Synthesis of jasmonoids : Synthesis of cyclic ketal-type insect pheromones

This thesis consists of two parts. Part I describes the preparation of cyclopentenones using trimethylsilylpropyne as an acetonyl unit, and its application to the synthesis of cis-jasmone (1) and dihydrojasmone (3) . β-Keto ester dianions were alkylated at the γ-carbon with 3-bromo-l-trimethylsilyl-1-propyne. The resulting alkynes were hydrated to give the 1,4-diketo compounds which were then cyclized to the corresponding cyclopentenones, cis-jasmone (1) and dihydrojasmone (3) . Part II describes the synthesis of various cyclic ketal insect pheromones, viz., frontalin (17) , endo-brevicomin (16) , exo-brevicomin (15) , (-)-α-multistriatin (18α) and lineatin (22) , which have considerable economic values due to their potential utility in the control of beetle population. The dianion of methyl acetoacetate was alkylated with homoallylic bromides. The resulting alkenes were epoxidized and then cyclized with a Lewis acid to produce esters containing the 6,8-dioxabicyclo[3.2.1]octane skeleton. These esters were hydrolyzed and decarboxylated. This methodology was utilized in a synthesis of frontalin (17) and stereospecific syntheses of endo-brevicomin (16), and exo-brevicomin (15). In addition, one of the intermediates in the synthesis of 15 was partially resolved, leading to optically active exo-brevicomin. A chiral synthesis of (-)-α-multistriatin (18α) was accomplished using methyl α-D-glucopyranoside (7i) as starting material. Methyl 4,6-0-benzylidene-2,3-dideoxy-2-C-methyl-α-D-arabino-hexopyranoside (197) was prepared in six steps from methyl α-D-glucopyranoside (71) . The benzylidene 197 was converted into the alkene 207. Then the methyl group at C-4 in the intermediate 201 was generated by a stereoselective hydro-genation of 207 using Wilkinson's catalyst. The dithiane derivative 219 was used to effect both introduction of the ethyl side-chain and the cyclization to form the bicyclic ketal skeleton. The spectral data of the synthetic (-)-α-multistriatin (18α) was found to be identical with those for the natural compound. A synthesis of lineatin (22) is also described. The cis fused bicyclo[4.2.0] ring system in 22 was constructed via a photocycloaddition reaction involving 3-methyl-5-hydroxy-2-pentenoic acid 6-lactone 119 and allene. A mixture of regio-isomers 233 and 234 was obtained in a ratio of 5:3. This mixture was used throughout the subsequent reactions and the final isomeric products lineatin (22) and 23 were separated by column chromatography. The present route to lineatin (22) represents a major improvement over the previous syntheses in terms of efficiency and stereoselectivity. [See thesis for Chemical Diagrams]Science, Faculty ofChemistry, Department ofGraduat

Synthetic studies using B-keto esters

The dianions of β-keto esters have been generated using one equivalent of sodium hydride and one equivalent of n-butyl1ithium or two equivalents of lithium diisopropyl-amide. The reaction of these dianions with various epoxides was studied. On treatment of lithio sodio methyl aceto-acetate (55) with one and one-half equivalents of ethylene oxide and on subsequent treatment with acid, a tetrahydro-furylidene acetate 95, formed from the cyclization of the intermediate alcohol. Methyl α-(tetrahydro-5-methy1-2-fury1i-dene) acetate (96) was also synthesized by treating the di-anion with propylene oxide. The alkylation of these dianions from β -keto esters with α-chloroethy1 ethyl ether or α-ch1oromethy1 methyl ether was also investigated. This provides a convenient preparation of the precursors to Nazarov's reagent which are very useful in the Robinson type annelation. The preparation of methyl 5 - methoxy-3-oxopentanoate (12 0) could be easily achieved in a single step synthesis by treating the lithio sodio methyl acetoacetate (55) with one equivalent of chloromethyl methyl ether. Similarly, methyl 5-ethoxy-3-oxohexanoate (12 9) and methyl 5-methoxy-3-oxohexanoate (130) were prepared using our dianion reaction. The synthesis of large ring compounds using the internal double alkylation of these dianions with dihalo-alkanes in a one-step reaction was found to be rather difficult. The isolated products were a mixture of the bis β -diketo esters and a y-alkylated mono-halo compound in approximately equal amounts. However, on treatment of the mono-halo compound with one equivalent of sodium methoxide in refluxing methanol, the intramolecular cyclization at the a-carbon was easily achieved. The mono-halo β-keto esters could also be cyclized via their dianions to the cyclic or spiro β-keto esters.Science, Faculty ofChemistry, Department ofGraduat

Efflux-Mediated Resistance to Tigecycline (GAR-936) in Pseudomonas aeruginosa PAO1

Pseudomonas aeruginosa strains are less susceptible to tigecycline (previously GAR-936; MIC, 8 μg/ml) than many other bacteria (P. J. Petersen, N. V. Jacobus, W. J. Weiss, P. E. Sum, and R. T. Testa, Antimicrob. Agents Chemother. 43:738-744, 1999). To elucidate the mechanism of resistance to tigecycline, P. aeruginosa PAO1 strains defective in the MexAB-OprM and/or MexXY (OprM) efflux pumps were tested for susceptibility to tigecycline. Increased susceptibility to tigecycline (MIC, 0.5 to 1 μg/ml) was specifically associated with loss of MexXY. Transcription of mexX and mexY was also responsive to exposure of cells to tigecycline. To test for the emergence of compensatory efflux pumps in the absence of MexXY-OprM, mutants lacking MexXY-OprM were plated on medium containing tigecycline at 4 or 6 μg/ml. Resistant mutants were readily recovered, and these also had decreased susceptibility to several other antibiotics, suggesting efflux pump recruitment. One representative carbenicillin-resistant strain overexpressed OprM, the outer membrane channel component of the MexAB-OprM efflux pump. The mexAB-oprM repressor gene, mexR, from this strain contained a 15-bp in-frame deletion. Two representative chloramphenicol-resistant strains showed expression of an outer membrane protein slightly larger than OprM. The mexCD-OprJ repressor gene, nfxB, from these mutants contained a 327-bp in-frame deletion and an IS element insertion, respectively. Together, these data indicated drug efflux mediated by MexCD-OprJ. The MICs of the narrower-spectrum semisynthetic tetracyclines doxycycline and minocycline increased more substantially than did those of tigecycline and other glycylcyclines against the MexAB-OprM- and MexCD-OprJ-overexpressing mutant strains. This suggests that glycylcyclines, although they are subject to efflux from P. aeruginosa, are generally inferior substrates for P. aeruginosa efflux pumps than are narrower-spectrum tetracyclines

Crystal structures of the two major aggrecan degrading enzymes, ADAMTS4 and ADAMTS5

Aggrecanases are now believed to be the principal proteinases responsible for aggrecan degradation in osteoarthritis. Given their potential as a drug target, we solved crystal structures of the two most active human aggrecanase isoforms, ADAMTS4 and ADAMTS5, each in complex with bound inhibitor and one wherein the enzyme is in apo form. These structures show that the unliganded and inhibitor-bound enzymes exhibit two essentially different catalytic-site configurations: an autoinhibited, nonbinding, closed form and an open, binding form. On this basis, we propose that mature aggrecanases exist as an ensemble of at least two isomers, only one of which is proteolytically active