Polyhydroxylated alkaloids -- natural occurrence and therapeutic applications

4'-(2,4-Dichloro-phen-yl)-1,1'-dimethyl-piperidine-3-spiro-3'-pyrrolidine-2'-spiro-3''-indoline-4,2''-dione

In the title compound, C₂₃H₂₃Cl₂N₃O₂, the pyrroline ring adopts an envelope conformation and the piperidinone ring assumes a slightly twisted chair form. In the crystal, inversion dimers linked by pairs of N—H⋯O hydrogen bonds generate an R₂⁸ graph-set motif and a short Cl⋯Cl contact of 3.478 (1) Å occurs.

Asymmetric total synthesis of 1-deoxy-7,8-di-epi-castanospermine

An efficient, stereocontrolled synthesis of (6S,7S,8S,8aR)-6,7,8-trihydroxyindolizidine (alias 1-deoxy-7,8-di-epi-castanospermine) (14) has been developed, which exploits an asymmetric vinylogous Mukaiyama aldol reaction (VMAR) between N-(tert-butoxycarbonyl)-2-(tert-butyldimethylsilyloxy)pyrrole (1) and 2,3-O-isopropylidene-D-glyceraldehyde (2) to construct the initial pyrrolidine building block 3, and an ene-ene ring closing metathesis reaction (RCM) (9 to 10) to install the indolizidine skeleton. The synthetic sequence was 13 steps, proceeding in 19.5% overall yield. The configurational and conformational structure of 14 was ascertained unambiguously and confronted to previously published assignments of rac-14 and ent-14.

2-Azido-3,4;6,7-di-O-isopropyl-idene-α-d-glycero-d-talo-heptopyran-ose

In the title compound, C(13)H(21)N(3)O(6), the six-membered ring adopts a twist-boat conformation with the azide group in the bowsprit position. The azide group is disordered over two sets of sites in a 0.642 (10):0.358 (10) ratio. The crystal structure consists of O-H⋯O hydrogen-bonded trimer units. The absolute configuration was determined from the use of d-mannose as the starting material.

Glycosidase inhibition: assessing mimicry of the transition state

Glycoside hydrolases, the enzymes responsible for hydrolysis of the glycosidic bond in di-, oligo- and polysaccharides, and glycoconjugates, are ubiquitous in Nature and fundamental to existence. The extreme stability of the glycosidic bond has meant these enzymes have evolved into highly proficient catalysts, with an estimated 10(17) fold rate enhancement over the uncatalysed reaction. Such rate enhancements mean that enzymes bind the substrate at the transition state with extraordinary affinity; the dissociation constant for the transition state is predicted to be 10(-22) M. Inhibition of glycoside hydrolases has widespread application in the treatment of viral infections, such as influenza and HIV, lysosomal storage disorders, cancer and diabetes. If inhibitors are designed to mimic the transition state, it should be possible to harness some of the transition state affinity, resulting in highly potent and specific drugs. Here we examine a number of glycosidase inhibitors which have been developed over the past half century, either by Nature or synthetically by man. A number of criteria have been proposed to ascertain which of these inhibitors are true transition state mimics, but these features have only be critically investigated in a very few cases.

The novel proapoptotic activity of nonnatural enantiomer of Lentiginosine

D-(-)-Lentiginosine [(-)-4], the nonnatural enantiomer of the iminosugar indolizidine alkaloid L-(+)-lentiginosine, acts as apoptosis inducer on tumor cells of different origin, in contrast to its natural enantiomer. Although D-(-)-4 exhibited a proapoptotic activity towards tumor cells at level lower than the chemotherapeutic agent, SN38, it was less proapoptotic towards normal cells and less cytotoxic. Apoptosis induced by D-(-)-4 was caspase-dependent, as shown by the increased expression and activity of caspase-3 and -8 in treated cells, and by inhibition following treatment with the pan caspase inhibitor, ZVAD-FMK. This study highlighted how a natural iminosugar alkaloid and its synthetic enantiomer, which were simply known for their inhibition against a fungal glucoamylase, could behave in a complete different way when tested towards cell growth and death of cells of different origin.

Protecting-group-free synthesis of 2-deoxy-aza-sugars

The protecting-group-free asymmetric synthesis of 1,2,4-trideoxy-1,4-imino-L-xylitol is readily achieved in five steps from 2-deoxy-D-ribose and with an overall yield of 48%. Key in this synthesis is the application of our recently developed Vasella-reductive amination and carbamate annulation methodologies to the synthesis of 2-deoxy-aza-sugars. The carbamate annulation occurred with excellent yield and diastereoselectively (>20:1 d.r.), in favour of the 3,4-cis isomer.

