7‐Oxanorbornane and Norbornane Mimics of a Distorted β‐D‐Mannopyranoside: Synthesis and Evaluation as β‐Mannosidase Inhibitors

The racemic 7‐oxanorbornanyl and norbornanyl aminoalcohols 3, 4, 42, 45 , and 46 were synthesized and tested as snail β‐mannosidase inhibitors. The amino tetraol 3 was obtained from the known sulfonyl acrylate 9 and furan 10 . Esterification provided 11 that underwent an intramolecular Diels–Alder reaction to the 7‐oxanorbornene 12 . Reduction of 12 to 13 , desulfonylation, isopropylidenation, and cis‐dihydroxylation gave 16 . A second isopropylidenation to 17 , followed by debenzylation and a Mitsunobu–Gabriel reaction provided 19 that was deprotected via 20 to 3 . Diels–Alder cycloaddition of furfuryl acetate and maleic anhydride to 21 , followed by alcoholysis of the anhydride, cis‐dihydroxylation, isopropylidenation, and Barton decarboxylation gave the ester 25 . Deacetylation to 26 and a Mitsunobu–Gabriel reaction led to 27 that was transformed into the N‐Boc analogue 29 , reduced to the alcohol 30 , and deprotected to 4 . The 1‐aminonorbornane 5 was obtained from Thiele's Acid 31 . Diels–Alder cycloaddition of the cyclopentadiene obtained by thermolysis of the diester 32 , methanolysis of the resulting anhydride 33 , dihydroxylation, isopropylidenation, Barton decarboxylation, and Curtius degradation led to the benzyl carbamate 39 that was reduced to the alcohol 40 , transformed into the N‐Boc carbamate 41 , and deprotected to 5 . The alcohol 40 was also transformed into the benzylamine 42 , aniline 45 , and hydroxylamine 46 . Snail β‐mannosidase was hardly inhibited by 3, 4, 42, 45 , and 46 . Only the amino triol 5 proved a stronger inhibitor. The inhibition by 5 depends on the pH value (at pH 3.5: K_i = 1900 μM ; at pH 4.5: K_i = 340 μm; at pH 5.5: K_i = 110 μm). The results illustrate the strong dependence of the inhibition by bicyclic mimics upon the precise geometry and orientation of the amino group as determined by the scaffold. It is in keeping with the hypothesis that the reactive conformation imposed by snail β‐mannosidase is close to a ^1,4B/¹S₃.     

Synthesis and Biological Evaluation of Andrographolide C‐Glycoside Derivatives as α‐Glycosidase Inhibitors

A series of new andrographolide C‐glycoside derivatives were synthesized by a facile route. The new compounds showed higher potency than the parent andrographolide evaluated as α‐glycosidase inhibitors in the preliminary study.     

Synthesis of 2‐Azabicyclo[3.2.2]nonane‐Derived Monosaccharide Mimics and Their Evaluation as Glycosidase Inhibitors

The racemic 2‐azabicyclo[3.2.2]nonanes 5 and 18 were synthesized and tested as β‐glycosidase inhibitors. The intramolecular Diels–Alder reaction of the masked o‐benzoquinone generated from 2‐(allyloxy)phenol ( 6 ) gave the α‐keto acetal 7 which was reduced with SmI₂ to the hydroxy ketone 8 . Dihydroxylation, isopropylidenation (→ 12 ), and Beckmann rearrangement provided lactam 15 . N‐Benzylation of this lactam, reduction to the amine 17 , and deprotection provided the amino triol 19 which was debenzylated to the secondary amine 5 . Both 5 and 19 proved weak inhibitors of snail β‐mannosidase (IC₅₀ > 10 mM ), Caldocellum saccharolyticum β‐glucosidase (IC₅₀ > 10 mM ), sweet almond β‐glucosidase (IC₅₀ > 10 mM ), yeast α‐glucosidase ( 5 : IC₅₀ > 10 mM ; 19 : IC₅₀ = 1.2 mM ), and Jack bean α‐mannosidase (no inhibition detected).     

