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Small molecules that target the spliceosome SF3B complex are potent inhibitors of cancer cell growth. The compounds affect an early stage of spliceosome assembly when U2 snRNP first engages the branch point sequence of an intron. Employing an inactive herboxidiene analog (iHB) as a competitor, we investigated factors that influence inhibitor interactions with SF3B to inhibit pre-mRNA splicing in vitro. Order-of addition experiments show that inhibitor interactions are long lasting and affected by both temperature and the presence of ATP. Our data are also consistent with the idea that not all SF3B conformations observed in structural studies are conducive to productive inhibitor interactions. Notably, SF3B inhibitors do not impact an ATP-dependent rearrangement in U2 snRNP that exposes the branch binding sequence for base pairing. We also report extended structure activity relationship analysis of the splicing inhibitor herboxidiene. We identified features of the tetrahydropyran ring that mediate its interactions with SF3B and its ability to interfere with splicing. Analyzing our data in the context of recent structural models of SF3B interactions with inhibitors, our results lead us to extend the model for early spliceosome assembly and inhibitor mechanism. We postulate that interactions between a carboxylic acid substituent of herboxidiene and positively charged SF3B1 sidechains in the inhibitor binding channel are required to maintain inhibitor occupancy while counteracting the SF3B transition to a closed state that is required for stable U2 snRNP interactions with the intron.

The ability to perform routine structure-guided drug design for selective BACE inhibitors has been limited because of the lack of robust platform for BACE2 expression, purification, and crystallization. To overcome this limitation, we developed a platform that produces 2–3 mg of pure BACE2 protein per liter of E. coli culture, and we used this protein to design macrocyclic compounds that potently and selectively inhibit BACE1 over BACE2. Compound 2 was found to potently inhibit BACE 1 (K(i) = 5 nM) with a selectivity of 214-fold over BACE2. The X-ray crystal structures of unbound BACE2 (2.2 Å) and BACE2 bound to compound 3 (3.0 Å and K(i) = 7 nM) were determined and compared to the X-ray structures of BACE1 revealing the S1–S3 subsite as a selectivity determinant. This platform should enable a more rapid development of new and selective BACE inhibitors for the treatment of Alzheimer’s disease or type II diabetes.

We describe an enantioselective synthesis of (3R,3aS,6aR)-hexahydrofuro[2,3-b]furan-3-ol which is a key subunit of darunavir, a widely used HIV-1 protease inhibitor drug for the treatment of HIV/AIDS patients. The synthesis was achieved in optically pure form utilizing commercially available sugar derivatives as the starting material. The key steps involve a highly stereoselective substrate-controlled hydrogenation, a Lewis acid catalyzed anomeric reduction of a 1,2-O-isopropylidene-protected glycofuranoside, and a Baeyer-Villiger oxidation of a tetrahydrofuranyl-2-aldehyde derivative. This optically active ligand alcohol was converted to darunavir efficiently.

Lewis acid-catalyzed rearrangements of 4,5-dihydro-1,3-dioxepines have been investigated. Rearrangement of vinyl acetals under a variety of conditions resulted in cis- and trans-2,3-disubstituted tetrahydrofuran derivatives in a highly stereoselective manner. Rearrangements at lower temperatures typically provided the cis-2,3-disubstituted tetrahydrofuran carbaldehydes. At higher temperatures, the corresponding trans-2,3-disubstituted tetrahydrofuran carbaldehydes are formed. The requisite substrates for the vinyl acetal rearrangement were synthesized via ring-closing olefin metathesis of bis(allyoxy)methyl derivatives using Grubbs second-generation catalyst followed by olefin isomerization using a catalytic amount of RuCl2(PPh3)3. We examined the substrate scope using substituted aromatic and aliphatic derivatives. Additionally, the rearrangement was utilized in the synthesis of a stereochemically-defined bis-tetrahydrofuran (bis-THF) derivative, which is one of the key structural elements of darunavir, an FDA-approved drug for the treatment of HIV/AIDS.

