Highly diastereoselective alkylations of chiral amide enolates: new routes to hydroxyethylene dipeptide isostere inhibitors of HIV-1 protease

Samarium-Mediated Iodine-Catalysed Reductive amination of the Adamantyl Methyl Ketone

Adamantyl amines were prepared by samarium-metal induced iodine-catalysed reduction of the imines in a one-pot operation.

Naturally Occurring Limonene to Cinnamyl-type γ-Butyrolactone Substituted Aldol Condensation Derivatives as Antioxidant Compounds

A method has been developed for the synthesis of γ-butyrolactone substituted cinnamyl type Aldol condensation products under milder proline and triethylamine basic conditions. The antioxidant activity of all the synthesized compounds was assessed using three different assays. The cinnamyl type derivatives of γ-butyrolactone bearing an -OH group in the benzene ring exhibited excellent antioxidant activity.

Amine and thiazole substituted γ-butyrolactones from naturally occurring limonene

Substituted γ-butyrolactones are important structural motifs in several natural products and pharmaceuticals. In this paper, we report the synthesis of novel γ-butyrolactone amine and thiazole derivatives from naturally occurring limonene. Regioselective bromination followed by nucleophilic substitution with different amines and thiourea gave desired products in moderate yield.

Design, asymmetric synthesis, and evaluation of pseudosymmetric sulfoximine inhibitors against HIV-1 protease

The HIV-1 protease is a validated drug target for the design of antiretroviral drugs to combat AIDS. We previously established the sulfoximine functionality as a valid transition state mimetic (TSM) in the HIV-1 protease inhibitors (PI) design and have identified a lead pseudosymmetric compound with nanomolar enzymatic inhibitory activity. Here, we report the asymmetric synthesis of this compound and its application in the synthesis of sulfoximine-based peptidomimetic HIV-1 protease inhibitors. Molecular modeling revealed the potential mode of binding of the sulfoximine inhibitor as a TSM. The predicted absolute binding free energies suggested similar inhibitory effect as observed in our enzymatic inhibitory studies.

Stereoselective formation of a functionalized dipeptide isostere by zinc carbenoid-mediated chain extension

The application of a zinc carbenoid-mediated chain-extension reaction to a functionalized peptide isostere is reported. The cleavage site of human CVM protease was utilized as a target for testing the synthetic methodology. The utility of this chain-extension reaction is demonstrated in the preparation of an amino acid-derived alpha-unsubstituted gamma-keto ester, which is incorporated into a framework that mimics a tetrapeptide. The identification of a suitable protecting group strategy facilitated the application of a tandem reaction for the incorporation of an alpha-side chain, and the use of an oxazolidinone auxiliary provided excellent diastereocontrol in a tandem chain-extension-aldol reaction. Stereoselectivity of the tandem chain-extension-aldol reaction was determined through application of a CAN-mediated oxidative cleavage reaction.

Discovery of potent HIV-1 protease inhibitors incorporating sulfoximine functionality

Based on the unique property of sulfoximine and the homodimeric C(2) structural symmetry of HIV-1 protease, a novel class of sulfoximine-based pseudosymmetric HIV-1 protease inhibitors was designed and synthesized. The sulfoximine moiety was demonstrated to be important for HIV-1 protease inhibitor potency. The most active stereoisomer (2S,2'S) displays a potency of 2.5 nM (IC(50)) against HIV-1 protease and an anti-HIV-1 activity of 408 nM (IC(50)). A possible mode of action is proposed.

Total synthesis of garsubellin A

A concise approach to the laboratory synthesis of garsubellin A is described. Garsubellin A, an effective inducer of choline acetyltransferase (ChAT), has been shown to have potential as a therapeutic agent for the treatment of Alzheimer's disease. Starting from 3,5-dimethoxyphenol, the synthesis has provided garsubellin A in an 18-step sequence. Notable transformations include dearomative allylation, diastereoselective vinylogous lactonization, iodocarbocyclization, transannular Wurtz, and bridgehead functionalization reactions.