Norbornane Mimics of Distorted β‐D‐Glucopyranosides – Inhibitors of β‐D‐Glucopyranosidases?

The racemic gluco‐configured norbornanes 4 and 16 were prepared and tested as inhibitors of β‐glucosidases. The known alcohol 5 was deprotected to provide the triol 6 . Silylation (→ 7 ), monobenzoylation (→ 8 / 9 ), and oxidation provided the regioisomeric ketones 10 and 11 . Reduction of 10 gave the desired endo‐alcohol 13 , albeit in low yield, while reduction of the isomeric ketone 11 provided mostly the altro‐configured endo‐alcohol 12 . The alcohol 13 was desilylated to 14 . Debenzoylation to 15 followed by hydrogenolytic deprotection gave the amino triol 4 that was reductively aminated to the benzylamine 16 . The amino triols 4 and 16 proved weak inhibitors of the β‐glucosidase from Caldocellum saccharolyticum ( 4 : IC₅₀ = 5.6 mm; 16 : IC₅₀ = 3.3 mm) and from sweet almonds ( 16 : IC₅₀ = 5.5 mm) . A comparison of 4 with the manno‐configured norbornane 3 shows that 3 is a better inhibitor of snail β‐mannosidase than 4 is of β‐glucosidases, in keeping with earlier results suggesting that these β‐glycosidases enforce a different conformational itinerary.     

Synthesis of Monosaccharide‐Derived Spirocyclic Cyclopropylamines and Their Evaluation as Glycosidase Inhibitors

The glucose‐, mannose‐, and galactose‐derived spirocyclic cyclopropylammonium chlorides 1a – 1d, 2a – 2d and 3a – 3d were prepared as potential glycosidase inhibitors. Cyclopropanation of the diazirine 5 with ethyl acrylate led in 71% yield to a 4 : 5 : 1 : 20 mixture of the ethyl cyclopropanecarboxylates 7a – 7d , while the Cu‐catalysed cycloaddition of ethyl diazoacetate to the exo‐glycal 6 afforded 7a – 7d (6 : 2 : 5 : 3) in 93–98% yield (Scheme 1). Saponification, Curtius degradation, and subsequent addition of BnOH or t‐BuOH led in 60–80% overall yield to the Z‐ or Boc‐carbamates 11a – 11d and 12a – 12d , respectively. Hydrogenolysis of 11a – 11d afforded 1a – 1d , while 12a – 12d was debenzylated to 13a – 13d prior to acidic cleavage of the N‐Boc group. The manno‐ and galacto‐isomers 2a – 2d and 3a – 3d , respectively, were similarly obtained in comparable yields (Schemes 2 and 4). Also prepared were the differentially protected manno‐configured esters 24a – 24d ; they are intermediates for the synthesis of analogous N‐acetylglucosamine‐derived cyclopropanes (Scheme 3). The cyclopropylammonium chlorides 1a – 1d, 2a – 2d and 3a – 3d are very weak inhibitors of several glycosidases (Tables 1 and 2). Traces of Pd compounds, however, generated upon catalytic debenzylation, proved to be strong inhibitors. PdCl is, indeed, a reversible, micromolar inhibitor for the β‐glucosidases from C. saccharolyticum and sweet almonds (non‐competitive), the β‐galactosidases from bovine liver and from E. coli (both non‐competitive), the α‐galactosidase from Aspergillus niger (competitive), and an irreversible inhibitor of the α‐glucosidase from yeast and the α‐galactosidase from coffee beans. The cyclopropylamines derived from 1a – 1d or 3a – 3d significantly enhance the inhibition of the β‐glucosidase from C. saccharolyticum by PdCl , lowering the K_i value from 40 μM (PdCl ) to 0.5 μM for a 1 : 1 mixture of PdCl and 1d . A similar effect is shown by cyclopropylamine, but not by several other amines.     

