quinone precursor 3. A stereoselective, organometallic coupling reaction was chosen for the addition of the THP unit to the rest of the molecule. In the final step, the oxidation to the free quinone was achieved by using cerium(IV) ammonium nitrate (CAN) as the oxidizing agent. Quinone-squamocin D was assembled in a similar manner, from the chiral side chain bromide 16, the central bis-THF core 17, and the quinone precursor 18. Inhibition of complex I (isolated from bovine heart mitochondria) by the quinone acetogenins and several smaller building blocks was examined; quinone mucocin and quinone-squamocin D act as strong inhibitors of complex I. These results and the data from the smaller substructures indicate that other substructures of the acetogenins besides the butenolide group, such as the polyether component and the lipophilic left-hand side chain, are necessary for the strong binding of the acetogenins to complex I.     

The first synthesis of substituted azepanes mimicking monosaccharides: a new class of potent glycosidase inhibitors

The synthesis of the first examples of seven-membered ring iminoalditols, molecules displaying an extra hydroxymethyl substituent on their seven-membered ring compared to the previously reported polyhydroxylated azepanes, has been achieved from d-arabinose in 10 steps using RCM of a protected N-allyl-aminohexenitol as a key step. While the (2R,3R,4R)-2-hydroxymethyl-3,4-dihydroxy-azepane 10, a seven-membered ring analogue of fagomine, is a weak inhibitor of glycosidases, the (2R,3R,4R,5S,6S)-2-hydroxymethyl-3,4,5,6-tetrahydroxy-azepane 9 selectively inhibits green coffee bean alpha-galactosidase in the low micromolar range (Ki = 2.2 muM) despite a D-gluco relative configuration.     

A practical and efficient synthesis of five-membered azasugar derivatives

A practical and efficient synthesis of 1,4-dideoxy-1,4-imino-D-arabinol and its partially protected derivatives has been described via the key intermediate with a norbornane-like structure using D-glucose as a starting material. __________ Translated from Chinese Journal of Organic Chemistry, 2007, 27(8): 1013–1017 [译自: 有机化学]     

A new approach to cyclic nitrones: Application to the synthesis of α,α′-disubstituted piperidines and pyrrolidines

A new synthesis of nitrones is described. This involves the Ag(I)-catalysed cyclisation of allenic oximes, the resulting nitrones being trapped by various 1,3-dipolarophiles.     

Intramolecular cyclization of alkoxyaminosugars: access to novel glycosidase inhibitor families

We report the synthesis of two novel families of iminosugars as glycosidase inhibitors involving an intramolecular cyclization between an N-alkoxyamino group and a latent aldehyde of a reducing sugar as the key step. Using this methodology we have prepared the hitherto unknown bicyclic polyhydroxylated N-(methoxy, benzyloxy)anhydroazepanes and N-benzyloxy-d-xylonojirimycin; all these novel compounds turned out to be moderate β-glucosidase inhibitors in a pH-dependent manner.     

A new route to 3,4-disubstituted piperidines: formal synthesis of (−)-paroxetine and (+)-femoxetine

A new route to 3,4-disubstituted piperidines was developed using chiral 1,4-dihydropyridines as key intermediates, the synthetic utility of which was demonstrated by formal synthesis of (−)-paroxetine and (+)-femoxetine.     

A new asymmetric synthetic route to substituted piperidines

An asymmetric synthesis of substituted piperidines has been described. β-Cyclodextrin- or oxazaborolidine-catalyzed asymmetric reduction of α-azido aryl ketones to the corresponding alcohols has been employed as the key step along with ring closing metathesis and selective dihydroxylation.     

A new access to dihydrotropones through ring expansion of spirocyclohexadienones: synthesis and mechanism

In this paper we report the rearrangement of spirocyclohexadienones into dihydrotropones in basic conditions as a new method for the preparation of seven-membered ring ketones, which are key building blocks for the synthesis of tropoloalkaloids. DFT calculations and deuterium labeling studies support the mechanism we propose for this rearrangement, involving the ring opening of a spirocyclopropane intermediate followed by successive base-catalyzed 1,3-hydrogen shifts. The X-ray structure of the resulting dihydrotropone shows near-perfect planarity and the conjugation gain is likely to be the driving force of the reaction.     

Synthesis of polyhydroxy piperidines and their analogues: a novel approach towards selective inhibitors of alpha-glucosidase

Various polyhydroxy piperidine azasugars have been synthesized from precursors 18a and 18b, obtained in both enantiomeric forms from d-ribose. Out of these polyhydroxy piperidine azasugars, 22, 39 and 20 were found to be potent as well as selective inhibitors of alpha-glucosidase with K(i) values ranging as low as 1.07 microM, 16.4 microM, and 88.2 microM, respectively. Replacement of the hydroxy methylene moiety of (K(i) 33% at 1 mM) by an amino methylene moiety (32, K(i) 36.8 microM) showed a remarkable increase in the activity (almost 30 times). Furthermore, increasing the lipophilicity of by N-alkylation with a dodecyl group (36) showed a three-fold enhancement in the activity (K(i) 217 microM to K(i) 72.3 microM).     

Diastereoselective access to nonracemic 2-cis-substituted and 2,6-cis-disubstituted piperidines

Access to nonracemic amino ketones via a hydrozirconation/transmetalation/acylation sequence applied to Boc-protected 1-aminobut-3-enes is presented. This method was applied to the stereoselective synthesis of cyclic imines (or iminiums) which were diastereoselectively converted into 2-cis-substituted and 2,6-cis-disubstituted piperidines. The potential of this approach in the field of alkaloid synthesis was illustrated by the synthesis of (-)-coniine and (-)-indolizidine 209D. Furthermore, access to indolizidines bearing a quaternary center could also be envisioned through this strategy.     

Homochiral carbon branched piperidines from carbon branched sugar lactones: 4-C-methyl-deoxyfuconojirimycin (DFJ) and its enantiomer—removal of glycosidase inhibition

The value of readily available 2-C-methyl aldonic acids in short syntheses of carbon branched piperidines containing quaternary centers is demonstrated. The effect of the introduction of a 4-C-methyl group into piperidine imino sugar inhibitors of l-fucosidases and d-galactosidases is reported.     

A Diels-Alder approach to the stereoselective synthesis of 2,3,5,6-tetra- and 2,3,4,5,6-pentasubstituted piperidines

[structures: see text] A stereoselective synthesis of 2,3,5,6-tetra- and 2,3,4,5,6-pentasubstituted piperidines was achieved from oxidative cleavage of 2-aza-bicyclo[2.2.2]octene Diels-Alder adducts derived from N-protected 2-methyl-1,2-dihydropyridine. A chiral auxiliary mediated asymmetric synthesis of the pentasubstituted piperidine is also demonstrated. This methodology incorporates orthogonal protecting groups, thus providing a piperidine scaffold with easily modified points of diversity.     

A practical asymmetric synthesis of 2,6-cis-disubstituted piperidines

A highly diastereoselective intramolecular Mannich reaction involving enantiopure α-methylamine 7 and achiral aldehydes is employed to prepare enantiomerically pure 2,6-cis-disubstituted piperidines. This methodology provides an efficient and selective route to 2,6-cis-disubstituted piperidines, 2,6-cis-disubstituted 4-piperidones and 2,6-cis-disubstituted 4-piperidinols.