Total synthesis of ciguatoxin CTX3C: a venture into the problems of ciguatera seafood poisoning

After a twelve-year struggle, the total synthesis of ciguatoxin CTX3C has been achieved. Annually, more than 20,000 people worldwide suffer from ciguatera seafood poisoning. The extremely small amounts of the causative neurotoxin, ciguatoxin, in fish hampered the isolation, structural elucidation, detailed biological study, and preparation of anti-ciguatoxin antibodies for detecting these toxins. The large (3 nanometers long) and complicated molecular structure of ciguatoxins hindered chemists from completing a total synthesis. The chemical synthesis of CTX3C, determination of the absolute configuration, and synthesis-based preparation of the monoclonal antibodies as well as the effect of synthetic CTX3C on voltage-sensitive sodium channels are outlined.

Design and Synthesis of Hydroxyethylene-Based Peptidomimetic Inhibitors of Human β-Secretase

The hydroxyethylene (HE) transition state isostere was developed as a scaffold to provide potent, small molecule inhibitors of human beta-secretase (BACE). The previous work on the statine series proved critical to the discovery of HE structure-activity relationships. Compound 20 with the N-terminal isophthalamide proved to be the most potent HE inhibitor (IC(50) = 30 nM) toward BACE. Unlike the statine series, we identified HE inhibitors without carboxylic acids on the C terminus, leading to enhanced cell penetration and making them attractive candidates for further drug development in Alzheimer's disease.

Using a Lipase as a High-Throughput Screening Method for Measuring the Enantiomeric Excess of Allylic Acetates

This report describes a high-throughput method for measuring the enantiomeric excess of allylic acetates. Such methods are useful tools for screening libraries of potential catalysts for enantioselective reactions. This technique, which is called EMDee for an enzymatic method for determining enantiomeric excess, uses the lipase from Pseudomonas cepacia to hydrolyze the (R) enantiomer of an allylic acetate, while the (S) enantiomer does not react. The rate of the reaction is monitored by measuring the acetic acid that is produced during the hydrolysis reaction with a pH indicator. Using the Michaelis-Menten equation, the rate of the reaction can be correlated with the concentration of the (R) enantiomer. This method can process 88 samples in less that 30 min.

Synthesis of 2,6-dideoxysugars via ring-closing olefinic metathesis

[formula: see text] Grubbs' RuCl2 (=CHPh)(PCy3)2 (catalyst 1) and RuCl2(=CHPh)(PCy3)(IMess) (catalyst 2) complexes have been successfully utilized in the construction of beta,gamma-unsaturated delta-lactones containing various substitution patterns of methyl groups. Asymmetric dihydroxylation followed by reduction leads to 3,4-cis-dihydroxy-2,6-dideoxypyranoses, which have proven to play very important biological roles as key components of natural products.

Linear non-competitive inhibition of solubilized human gamma-secretase by pepstatin A methylester, L685458, sulfonamides, and benzodiazepines

Cerebral deposition of amyloid beta-protein (A beta) is believed to play a key role in the pathogenesis of Alzheimer's disease. Because A beta is produced from the processing of amyloid beta-protein precursor (APP) by beta- and gamma-secretases, these enzymes are considered important therapeutic targets for identification of drugs to treat Alzheimer's disease. Unlike beta-secretase, which is a monomeric aspartyl protease, gamma-secretase activity resides as part of a membrane-bound, high molecular weight, macromolecular complex. Pepstatin and L685458 are among several structural classes of gamma-secretase inhibitors identified so far. These compounds possess a hydroxyethylene dipeptide isostere of aspartyl protease transition state analogs, suggesting gamma-secretase may be an aspartyl protease. However, the mechanism of inhibition of gamma-secretase by pepstatin and L685458 has not been elucidated. In this study, we report that pepstatin A methylester and L685458 unexpectedly displayed linear non-competitive inhibition of gamma-secretase. Sulfonamides and benzodiazepines, which do not resemble transition state analogs of aspartyl proteases, also displayed potent, non-competitive inhibition of gamma-secretase. Models to rationalize how transition state analogs inhibit their targets by non-competitive inhibition are discussed.