Synthesis of N‐Acetylglucosamine‐Derived Nagstatin Analogues and Their Evaluation as Glycosidase Inhibitors

The gluco‐configured analogue 15 of nagstatin ( 1 ) and the methyl ester 14 were synthesized via condensation of the thionolactams 17 or 18 with the β‐amino ester 19 . The silyl ethers 20 and 21 resulting from 17 were desilylated to 22 and 23 ; these alcohols were directly obtained by condensing 18 and 19 . The attempted substitution of the C(8)? OH group of 22 by azide under Mitsunobu conditions led unexpectedly to the deoxygenated α‐azido esters 24 . The desired azide 25 was obtained by treating the manno‐configured alcohol 23 with diphenyl phosphorazidate. The azide was transformed to the debenzylated acetamido ester 14 that was hydrolyzed to the nagstatin analogue 15 . The imidazole‐2‐acetates 14 and 15 are nanomolar inhibitors of the N‐acetyl‐β‐glucosaminidases from Jack beans and from bovine kidney, submicromolar to micromolar inhibitors of the β‐glucosidase from Caldocellum saccharolyticum, and rather weak inhibitors of the snail β‐mannosidase. In all cases, the ester was a stronger inhibitor than the corresponding acid. As expected from their gluco‐configuration, both imidazopyridines 14 and 15 are stronger inhibitors of the β‐N‐acetylglucosaminidase from bovine kidney than nagstatin.     

Synthesis of C(2)‐Substituted manno‐Configured Tetrahydroimidazopyridines and Their Evaluation as Inhibitors of Snail β‐Mannosidase

It was shown that retaining β‐glucosidases and galactosidases of families 1–3 feature a strong interaction between C(2)OH of the substrate and the catalytic nucleophile. An analogous interaction can hardly take place for retaining β‐mannosidases. A structure? activity comparison between the inhibition of the β‐glucosidase from Caldocellum saccharolyticum (family 1) and β‐glucosidase from sweet almonds by the gluco‐imidazoles 1 – 6 , and the inhibition of snail β‐mannosidase by the corresponding manno‐imidazoles 8 – 13 does not show any significant difference, suggesting that also the mechanisms of action of these glycosidases do not differ significantly. For this comparison, we synthesized and tested the manno‐imidazoles 9 – 13, 28, 29, 32, 35, 40, 41, 43, 46, 47 , and 50 . Among these, the alkene 29 is the strongest known inhibitor of snail β‐mannosidase (K_i=6 nM , non‐competitive); the aniline 35 is the strongest competitive inhibitor (K_i=8 nM ).     

Synthesis and Evaluation as Glycosidase Inhibitors of 1H-Imidazol-2-yl C-Glycopyranosides

The (1H-imidazol-2-yl)ulose 8 and the 1H-imidazol-2-yl C-glycopyranosides 23 and 24 have been prepared from tetra-O-benzylgluconolactone 6 in two and six steps, respectively. The imidazoles 8 and 24 are moderate competitive inhibitors of sweet-almond β-glucosidase (pH 6.8, K_i ≈ 0.79 and 0.64 mM , respectively), while 23 is a competitive inhibitor of yeast α-glucosidase (pH 6.8, K_i ≈ 0.26 mM ). Addition of 2-lithiated 1-[(dimethylamino)methyl]-1H-imidazole to 6 gave the ulose 7 (68%), which was deprotected to 8 . Reduction of 7 with NaBH₄ yielded a 12:88 mixture 10/11 . Attempts to selectively mesylate HO? C(1) of these diols failed, while dinitrobenzoylation led to 19/20 , which cyclized easily (NaH) to a 25:75 mixture of 21 and 22 which were separated and debenzylated to the C-glycosides 23 and 24 .     

