Tacrines as Therapeutic Agents for Alzheimer's Disease. IV. The Tacripyrines and Related Annulated Tacrines

Notwithstanding the clinical use of tacrine was hampered by severe hepatotoxicity, tacrine still remains a reference scaffold in the search for new efficient drugs for Alzheimer's disease therapy. In this account we summarize the efforts toward the development and characterization of non‐hepatotoxic tacripyrines and related tacrine analogues resulting from the substitution of the benzene ring by a 1,4‐dihydropyridine, a 1,2,3,4‐tetrahydropyrimidine or a pyridone nucleus. These efforts have successfully led to the identification of a number of promising hits endowed with interesting multifunctional profiles. These include the 4′‐metoxytacripyrine (S)‐ ITH122 , able to target cholinesterases (ChEs), beta‐amyloid (Aβ) and Ca²⁺ channels, the racemic 3′‐methoxytacripyrimidine EB65F2 , the first fully balanced micromolar inhibitor of ChEs and Ca²⁺ channels, and tacripyrine (?)‐SCR1693 a GSK‐3β (enzyme involved in tau phosphorylation) inhibitor able to also lower Aβ production in N2a cells.     

Inhibitors Designed for Presenilin 1 by Means of Aspartic Acid Activation. Preliminary Communication

γ‐Secretase, a multiprotein aspartic protease crucial to Alzheimer's dementia, is not available for NMR experiments and has, so far, escaped crystallization. A positional scan of the aspartic protease by reactive probes may provide the necessary structural information for drug development. We describe here the synthesis of acid‐labile compounds based on the known inhibitor DAPT ( 1 ), e.g., the N‐terminally functionalized diazo compound 4 or the C‐terminally acid‐labile (cyclopropylmethyl)ester 11 , which were designed to react in the specific acidic active‐site environment of the aspartic protease presenilin 1. The acid‐labile DAPT analogues 11 – 13 , indeed, displayed strong inhibition in a cell‐free γ‐secretase assay.     

Structure‐Reactivity Relationships as Probes for the Inhibition Mechanism of Cholesterol Esterase by Aryl Carbamates. I. Steady‐State Kinetics

For substituted phenyl‐N‐butyl carbamates (1) and 4‐nitrophenyl‐N‐substituted carbamates (2), linear relationships between values of NH proton chemical shift (δ_NH), pKa, and logk_[OH] and Hammett substituent constant (σ) or Taft substituent constant (σ*) are observed. Carbamates 1 and 2 are pseudo‐substrate inhibitors of porcine pancreatic cholesterol esterase. Thus, the mechanism of the reaction necessitates that the inhibitor molecule and the enzyme form the enzyme‐inhibitor tetrahedral species at the K_i step of the reaction and then form the carbamyl enzyme at the k_c step of the reaction. Linear relationships between the logarithms of K_i and k_c for cholesterol esterase by carbamates 1 and σ are observed, and the reaction constants (ρs) are ?3.4 and ?0.13, respectively. Therefore, the above reaction forms the negative‐charge tetrahedral species and follows the formation of the relatively neutral carbamyl enzymes. For the inhibition of cholesterol esterase by carbamates 2 except 4‐nitrophenyl‐N‐phenyl carbamate and 4‐nitrophenyl‐N‐t‐butyl carbamate, linear relationships of ‐logK_i and logk_c with σ* are observed and the ρ* values are ?0.50 and 1.03, respectively. Since the above reaction also forms the negative‐charge tetrahedral intermediate, it is possible that the K_i step of this reaction is further divided into two steps. The first K_i step is the development of the positive‐charge at the carbamate nitrogen from the protonation of the carbamate nitrogen. The second K_i step is the formation of the tetrahedral intermediate with the negative‐charge at the carbonyl oxygen. From Arrhenius plots of a series of inhibition reactions by carbamates 1 and 2, the isokinetic and isoequilibrium temperatures are different from the reaction temperature (25°C). Therefore, the observed ρ and ρ* values only depend upon the electronic effects of the substituents. Taken together, the cholesterol esterase inhibition mechanism by carbamates 1 and 2 is proposed.     