BACE1 (Beta-site Amyloid Precursor Protein (APP) Cleaving Enzyme 1) is a promising therapeutic target for Alzheimer’s Disease (AD). However, efficient expression, purification, and crystallization systems are not well described or detailed in the literature nor are approaches for treatment of enzyme kinetic data for potent inhibitors well described. We therefore developed a platform for expression and purification of BACE1, including protein refolding from E.coli inclusion bodies, in addition to optimizing a reproducible crystallization procedure of BACE1 bound with inhibitors. We also report a detailed approach to the proper analysis of enzyme kinetic data for compounds that exhibit either rapid-equilibrium or tight-binding mechanisms. Our methods allow for the purification of ~15 mg of BACE1 enzyme from 1 L of culture which is higher than reported yields in the current literature. To evaluate the data analysis approach developed here, a well-known potent inhibitor and two of its derivatives were tested, analyzed, and compared. The inhibitory constants (K(i)) obtained from the kinetic studies are in agreement with dissociation constants (K(d)) that were also determined using isothermal titration calorimetry (ITC) experiments. The X-ray structures of these three compounds in complex with BACE1 were readily obtained and provide important insight into the structure and thermodynamics of the BACE1-inhibitor interactions.

We have designed, synthesized, and evaluated a new class of potent HIV-1 protease inhibitors with novel bicyclic oxazolidinone derivatives as the P2 ligand. We have developed an enantioselective synthesis of these bicyclic oxazolidinones utilizing a key o-iodoxybenzoic acid mediated cyclization. Several inhibitors displayed good to excellent activity toward HIV-1 protease and significant antiviral activity in MT-4 cells. Compound 4k has shown an enzyme K(i) of 40 pM and antiviral IC(50) of 31 nM. Inhibitors 4k and 4l were evaluated against a panel of highly resistant multidrug-resistant HIV-1 variants, and their fold-changes in antiviral activity were similar to those observed with darunavir. Additionally, two X-ray crystal structures of the related inhibitors 4a and 4e bound to HIV-1 protease were determined at 1.22 and 1.30 Å resolution, respectively, and revealed important interactions in the active site that have not yet been explored.

The design, synthesis, and biological evaluation of a new class of HIV-1 protease inhibitors containing stereochemically defined fused tricyclic polyethers as the P2 ligands and a variety of sulfonamide derivatives as the P2′ ligands, are described. A number of ring sizes and various substituent effects were investigated to enhance the ligand-backbone interactions in the protease active site. Inhibitors 5c and 5d containing this unprecedented fused 6-5-5 ring system as the P2 ligand, an aminobenzothiazole as the P2′ ligand and a difluorophenylmethyl as the P1 ligand exhibited exceptional enzyme inhibitory potency and maintained excellent antiviral activity against a panel of highly multidrug-resistant HIV-1 variants. The umbrella-like P2 ligand for these inhibitors has been synthesized efficiently in an optically active form using a Pauson-Khand cyclization reaction as the key step. The racemic alcohols were resolved efficiently using a lipase catalyzed enzymatic resolution. Two high resolution X-ray structures of inhibitor-bound HIV-1 protease revealed extensive interactions with the backbone atoms of HIV-1 protease and provided molecular insight into the binding properties of these new inhibitors.

Thailanstatin A has been isolated recently from the fermentation broth of B. thailandensis MSMB43. We describe here an enantioselective convergent synthesis of thailanstatin A methyl ester and evaluation of its splicing activity. Synthesis of both highly functionalized tetrahydropyran rings were carried out from commercially available tri- O-acetyl-d-glucal as the key starting material. Our convergent synthesis involved the synthesis of both tetrahydropyran fragments in a highly stereoselective manner. The fragments were then coupled using cross-metathesis as the key step. The synthesis of the diene subunit included a highly stereoselective Claisen rearrangement, a Cu(I)-mediated conjugate addition of MeLi to set the C-14 methyl stereochemistry, a reductive amination reaction to install the C16-amine functionality, and a Wittig olefination reaction to incorporate the diene unit. The epoxy alcohol subunit was synthesized by a highly selective anomeric allylation, a Peterson olefination, and a vanadium catalyzed epoxidation that installed the epoxide stereoselectively. Cross-metathesis of the olefins provided the methyl ester derivative of thailanstatin A. We have carried out in vitro splicing studies of the methyl ester derivative, which proved to be a potent inhibitor of the spliceosome.