Synthesis and Enzymatic Evaluation of Substrates and Inhibitors of β-Glucuronidases

The phosphono and the tetrazolyl analogues 4 and 5 of 4-methylumbelliferyl β-D -glucuronide (=(4-methyl-2-oxo-2H-1-benzopyran-7-yl β-D -glucopyranosid)uronic acid; 6 ) were synthesized and evaluated as substrates of β-glucuronidases. Similarly, the phenylcarbamate 7 and its phosphono analogue 8 were prepared and evaluated as inhibitors. To examine the diastereoselectivity of the phosphorylation, we also synthesized the protected L -ido-D -gluco-, and D -galacto-configurated phospha-glycopyranuronates 12, 13, 21, 22, 34 and 35 . Two strategies were followed. In the first one, the glucuronic acid 19 was decarboxylated to 11 and further transformed, via 20 , into the trichloroacetimidate 10 (Scheme 2). Phosphorylation of 10 with (MeO)₃P yielded the diastereoisomers 12 and 13 , the diastereoselectivity depending on the solvent. In MeCN, 12 and 13 were obtained in a ratio of 1:1, while in non-participating solvents the L -ido 12 was by far the major diastereoisomer. The acetate 11 was inert to (MeO)₃P, but reacted with (PhO)₃P to the anomeric mixture 21/22 , in keeping with a stabilizing 1,3-interaction in the intermediate phosphonium salt. Similarly, the phospha-galacturonates 34 and 35 were prepared from the galactoside 23 via the enol ether 26 , the lactone 27 , and the acetates 28/29 that were also transformed into the trichloroacetimidate 33 (Scheme 3). In the second, higher-yielding strategy, phosphorylation of the pentodialdehyde 39 to 40/41 was followed by hydrolysis and acetylation to the phospha-glucuronates 43/44 (Scheme 4). Transesterification to 45/46 , selective deacetylation to 48/49 , and formation of the trichloroacetimidates 50/51 were followed by glycosidation and deprotection to 4 . The tetrazole 5 was prepared from the lactones 54/55 via the N-benzylamides 57/58 that were treated with TfN₃ to give the N-benzyltetrazoles 59/60 (Scheme 4). These were transformed into the trichloroacetimidates 63/64 , glycosylated to 65 , and deprotected. The O-carbamoylhydroximo-lactone 7 derived from the glucuronate 67/68 , and the phosphonate analogue 8 were prepared by established methods. The phosphonate 4 is slowly hydrolyzed by the E. coli β-glucuronidase, but neither 4 nor the tetrazole 5 are affected by the bovine liver β-glucuronidase (Table 4). The phenylcarbamate 7 of D -glucarhydroximo-1,5-lactone, but not its phosphonate analogue 8 , is an inhibitor (K_I = 8 m?M ) of the E. coli β-glucuronidase. The bovine liver β-glucuronidase is inhibited strongly by 7 (IC₅₀ = 0.2 m?M ) and weakly by 8 (IC₅₀ = 2mM ).     

Synthesis and Evaluation as Glycosidase Inhibitors of Carbasugar‐Derived Spirodiaziridines,Spirodiazirines, and Spiroaziridines