Intake of 1-deoxynojirimycin suppresses lipid accumulation through activation of the beta-oxidation system in rat liver

It was recently shown that administration of 1-deoxynojirimycin (DNJ) extracted from mulberry suppresses an increase in postprandial blood glucose in humans. These findings are of interest, but other physiological functions of DNJ are unknown. This study examined the effects of oral administration of DNJ (1 mg/kg of body weight/day) or mulberry extracts enriched in DNJ (meDNJ; 100 or 200 mg of extract/kg of body weight/day, equivalent to 0.53 or 1.06 mg of DNJ/kg of body weight/day) in male Sprague-Dawley rats for 4 weeks. DNJ and meDNJ enhanced expression of adiponectin mRNA in white adipose tissue; increased plasma adiponectin levels, enhanced expression of AMPK mRNA, activated the beta-oxidation system, and suppressed lipid accumulation in the liver. Intake of DNJ and meDNJ did not cause hepatic dysfunction and led to a reduction of oxidative stress. These results indicate the efficacy and safety of DNJ and meDNJ.

Steviamine, a new class of indolizidine alkaloid [(1R,2S,3R,5R,8aR)-3-hydroxy-meth-yl-5-methyl-octa-hydro-indolizine-1,2-diol hydro-bromide]

X-ray crystallographic analysis of the title hydro­bromide salt, C₁₀H₂₀N⁺·Br⁻, of (1R,2S,3R,5R,8aR)-3-hydroxy­meth­yl-5-methyl­octa­hydro­indolizine-1,2-diol defines the absolute and relative stereochemistry at the five chiral centres in steviamine, a new class of polyhydroxy­lated indolizidine alkaloid isolated from Stevia rebaudiana (Asteraceae) leaves. In the crystal structure, mol­ecules are linked by inter­molecular O—H⋯Br and N—H⋯Br hydrogen bonds, forming double chains around the twofold screw axes along the b-axis direction. Intra­molecular O—H⋯O inter­actions occur.

Synthesis and evaluation of D-gluco-pyranocyclopropyl amines as potential glucosidase inhibitors

In the recent past sugar-derived cyclopropylamines were proposed as structurally new glycosidase inhibitors. In this Letter we report our efforts in the synthesis of a set of alpha-glucose configured oxabicyclo[4.1.0] heptanes, based on this hypothesis, bearing an amine substituent on the propyl ring and reveal that their inhibitory potential towards a range of mammalian glucosidases is modest.

(2S,3R,4R,5R)-3,4-Dihydr-oxy-5-(hydroxy-meth-yl)pyrrolidine-2-carboxylic acid [(2S,3R,4R,5R)-3,4-dihydr-oxy-5-(hydroxy-meth-yl)proline]

The crystal structure of the title compound, C(6)H(11)NO(5), establishes the relative configuration at the four stereogenic centres; the absolute configuration is determined by the use of d-glucuronolactone as the starting material for the synthesis. Mol-ecules are linked by inter-molecular O-H⋯O and N-H⋯O hydrogen bonds into a three-dimensional network, with each mol-ecule acting as a donor and acceptor for five hydrogen bonds.

Pochonicine, a polyhydroxylated pyrrolizidine alkaloid from fungus Pochonia suchlasporia var. suchlasporia TAMA 87 as a potent beta-N-acetylglucosaminidase inhibitor

A new polyhydroxylated pyrrolizidine alkaloid designated as pochonicine (1) was isolated from a solid fermentation culture of the fungal strain Pochonia suchlasporia var. suchlasporia TAMA 87. The structure of 1 was determined using NMR and MS techniques as (1R*, 3S*, 5S*, 6S*, 7R*, 7a S*)-5-acetamidomethyl-3-hydroxymethyl-1,6,7-trihydroxypyrrolizidine. Pochonicine (1) showed potent inhibition against beta-N-acetylglucosaminidases (GlcNAcases) of various organisms including insects, fungi, mammals, and a plant but no inhibition against beta-glucosidase of almond, alpha-glucosidase of yeast, or chitinase of Bacillus sp. The GlcNAcase inhibitory activity of pochonicine (1) was comparable to nagstatin, a potent GlcNAcase inhibitor of natural origin.