A Strategy for Neuraminidase Inhibitors Using Mechanism‐Based Labeling Information

A potent inhibitor for Vibrio cholerae neuraminidase (VCNA) was developed by using a novel two‐step strategy, a target amino acid validation using mechanism‐based labeling information, and a potent inhibitor search using a focused library. The labeling information suggested the hidden dynamics of a loop structure of VCNA, which can be a potential target of the novel inhibitor. A focused library composed of 187 compounds was prepared from a 9‐azide derivative of 2,3‐dehydro‐N‐acetylneuraminic acid (DANA) to interrupt the function of the loop of the labeled residues. Inhibitor 3c showed potent inhibition properties and was the strongest inhibitor with FANA, a N‐trifluoroacetyl derivative of DANA. Validation studies of the inhibitor with a detergent and a Lineweaver–Burk plot suggested that the 9‐substitution group would interact hydrophobically with the target loop moiety, adding a noncompetitive inhibition property to the DANA skeleton. This information enabled us to design compound 4 having the combined structure of 3c and FANA. Compound 4 showed the most potent inhibition (K_i=73 nM , mixed inhibition) of VCNA with high selectivity among the tested viral, bacterial, and mammal neuraminidases.     

Structure‐Reactivity Relationships as Probes to Acetylcholinesterase Inhibition Mechanisms by Aryl Carbamates. I. Steady‐State Kinetics

From enzyme kinetics, 4‐nitrophenyl‐N‐substituted carbamates 1 are characterized as pseudo‐substrate inhibitors of acetylcholinesterase. However, the activity of the carbamyl enzyme does not recover in the presence of a competitive inhibitor, edrophonium. Therefore, carbamates 1 should be called as the “pseudo‐pseudo‐substrate” inhibitors of the enzyme. Moreover, the ‐logK_i, logk_c, and logk_i values are linearly correlated with Taft‐Ingold equation, log (k/k_o) = ρ*σ* + δ E_s. A three‐step AChE inhibition mechanism by carbamates 1 is proposed. The first step is the pre‐equilibrium protonations of carbamates 1 with ρ* value of ?1.4 from pK_a‐σ*‐correlation. The second step is the enzyme‐carbamates 1 tetrahedral intermediate formation from nucleophilic attack of the active site Ser200 on the protonated carbamates 1 . The ρ* value for the ‐logK_i‐σ*‐E_s‐correlation indicates that the true ρ* value for the second step is 0.5 [= ?0.9 ‐ (‐1.4)]. The δ value of 0.56 for the ‐logK_i‐σ*‐E_s‐correlation indicates that carbamates 1 with bulky substituents retarded the formation of enzyme‐inhibitor tetrahedral intermediates. The third step (k_c step) is the carbamylation step and is the carbamyl enzyme conjugate formation from the enzyme‐carbamates 1 tetrahedral intermediate. The ρ* value of 0.21 for the logk_c‐correlation indicates that the transition state for the carbamylation step is more negative charge than the enzyme‐carbamates 1 tetrahedral intermediate. Moreover, the k_c step is insensitive to substituent effects since there is a cancellation of electronic demands for bond‐making and bond‐breaking components, like S_N2 reactions. The δ value of 0.00 for the logk_c‐correlation indicates that the k_c step is independent of substituent steric effect. Therefore, the product of this step carbamyl enzyme conjugate is as crowded as the enzyme‐carbamates 1 tetrahedral intermediate and is likely bound to the leaving group, p‐nitrophenol.     

(1S,2S,3R,6R)‐6‐Aminocyclohex‐4‐ene‐1,2,3‐triol (= (−)‐Conduramine B‐1) Is a Selective Inhibitor of α‐Mannosidases. Its Inhibitory Activity Is Enhanced by N‐Benzylation