Synthesis of different chiral amino gamma-butyrolactones and amino gamma-butenolides

Different transformations of chiral epoxy esters 1 afford two different amino gamma-butyrolactones 2 and 6, and amino gamma-butenolides 8, by different nucleophilic opening-closing processes. [reaction: see text]

Design and synthesis of a series of (2R)-N(4)-hydroxy-2-(3-hydroxybenzyl)-N(1)- [(1S,2R)-2-hydroxy-2,3-dihydro-1H-inden-1-yl]butanediamide derivatives as potent, selective, and orally bioavailable aggrecanase inhibitors

A pharmacophore model of the P1' site, specific for aggrecanase, was defined using the specificity studies of the matrix metalloproteinases and the similar biological activity of aggrecanase and MMP-8. Incorporation of the side chain of a tyrosine residue into compound 1 as the P1' group provided modest selectivity for aggrecanase over MMP-1, -2, and -9. A cis-(1S)(2R)-amino-2-indanol scaffold was incorporated as a tyrosine mimic (P2') to conformationally constrain 2. Further optimization resulted in compound 11, a potent, selective, and orally bioavailable inhibitor of aggrecanase.

Asymmetric synthesis of chiral organofluorine compounds: use of nonracemic fluoroiodoacetic acid as a practical electrophile and its application to the synthesis of monofluoro hydroxyethylene dipeptide isosteres within a novel series of HIV protease inhibitors

Two stereoselective routes to a series of diastereomeric inhibitors of HIV protease, monofluorinated analogues of the Merck HIV protease inhibitor indinavir, are described. The two routes feature stereoselective construction of the fluorinated core subunits by asymmetric alkylation reactions. The first-generation syntheses were based on the conjugate addition of the lithium enolate derived from pseudoephedrine alpha-fluoroacetamide to nitroalkene 12, a modestly diastereoselective transformation. A more practical second-generation synthetic route was developed that is based on a novel method for the asymmetric synthesis of organofluorine compounds, by enolate alkylation using optically active fluoroiodoacetic acid as the electrophile in combination with a chiral amide enolate. Resolution of fluoroiodoacetic acid with ephedrine provides either enantiomeric form of the electrophile in > or = 96% ee. Alkylation reactions with this stable and storable chiral fluorinated precursor are shown to proceed in a highly stereospecific manner. With the development of substrate-controlled syn- or anti-selective reductions of alpha-fluoro ketones 44 and 45 (diastereomeric ratios 12:1-84:1), efficient and stereoselective routes to each of the four targeted inhibitors were achieved. The optimized synthetic route to the most potent inhibitor (syn,syn-4, K(i) = 2.0 nM) proceeded in seven steps (87% average yield per step) from aminoindanol hydrocinnamide 40 and (S)-fluoroiodoacetic acid, and allowed for the preparation of more than 1 g of this compound. The inhibition of HIV-1 protease by each of the fluorinated inhibitors was evaluated in vitro, and the variation of potency as a function of inhibitor stereochemistry is discussed.

Synthesis and ring contraction reactions of polyazamacrolides

The synthesis of three analogues of the single most abundant component of a ladybird beetle (Epilachna borealis) defensive secretion, the trimeric 42-membered polyazamacrolide PAML 681, is described. Construction of the nonnatural macrocyclic trimers began with the preparation of the corresponding monomeric segments, followed by their oligomerization and a final macrolactonization step of the activated linear trimeric hydroxy acid. The relative rates of the O-to-N acyl migrations that are characteristic of PAML 681 itself, as well as of the synthetic analogues, were investigated. These studies showed that changes in the substitution pattern adjacent to the nucleophilic nitrogen atom, along with changes in the size of the oxaazacyclic intermediates, have substantial effects on the polyazamacrolide rearrangement rates.