The spirodiaziridines 6 and 9 , potential inhibitors of α‐ and β‐glucosidases, were prepared from the validoxylamine A‐derived cyclohexanone 5 . The trimethylsilyl protecting groups of 5 are crucial for the formation of 6 in good yields. Oxidation of 6 gave 7 . The diaziridine 6 (pK_HA=2.6) and the diazirine 7 did not inhibit the β‐glucosidases from almonds, the β‐glucosidase from Caldocellum saccharolyticum, and the α‐glucosidase from yeast. The N‐benzyl diaziridine 9 is a very weak inhibitor of the α‐glucosidase, but did not inhibit the β‐glucosidases. To see whether the weak inhibition is due to the low basicity of the diaziridines or to geometric factors, we prepared the spiro‐aziridines 21 and 25 and 1‐epivalidamine ( 32 ). The known cyclohexanone 10 was methylenated and epoxidised to 16 and 17 . Azide opening of 16 and 17 , mesylation, LiAlH₄ reduction, and deprotection gave the aziridines 21 and 25 respectively. 1‐Epivalidamine ( 32 ) was prepared from the known carba‐glucose 29 . The aziridine 25 (pK_HA=6.8) is a weak irreversible inhibitor of the β‐glucosidase from Caldocellum saccharolyticum and a weak reversible inhibitor of the α‐glucosidase from yeast, but did not inhibit the β‐glucosidases from almonds. The poorly stable aziridine 21 weakly inhibited the three enzymes. Similarly, 1‐epivalidamine (pK_HA=8.4) proved only a weak inhibitor. The known cyclopentylamine 34 (pK_HA=7.9), however, is a micromolar inhibitor of these enzymes. The much stronger inhibition by 34 is related to the pseudoaxial orientation of its amino group.     

Indolo‐Phakellins as β5‐Specific Noncovalent Proteasome Inhibitors

The proteasome represents an invaluable target for the treatment of cancer and autoimmune disorders. The application of proteasome inhibitors, however, remains limited to blood cancers because their reactive headgroups and peptidic scaffolds convey unfavorable pharmacodynamic properties. Thus, the discovery of more drug‐like lead structures is indispensable. In this study, we present the first structure of the proteasome in complex with an indolo‐phakellin that exhibits a unique noncovalent binding mode unparalleled by all hitherto reported inhibitors. The natural product inspired pentacyclic alkaloid binds solely and specificially into the spacious S3 subpocket of the proteasomal β5 substrate binding channel, gaining major stabilization through halogen bonding with the protein backbone. The presented compound provides an ideal scaffold for the structure‐based design of subunit‐specific nonpeptidic proteasome‐blockers.     

5-雄烯-3β, 7α, 17β-三醇和5-雄烯-3β, 7β, 17β-三醇的合成

以5-雄烯二醇为原料,用微生物转化的方法合成了两个重要的神经甾体5-雄烯-3β, 7α, 17β-三醇和5-雄烯-3β, 7β, 17β-三醇。所用菌种总枝毛霉为我们自己筛选,并首次应用于5-雄烯-3β, 7α, 17β-三醇和5-雄烯-3β, 7β, 17β-三醇的合成中。     

2‐Phenylquinoline–Sugar Hybrids as Photoswitchable α‐Glucosidase Inhibitors

Purpose‐designed 2‐phenylquinoline (PQ)‐sugar hybrids 1 and 2 were synthesized and evaluated for their photodegradation activities against an α‐glucosidase target. The results indicated that PQ‐mannose hybrid 2 selectively and effectively photodegraded α‐glucosidase and significantly inhibited its enzymatic activity upon irradiation with long‐wavelength UV light in the absence of any additives under neutral and aqueous conditions. Furthermore, 2 selectively and effectively inhibited α‐glucosidase activity only with photo‐irradiation even in complex cell lysate.     

Synthesis of β‐Arylacyl/β‐Heteroarylacyl‐β‐alkylidene Malonates and Their Application in Substituted Pyridone Synthesis

A novel approach has been developed for the synthesis of β‐arylacyl/β‐heteroarylacyl‐β‐alkylidine malonates in moderate to good yields by the reaction of Stork aryl and heteroaryl enamine with β‐chloroalkylidene malonates. The reaction involves conjugate (Michael) addition of Stork enamine on β‐chloroalkylidene malonates and elimination of chloride ion. These Michael adducts were utilized as intermediates for the synthesis of highly substituted 1,4‐dialkyl‐2‐oxo‐6‐aryl/hetreoaryl‐1,2‐dihydro‐pyridine‐3‐carboxylic acid ethyl esters via 5 + 1 ring annulation protocol.