(−)‐ and (+)‐Conduramine B‐1 ((−)‐ and (+)‐ 5 , resp.) have been derived from (+)‐ and (−)‐7‐oxabicyclo[2.2.1]hept‐5‐en‐2‐one (‘naked sugars’ of the first generation). Although (−)‐ 5 imitates the structure of β‐glucosides, it does not inhibit β‐glucosidases but inhibits α‐mannosidases selectively. N‐Benzylation of (−)‐ 5 improves the potency of conduramine B‐1 as α‐mannosidase inhibitor and also generates compounds inhibiting β‐glucosidases. For instance, (−)‐N‐benzyl‐conduramine B‐1 ((−)‐ 19a ) is a competitive inhibitor of β‐glucosidase from almonds (IC₅₀ = 32 μM , K_i = 10 μM ) and a weak inhibitor of α‐mannosidases from jack bean (IC₅₀ = 171 μM ) and from almonds (IC₅₀ = 225 μM ) whereas (−)‐N‐(4‐phenylbenzyl)conduramine B‐1 ((−)‐ 19g ) is a good inhibitor of α‐mannosidase from jack beans (IC₅₀ = 29 μM , K_i = 4.8 μM ) and a weaker inhibitor of β‐glucosidase from almonds (IC₅₀ = 32 μM , K_i = 7.8 μM ) (Table 1).     

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.     

Inhibition of Diamino Pelargonic Acid Aminotransferase,an Enzyme of the Biotin Biosynthetic Pathway,by Amiclenomycin: A Mechanistic Study

The mechanism of action of amiclenomycin ( 1a ), a naturally occuring inhibitor of diaminopelargonic acid aminotransferase, has been established. The enzyme catalyzes the formation of an aromatic adduct between the inhibitor and pyridoxal‐5′‐phosphate. The structure of the adduct, determined by mass spectrometry, is in agreement with the reported X‐ray crystal structure. Kinetic parameters, characteristic of k_cat inhibitors, have been observed, with a K_I value of 2 μM and a k_inact value of 0.4 min^?1. The irreversibility of the inactivation observed, in spite of the absence of covalent bond between the inhibitor and the protein, reveals the high affinity of the adduct for the active site. Two other cis‐1‐amino‐4‐substituted‐cyclohexa‐2,5‐dienes, 3a and 4a , were also found to efficiently inhibit the enzyme. The trans‐isomers were either much less potent ( 1b ) or inactive ( 3b and 4b ). The aminocyclohexadiene moiety, which is, apparently, responsible for the inhibition, could constitute an original pharmacophore for the design of new herbicides.     

Insights into the Synthesis of Steroidal A‐Ring Olefins

The classical synthesis, followed by purification of the steroidal A‐ring Δ¹‐olefin, 5α‐androst‐1‐en‐17‐one ( 5 ), from the Δ¹‐3‐keto enone, (5α,17β)‐3‐oxo‐5‐androst‐1‐en‐17‐yl acetate ( 1 ), through a strategy involving the reaction of Δ¹‐3‐hydroxy allylic alcohol, 3β‐hydroxy‐5α‐androst‐1‐en‐17β‐yl acetate ( 2 ), with SOCl₂, was revisited in order to prepare and biologically evaluate 5 as aromatase inhibitor for breast cancer treatment. Surprisingly, the followed strategy also afforded the isomeric Δ²‐olefin 6 as a by‐product, which could only be detected on the basis of NMR analysis. Optimization of the purification and detection procedures allowed us to reach 96% purity required for biological assays of compound 5 . The same synthetic strategy was applied, using the Δ⁴‐3‐keto enone, 3‐oxoandrost‐4‐en‐17β‐yl acetate ( 8 ), as starting material, to prepare the potent aromatase inhibitor Δ⁴‐olefin, androst‐4‐en‐17‐one ( 15 ). Unexpectedly, a different aromatase inhibitor, the Δ^3,5‐diene, androst‐3,5‐dien‐17‐one ( 12 ), was formed. To overcome this drawback, another strategy was developed for the preparation of 15 from 8 . The data now presented show the unequal reactivity of the two steroidal A‐ring Δ¹‐ and Δ⁴‐3‐hydroxy allylic alcohol intermediates, 3β‐hydroxy‐5α‐androst‐1‐en‐17β‐yl acetate ( 2 ) and 3β‐hydroxyandrost‐4‐en‐17β‐yl acetate ( 9 ), towards SOCl₂, and provides a new strategy for the preparation of the aromatase inhibitor 12 . Additionally, a new pathway to prepare compound 15 was achieved, which avoids the formation of undesirable by‐products.     

