Recent Advances in the Total Synthesis of Piperidine Azasugars

Synthesis and evaluation of sulfamide-type indolizidines as glycosidase inhibitors

A practical synthesis of reducing sulfamide-derived iminosugar glycomimetics related to the indolizidine glycosidase inhibitor family is reported. The polyhydroxylated bicyclic system was built from readily accessible hexofuranose derivatives through a synthetic scheme that involves 5,6-cyclic sulfamides. Further intramolecular nucleophilic addition of the sulfamide nitrogen atom to the masked aldehyde group of the monosaccharide in the open chain form afforded the target sugar mimics. By starting from d-glucose and d-mannose precursors, 2-aza-3,3-dioxo-3-thiaindolizidine derivatives with hydroxylation profiles that matched those of (+)-castanospermine and 6-epi-(+)-castanospermine were obtained. In vitro screening against a panel of glycosidases evidenced a high selectivity towards alpha-mannosidase.

exo-Imino to endo-iminocyclitol rearrangement. A general route to five-membered antiviral azasugars

A facile synthesis is reported for five-membered iminocyclitols which allows for variation in stereochemistry at all the chiral centers, diverse C1- and N-substitution, and the potential for a three-component combinatorial process. The key step is inversion at the C4 stereocenter (L-lyxo sugar --> D-ribono azasugar). The exo-imino to endo-iminocyclitol process was extended to the D-lyxo and the D- and L-hexose series. Some analogues were found to be more potent than N-butyl DNJ and N-nonyl DNJ in antiviral activity.

D-Glucosylated Derivatives of Isofagomine and Noeuromycin and Their Potential as Inhibitors of β-Glycoside Hydrolases

While isofagomine and noeuromycin have previously been demonstrated to be effective inhibitors of a range of exo-acting glycosidases, they are usually only very weak inhibitors of endo-glycosidases. However, the disaccharide-like 3- and 4-O-β-d-glucopyranosylisofagomines have proven to be strong inhibitors of these endo-acting enzymes that utilize multiple sub-sites. In an attempt to emulate these successes, we have prepared 3- and 4-O-β-d-glucopyranosylnoeuromycin, the former by a selective glycosylation (at O2) of benzyl 4-C-cyano-4-deoxy-α-d-arabinoside (also leading to another synthesis of 3-O-β-d-glucopyranosylisofagomine), the latter by a non-selective glycosylation of benzyl 4-O-allyl-β-l-xyloside with subsequent introduction of the required nitrile group (also leading to another synthesis of 4-O-β-d-glucopyranosylisofagomine). 3-O-β-d-Glucopyranosylnoeuromycin was evaluated as an inhibitor of a family 26 lichenase from Clostridium thermocellum, and 4-O-β-d-glucopyranosylnoeuromycin as an inhibitor of both a family 5 endo-glucanase from Bacillus agaradhaerans and a family 10 endo-xylanase from Cellulomonas fimi. We also report X-ray structural investigations of 3- and 4-O-β-d-glucopyranosylnoeuromycin in complex with the family 26 and family 5 β-glycoside hydrolases, respectively. The two d-glucosylated noeuromycins were indeed able to harness the additional binding energy from the sub-sites of their endo-glycoside hydrolase targets, and were thus excellent inhibitors (in the nanomolar range), binding as expected in the –1 and –2 sub-sites of the enzymes.

Chiral Oxazolopiperidone Lactams: Versatile Intermediates for the Enantioselective Synthesis of Piperidine-Containing Natural Products

Phenylglycinol-derived oxazolopiperidone lactams are exceptionally versatile building blocks for the enantioselective construction of structurally diverse piperidine-containing natural products and bioactive compounds. These lactams are readily available in both enantiomeric series by cyclocondensation of the chiral amino alcohol with a delta-oxo acid derivative and allow the substituents to be introduced at the different ring positions in a regio- and stereocontrolled manner, providing access to enantiopure polysubstituted piperidines bearing virtually any type of substitution pattern, and also quinolizidines, indolizidines, perhydroquinolines, hydroisoquinolines, as well as complex indole alkaloids. Of particular interest are cyclocondensation reactions with racemic or prochiral delta-oxo (di)acid derivatives in processes involving dynamic kinetic resolution and/or differentiation of enantiotopic or diastereotopic ester groups, as they directly lead to lactams that already incorporate the carbon substituents on the heterocyclic ring. The use of (S)-3,4-dimethoxyphenylalaninol or (S)-tryptophanol in the above cyclocondensation reactions expands the potential and the scope of the methodology, providing a straightforward route to enantiopure benzo[a]- and indolo[2,3-a]quinolizidines.

Tandem Staudinger-azaWittig mediated ring expansion: rapid access to new isofagomine-tetrahydroxyazepane hybrids

New seven-membered iminosugars with potent and selective inhibition towards glycosidases have been prepared as 1-N-iminosugar homologues via a tandem Staudinger-azaWittig mediated ring expansion.