A stereocontrolled, efficient synthetic route to bioactive sphingolipids: synthesis of phytosphingosine and phytoceramides from unsaturated ester precursors via cyclic sulfate intermediates

An efficient and highly enantioselective method for the preparation of D-ribo- and L-lyxo-phytosphingosines (1a,b, respectively) and phytoceramides (2a,b) has been developed. The key steps in the syntheses are as follows: (i) osmium-catalyzed asymmetric dihydroxylation of 4-O-protected (E)-alpha,beta-unsaturated ester 5 (generated by dihydroxylation of 1-hexadecene, followed by oxidation to the aldehyde and Horner-Wadsworth-Emmons olefination), (ii) conversion to cyclic sulfate intermediate 7, and (iii) regioselective alpha-azidation of 7. Reduction of 4-O-protected 2-azido ester 8 via alpha-azidolactone 9 afforded phytosphingosine 1a. Staudinger reduction of the azido group of 8, followed by in situ N-acylation in aqueous media and reduction of the ester functionality with NaBH(4)/LiBr, provided phytoceramide 2a. By using a similar approach, phytosphingosine 1b was synthesized. D-erythro-4, 5-Dihydrosphingosine 1c and D-erythro-4,5-dihydroceramide 2c were synthesized in high yield from 1-hexadecanol via cyclic sulfate intermediate 15. The desired configurations at C-2, C-3, and C-4 of the sphingoid chain can be accessed readily by the route described here.

Stereoselective preparation of ceramide and its skeleton backbone modified analogues via cyclic thionocarbonate intermediates derived by catalytic asymmetric dihydroxylation of alpha,beta-unsaturated ester precursors

A novel and efficient synthetic route to ceramide 1a and skeleton backbone modified ceramide analogues 1b,c is reported. The syntheses utilize osmium-catalyzed asymmetric dihydroxylation of (E)-alpha, beta-unsaturated ester 5a-c as the chiral induction step, with the desired configurations in the products 1a-c, 2a, and 13 being generated by regioselective azide substitution at the alpha position of alpha,beta-dihydroxyesters 6a-c via a cyclic thionocarbonate intermediate. Azido esters 10a-c are converted to the corresponding ceramides 1a-c by a sequence of azide reduction, N-acylation, ester reduction (NaBH(4)/LiBr), and Birch reduction of the triple bond (Li, EtNH(2)). These seven- to eight-step syntheses afford the target compounds 1a-c with excellent stereocontrol and in 30-42% overall yields. Furthermore, propargylic alpha-azido-beta-hydroxyester 10a is converted to D-erythro-sphingosine 2a via simultaneous reduction of the triple bond, azido, and ester functional groups with LiAlH(4), providing a highly concise and practical four-step synthesis of this key naturally occurring sphingolipid. The L-erythro stereoisomers are also available in high enantiomeric purity by the method described herein.

Synthesis of ceramide analogues having the C(4)-C(5) bond of the long-chain base as part of an aromatic or heteroaromatic system

Two efficient and stereoselective methods are described for the preparation of aryl and heteroaryl ceramide analogues 2 and 3. The first route involves the addition of an aryllithium or a heteroaryllithium reagent (7a or 25a, respectively) to the L-serine-derived aldehyde 4, followed by hydrolysis of the oxazolidine, liberation of the amino group, and N-acylation. The second route, which was used to prepare arylceramide analogue 2 in eight steps and 28% overall yield starting with 3-bromobenzaldehyde, utilizes a Heck reaction to afford (E)-alpha,beta-unsaturated ester 16, then osmium-catalyzed asymmetric dihydroxylation for the introduction of the desired chirality at C-2 and C-3. Regioselective alpha-azidation of alpha-O-nosyl-beta-hydroxyester 18 with sodium azide, followed by LiAlH(4) reduction of the azido and ester groups and N-acylation, complete the synthesis of arylceramide analogue 2.