Inhibition effect of azole derivate on corrosion activity of copper in rolling oil

Corrosion rates are influenced by the formation of inhibitor aggregates on the copper surface. Laser scanning confocal microscope was used to investigate the adsorbed structures of benzotriazole, N‐((6‐methyl‐1H‐benzo[d][1,2,3]triazol‐1‐yl)methyl)‐N‐octyloctan‐1‐amine (MBA) and 2,5‐bis (ethyldisulfanyl)‐1,3,4‐thiadiazole at copper surface in relation to their performance as a corrosion inhibitor. The increase of water contact angle in the presence of inhibitor indicates its strong adsorption to the copper, and laser scanning confocal microscope visualization confirms the formation of MBA aggregates. The aggregates change from hemispherical to cylindrical shape with MBA concentration increasing in rolling oil, resulting in a decrease in corrosion rates as determined by mass loss measurements. Compared with 2,5‐bis (ethyldisulfanyl)‐1,3,4‐thiadiazole, oil‐added MBA has a quicker adsorption and formation speed of cylindrical aggregates and a lower corrosion rate. The adsorption of inhibitors on copper surface obeys Langmuir isotherm and physisorption and chemisorption mechanism. Copyright © 2015 John Wiley & Sons, Ltd.     

Structural and Kinetic Dissection of the endo‐α‐1,2‐Mannanase Activity of Bacterial GH99 Glycoside Hydrolases from Bacteroides spp.

Glycoside hydrolase family 99 (GH99) was created to categorize sequence‐related glycosidases possessing endo‐α‐mannosidase activity: the cleavage of mannosidic linkages within eukaryotic N‐glycan precursors (Glc_1–3Man₉GlcNAc₂), releasing mono‐, di‐ and triglucosylated‐mannose (Glc_1–3‐1,3‐Man). GH99 family members have recently been implicated in the ability of Bacteroides spp., present within the gut microbiota, to metabolize fungal cell wall α‐mannans, releasing α‐1,3‐mannobiose by hydrolysing αMan‐1,3‐αMan→1,2‐αMan‐1,2‐αMan sequences within branches off the main α‐1,6‐mannan backbone. We report the development of a series of substrates and inhibitors, which we use to kinetically and structurally characterise this novel endo‐α‐1,2‐mannanase activity of bacterial GH99 enzymes from Bacteroides thetaiotaomicron and xylanisolvens. These data reveal an approximate 5 kJ mol^?1 preference for mannose‐configured substrates in the ?2 subsite (relative to glucose), which inspired the development of a new inhibitor, α‐mannopyranosyl‐1,3‐isofagomine (ManIFG), the most potent (bacterial) GH99 inhibitor reported to date. X‐ray structures of ManIFG or a substrate in complex with wild‐type or inactive mutants, respectively, of B. xylanisolvens GH99 reveal the structural basis for binding to D ‐mannose‐ rather than D ‐glucose‐configured substrates.     

Synthesis and Evaluation as Glycosidase Inhibitors of Isoquinuclidines Mimicking a Distorted β‐Mannopyranoside

Racemic and enantiomerically pure manno‐configured isoquinuclidines were synthesized and tested as glycosidase inhibitors. The racemic key isoquinuclidine intermediate was prepared in high yield by a cycloaddition (tandem Michael addition/aldolisation) of the 3‐hydroxy‐1‐tosyl‐pyridone 10 to methyl acrylate, and transformed to the racemic N‐benzyl manno‐isoquinuclidine 2 and the N‐unsubstituted manno‐isoquinuclidine 3 (twelve steps; ca. 11% from 10 ). Catalysis by quinine of the analogous cycloaddition of 10 to (?)‐8‐phenylmenthyl acrylate provided a single diastereoisomer in high yield, which was transformed to the desired enantiomerically pure D ‐manno‐isoquinuclidines (+)‐ 2 and (+)‐ 3 (twelve steps; 23% from 10 ). The enantiomers (?)‐ 2 and (?)‐ 3 were prepared by using a quinidine‐promoted cycloaddition of 10 to the enantiomeric (+)‐8‐phenylmenthyl acrylate. The N‐benzyl D ‐manno‐isoquinuclidine (+)‐ 2 is a selective and slow inhibitor of snail β‐mannosidase. Its inhibition strength and type depends on the pH (at pH 4.5: K_i=1.0 μM , mixed type, α=1.9; at pH 5.5: K_i=0.63 μM , mixed type, α=17). The N‐unsubstituted D ‐manno‐isoquinuclidine (+)‐ 3 is a poor inhibitor. Its inhibition strength and type also depend on the pH (at pH 4.5: K_i=1.2?10³ μM , mixed type, α=1.1; at pH 5.5: K_i=0.25?10³ μM , mixed type, α=11). The enantiomeric N‐benzyl L ‐manno‐isoquinuclidine (?)‐ 2 is a good inhibitor of snail β‐mannosidase, albeit noncompetitive (at pH 4.5: K_i=69 μM ). The N‐unsubstituted isoquinuclidine (?)‐ 2 is a poor inhibitor (at pH 4.5: IC₅₀=7.3?10³ μM ). A comparison of the inhibition by the pure manno‐isoquinuclidines (+)‐ 2 and (+)‐ 3 , (+)‐ 2 /(?)‐ 2 1 : 1, and (+)‐ 3 /(?)‐ 3 1 : 1 with the published data for racemic 2 and 3 led to a rectification of the published data. The inhibition of snail β‐mannosidase by the isoquinuclidines 2 and 3 suggests that the hydrolysis of β‐D ‐mannopyranosides by snail β‐mannosidase proceeds via a distorted conformer, in agreement with the principle of stereoelectronic control.     