Single Diastereomers of Polyhydroxylated 9-Oxa-1-azabicyclo[4.2.1]nonanes from Intramolecular 1,3-Dipolar Cycloaddition of ω-Unsaturated Nitrones

8-Benzyloxymethyl-3,4,5-tribenzoyloxy-9-oxa-1-azabicyclo[4.2.1]nonane has been prepared as the single diastereoisomer 8 from an intramolecular 1,3-dipolar cycloaddition involving 2-(benzyloxy)acetaldehyde and omega-unsaturated hydroxylamine 7 derived from methyl alpha-D-glucopyranoside. The analogous 8-methoxycarbonyl 9-oxa-1-azabicyclo[4.2.1]nonane was afforded in a similar manner, from methyl D-galactopyranoside and methyl glyoxylate, as a 3:1 mixture of diastereoisomers 15 and 16. When conducted in achiral ionic liquid 17 this ratio increased to 8:1, and in chiral ionic liquid 18, compound 15 was formed exclusively.

Convergent Approach toward the Synthesis of the Stereoisomers of C-6 Homologues of 1-Deoxynojirimycin and Their Analogues: Evaluation as Specific Glycosidase Inhibitors

A new and stereoselective strategy is developed to synthesize an appropriate template 9 to obtain C-6 homologues of 1-deoxyazasugars such as 1-deoxy-D-galactohomonojirimycin (5), 1-deoxy-4-hydroxymethyl-D-glucohomonojirimycin (6), and their enantiomers. The template 9 is also used to obtain neutral nonbasic pseudo-glyconolactam (8), C-4 amino, and methyl analogues of 1-deoxy-homonojirimycin as new analogues of 1-deoxyhomoazasugars. Compound 5 is found to be a potent and specific inhibitor to alpha-galactosidase (Ki = 1.7 microM). Similarly compounds 6 (Ki= 28 microM), ent-5 (Ki= 129 microM), and ent-6 (Ki= 12 microM) exhibited specific inhibition of beta-glucosidase.

A facile synthesis of 1,6-dideoxynojirimycin from l-sorbose

A practical synthesis of 1,6-dideoxynojirimycin, a potent glycosidase inhibitor, starting from L-sorbose, is described.

Synthesis of New Nitrogen Analogues of Salacinol and Deoxynojirimycin and Their Evaluation as Glycosidase Inhibitors

The synthesis of two enantiomerically pure iminosugars, analogues of 1-L-deoxynojirimycin (l-DNJ) and 1-D-deoxymannojirimycin (DMJ), was achieved using cyclic sulfate substituted isoxazoline derivatives. The piperidine ring was formed via the reduction of an isoxazoline into an amine which underwent a spontaneous intramolecular cyclization by reaction with the cyclic sulfate moiety. The nucleophilic attack of these two trisubstituted piperidines and morpholine on L- and D-erythritol-1,3-cyclic sulfates gave six new nitrogen analogues of salacinol. The inhibitory properties of the synthesized salacinol analogues were evaluated on several commercial glycosidases.

Synthesis of naturally occurring iminosugars from d-fructose by the use of a zinc-mediated fragmentation reaction

A short synthesis of 1,4-dideoxy-1,4-imino-D-arabinitol (DAB) and a formal synthesis of australine are described. In both cases, D-fructose is employed as the starting material and converted into a protected methyl 6-deoxy-6-iodo-furanoside. Zinc-mediated fragmentation produces an unsaturated ketone which serves as a key building block for both syntheses. Ozonolysis, reductive amination with benzylamine and deprotection affords 1,4-dideoxy-1,4-imino-D-arabinitol in only 7 steps and 11% overall yield from D-fructose. Alternatively, reductive amination with homoallylamine, ring-closing metathesis and protecting group manipulations give rise to an intermediate which can be converted into australine in 3 steps. The intermediate is prepared by two different strategies both of which use a total of 9 steps. The first strategy utilizes benzyl ethers for protection of fructose while the second and more effective strategy employs an isopropylidene acetal.

A concise enantioselective synthesis of iminosugar derivatives

The concise de novo synthesis of amino- and iminosugar derivatives is presented; the four stereocenters of the iminosugar derivatives are created in two-steps with high chemoselectivity and excellent enantioselectivity.

Nucleophilic opening of epoxyazepanes: expanding the family of polyhydroxyazepane-based glycosidase inhibitors

A range of new tetra- and pentahydroxylated seven-membered iminoalditols has been efficiently synthesized from epoxyazepane precursors via nucleophilic opening with hydride or oxygenated species and subsequent hydrogenolysis. One tetrahydroxylated azepane, a ring homologue of deoxymannojirimycin, displays a selective and fairly good inhibition of alpha-L-fucosidase.