2,6-Dide-oxy-2,6-imino-l-glycero-d-ido-heptitol

The title mol­ecule, C₇H₁₅NO₅, the major product from selective enzymatic oxidation followed by hydrogeno­lysis of the corresponding azido­heptitol, was found by X-ray crystallography to exisit in a chair conformation with three axial hydroxyl groups. One of the hydroxymethyl groups is disordered over two sets of sites in a 0.590 (3):0.410 (3) ratio. In the crystal, O—H⋯O, O—H⋯(O,O), O—H⋯N and N—H⋯O hydrogen bonding occurs.

Design, synthesis, and biological evaluation of enantiomeric beta-N-acetylhexosaminidase inhibitors LABNAc and DABNAc as potential agents against Tay-Sachs and Sandhoff disease

N-Acetylhexosaminidases are of considerable importance in mammals and are involved in various significant biological processes. In humans, deficiencies of these enzymes in the lysosome, resulting from inherited genetic defects, cause the glycolipid storage disorders Tay-Sachs and Sandhoff diseases. One promising therapy for these diseases involves the use of beta-N-acetylhexosaminidase inhibitors as chemical chaperones to enhance the enzyme activity above sub-critical levels. Herein we describe the synthesis and biological evaluation of a potent inhibitor, 2-acetamido-1,4-imino-1,2,4-trideoxy-L-arabinitol (LABNAc), in a high-yielding 11-step procedure from D-lyxonolactone. The N-benzyl and N-butyl analogues were also prepared and found to be potent inhibitors. The enantiomers DABNAc and NBn-DABNAc were synthesised from L-lyxonolactone, and were also evaluated. The L-iminosugar LABNAc and its derivatives were found to be potent noncompetitive inhibitors of some beta-N-acetylhexosaminidases, while the D-iminosugar DABNAc and its derivatives were found to be weaker competitive inhibitors. These results support previous work postulating that D-iminosugar mimics inhibit D-glycohydrolases competitively, and that their corresponding L-enantiomers show noncompetitive inhibition of these enzymes. Molecular modelling studies confirm that the spatial organisation in enantiomeric inhibitors leads to a different overlay with the monosaccharide substrate. Initial cell-based studies suggest that NBn-LABNAc can act as a chemical chaperone to enhance the deficient enzyme's activity to levels that may cause a positive pharmacological effect. LABNAc, NBn-LABNAc, and NBu-LABNAc are potent and selective inhibitors of beta-N-acetylhexosaminidase and may be useful as therapeutic agents for treating adult Tay-Sachs and Sandhoff diseases.

An atom economic synthesis and antitubercular evaluation of novel spiro-cyclohexanones

The 1,3-dipolar cycloaddition of azomethine ylides derived from acenaphthenequinone and alpha-amino acids viz. sarcosine, phenylglycine, 1,3-thiazolane-4-carboxylic acid and proline to a series of 2,6-bis[(E)-arylmethylidene]cyclohexanones afforded novel spiro-heterocycles chemo-, regio- and stereoselectively in quantitative yields. These compounds were screened for their in vitro activity against Mycobacterium tuberculosis H37Rv (MTB) using agar dilution method. Two compounds, 4-(2,4-dichlorophenyl)-5-phenylpyrrolo(spiro[2.2'']acenaphthene-1''-one)spiro[3.2']-6'-(2,4-dichlorophenylmethylidene)cyclohexanone (4i) and spiro[5.2'']acenaphthene-1''-onespiro[6.2']-6'-(2,4-dichlorophenylmethylidene)cyclohexanone-7-(2,4-dichlorophenyl)tetrahydro-1H-pyrrolo[1,2-c][1,3]thiazole (5i) display maximum activity in vitro with a MIC value of 0.40microg/mL against MTB and were 4 and 15.6 times more potent than ethambutol and pyrazinamide, respectively.

(4S,5R,6R)-Methyl 4-hydroxy-4,5-isopropylidenedioxy-4,5,6,7-tetrahydro-1,2,3-triazolo[1,5-a]pyridine-3-carboxylate

X-ray crystallography confirmed the structure of the title triazole, C₁₁H₁₅N₃O₅, formed from a single-step reaction of a sugar azide with a brominated ylid. The absolute configuration was determined by the use of d-ribose as the starting material. The six-membered ring is in a half-chair conformation. The crystal structure exists as chains of O—H⋯O hydrogen-bonded moleclues running parallel to the b axis.