Terbogrel,a dual‐acting agent for thromboxane receptor antagonism and thromboxane synthase inhibition

Terbogrel, (E)‐6‐[4‐(3‐tert‐butyl‐2‐cyano­guanidino)­phenyl]‐6‐(3‐pyridyl)­hex‐5‐enoic acid, C₂₃H₂₇N₅O₂, a mixed thromboxane A₂ receptor antagonist and thromboxane A₂ synthase inhibitor, shows a hairpin‐like conformation stabilized by an intramolecular hydrogen bond. A structural feature characteristic of the thromboxane A₂ synthase inhibitor mode is observed: a distance of 8.4257 (19) Å between the pyridine N atom and the carboxyl group.     

Synthesis of Enantiomerically Pure 1,5,5‐Trideuterated cis‐ and trans‐2,4‐Dioxa‐3‐phosphadecalins. 31P‐NMR Evidence of Covalent‐Bond Formation and the Stereochemical Implications in the Course of the Inhibition of δ‐Chymotrypsin

The irreversible inhibition of δ‐chymotrypsin with the enantiomerically pure, P(3)‐axially and P(3)‐equatorially X‐substituted cis‐ and trans‐configurated 2,4‐dioxa‐3‐phospha(1,5,5‐²H₃)bicyclo[4.4.0]decane 3‐oxides (X=F, 2,4‐dinitrophenoxy) was monitored by ³¹P‐NMR spectroscopy. ¹H‐Correlated ³¹P{²H}‐NMR spectra enabled the direct observation of the vicinal coupling (³J) between the P‐atom of the inhibitor and the CH₂O moiety of Ser¹⁹⁵ (=‘Ser¹⁹⁵’(CH₂O)), thus establishing the covalent nature of the ‘Ser¹⁹⁵’(CH₂O? P) bond in the inhibited enzyme. The stereochemical course of the phosphorylation is dependent on the structure of the inhibitor, and neat inversion, both inversion and retention, as well as neat retention of the configuration at the P‐atom was found.     

Collagenase produced from Aspergillus sp. (UCP 1276) using chicken feather industrial residue

An extracellular collagenolytic serine protease was purified from Aspergillus sp., isolated from the Caatinga biome in northeast Brazil by a two‐step chromatographic procedure, using an anion‐exchanger and gel filtration. The enzyme was produced by submerged fermentation of feather residue as a substrate. The purified collagenase showed a 2.09‐fold increase in specific activity and 22.85% yield. The enzyme was a monomeric protein with a molecular mass of 28.7 kDa, estimated by an SDS–PAGE and AKTA system. The optimum temperature and pH for enzyme activity were around 40°C and pH 8.0, respectively. The enzyme was strongly inhibited by phenyl‐methylsulfonyl fluoride, a serine protease inhibitor, and was thermostable until 65°C for 1 h. We then evaluated the enzyme's potential for degradation of Type I and Type V collagens for producing peptides with antifungal activity. Our results revealed that the cleavage of Type V collagen yielded more effective peptides than Type I, inhibiting growth of Aspergillus terreus , Aspergillus japonicus and Aspergillus parasiticus . Both groups of peptides (Type I and Type V) were identified by SDS–PAGE. To conclude, the thermostable collagenase we purified in this study has various potentially useful applications in the fields of biochemistry, biotechnology and biomedical sciences.     

Design,Docking, and Synthesis of Some New Pyrazoline and Pyranopyrazole Derivatives as Anti‐inflammatory Agents

Design and synthesis of some novel pyrazoline and pyranopyrazole derivatives as potential anti‐inflammatory agents are described. Most of the compounds were tested for their anti‐inflammatory (in vitro and in vivo) and ulcerogenic activities. In all tested compounds, it was found that pyrazolines, 2a , and pyrazolopyrano[2,3‐d]pyrimidine 9 are the potent anti‐inflammatory and selective cyclooxygenase‐2 (COX‐2) inhibitor. All compounds are mainly in the safe level. Docking study of 2a and 9 revealed higher affinity for binding with the active site of COX‐2 enzyme like SC‐558, a selective COX‐2 inhibitor.     

An Improved Synthesis of 2‐Acetamido‐2‐deoxy‐ D‐gluconohydroximolactone (PUGNAc), A Strong Inhibitor of β‐N‐Acetylglucosaminidases

O‐(2‐Acetamido‐3,4,6‐tri‐O‐acetyl‐D ‐glucopyranosylidene)amino N‐phenylcarbamate ( 1 ), an established inhibitor of β‐N‐acetylglucosaminidases, has been prepared by an improved six‐step synthesis from N‐acetyl‐D ‐glucosamine.     

Dispersive solid‐phase extraction combined with dispersive liquid‐liquid microextraction for simultaneous determination of seven succinate dehydrogenase inhibitor fungicides in watermelon by ultra high performance liquid chromatography with tandem mass spectrometry

In this study, a simple and accurate sample preparation method based on dispersive solid‐phase extraction and dispersive liquid‐liquid microextraction has been developed for the determination of seven novel succinate dehydrogenase inhibitor fungicides (isopyrazam, fluopyram, pydiflumetofen, boscalid, penthiopyrad, fluxapyroxad, and thifluzamide) in watermelon. The watermelon samples were extracted with acetonitrile, cleaned up by dispersive solid‐phase extraction procedure using primary secondary amine, extracted and concentrated by the dispersive liquid‐liquid microextraction procedure with 1,1,2,2‐tetrachloroethane, and then analyzed by ultra high performance liquid chromatography with tandem mass spectrometry. The main experimental factors affecting the performance of dispersive solid‐phase extraction and dispersive liquid‐liquid microextraction procedure on extraction efficiency were investigated. The proposed method had a good linearity in the range of 0.1–100 µg/kg with correlation coefficients (r) of 0.9979–0.9999. The limit of quantification of seven fungicides was 0.1 µg/kg in the method. The fortified recoveries of seven succinate dehydrogenase inhibitor fungicides at three levels ranged from 72.0 to 111.6% with relative standard deviations of 3.4–14.1% (n = 5). The proposed method was successfully used for the rapid determination of seven succinate dehydrogenase inhibitor fungicides in watermelon.     

Hypoiodite‐Mediated Cyclopropanation of Alkenes

An efficient, transition metal‐free procedure for the cyclopropanation of alkenes using malononitrile and the LiI‐tBuOCl combination under mild reaction conditions is described. The reaction mechanism most likely involves tBuOI generated in situ from LiI and tBuOCl. The utility of this new methodology has been demonstrated by the synthesis of a potential HIV‐1 RT inhibitor.     

Synthesis and Glycosidase Inhibitory Activity of 7‐Deoxycasuarine

Reaction of 1,4‐anhydro‐2,3,5‐tri‐O‐benzyl‐1‐deoxy‐1‐imino‐D ‐arabinitol N‐oxide ( 8 ) with allyl alcohol produced a 3.6 : 1 mixture of the two pyrrolo[1,2‐b]isoxazole derivatives 13 and 14 . The major adduct 13 was converted to 7‐deoxycasuarine ( 7 ), a potent, specific, and competitive inhibitor of amyloglucosidase from Rhizopus mold (see Table).