Substituted oxocene derivatives have been synthesized by Lewis acid catalyzed reactions of ε-hydroxy-alkene and substituted aromatic aldehydes. The Cu(OTf)2-bis-phosphine catalyzed reaction typically provides substituted dihydropyran derivatives through an olefin-migration, followed by a Prins cyclization. The corresponding reaction catalyzed by TMSOTf or BF3·OEt2 provided eight-membered cyclic ethers, oxocenes, selectively. This methodology provides convenient access to a variety of 2,4,8-trisubstituted oxocenes in good yields and excellent diastereoselectivities.

HIV-1 protease inhibitors continue to play an important role in the treatment of HIV/AIDS, transforming this deadly ailment into a more manageable chronic infection. Over the years, intensive research has led to a variety of approved protease inhibitors for the treatment of HIV/AIDS. In this review, we outline current drug design and medicinal chemistry efforts toward the development of next-generation protease inhibitors beyond the currently approved drugs.

An extremely drug resistant mutant of HIV-1 protease (PR) bearing 20 mutations (PR20) has been studied with two potent antiviral investigational inhibitors. GRL-5010A and GRL-4410A were designed to introduce hydrogen bond interactions with the flexible flaps of the PR by incorporating gem-difluorines and alkoxy, respectively, at the C4 position of the bis-THF of darunavir. PR20 provides an excellent model for high level resistance, since clinical inhibitors are >1000-fold less active on PR20 than on wild-type enzyme. GRL-5010A and GRL-4410A show inhibition constants of 4.3 ± 7.0 and 1.7 ± 1.8 nM, respectively, for PR20, compared to the binding affinity of 41 ± 1 nM measured for darunavir. Crystal structures of PR20 in complexes with the two inhibitors confirmed the new hydrogen bond interactions with Gly 48 in the flap of the enzyme. The two new compounds are more effective than darunavir in inhibiting mature PR20 and show promise for further development of antiviral agents targeting highly resistant PR mutants.

The Middle East Respiratory Syndrome coronavirus (MERS-CoV) papain-like protease (PLpro) blocking loop 2 (BL2) structure differs significantly from that of SARS-CoV PLpro, where it has been proven to play a crucial role in SARS-CoV PLpro inhibitor binding. Four SARS-CoV PLpro lead inhibitors were tested against MERS-CoV PLpro, none of which were effective against MERS-CoV PLpro. Structure and sequence alignments revealed that two residues, Y269 and Q270, responsible for inhibitor binding to SARS-CoV PLpro, were replaced by T274 and A275 in MERS-CoV PLpro, making critical binding interactions difficult to form for similar types of inhibitors. High-throughput screening (HTS) of 25 000 compounds against both PLpro enzymes identified a small fragment-like noncovalent dual inhibitor. Mode of inhibition studies by enzyme kinetics and competition surface plasmon resonance (SPR) analyses suggested that this compound acts as a competitive inhibitor with an IC50 of 6 μM against MERS-CoV PLpro, indicating that it binds to the active site, whereas it acts as an allosteric inhibitor against SARS-CoV PLpro with an IC50 of 11 μM. These results raised the possibility that inhibitor recognition specificity of MERS-CoV PLpro may differ from that of SARS-CoV PLpro. In addition, inhibitory activity of this compound was selective for SARS-CoV and MERS-CoV PLpro enzymes over two human homologues, the ubiquitin C-terminal hydrolases 1 and 3 (hUCH-L1 and hUCH-L3).

An enantioselective total synthesis of GEX1Q1 has been accomplished in a convergent manner. The C-5 asymmetric center has now been assigned through synthesis. GEX1Q1 displayed slightly better spliceosome inhibitory activity over its C-5 epimer. The salient features of this synthesis include an asymmetric hetero-Diels-Alder reaction to construct the tetrahydropyran ring and a Suzuki cross-coupling to assemble the key segments. © 2014 American Chemical Society.

The design, synthesis, and biological evaluation of a series of HIV-1 protease inhibitors incorporating stereochemically defined fused tricyclic P2-ligands are described. Various substituent effects were investigated in order to maximize the ligand-binding site interactions in the protease active site. Inhibitors 16a and 16f showed excellent enzyme inhibitory and antiviral activity while incorporation of sulfone functionality resulted in a decrease in potency. Both inhibitors 16a and 16f have maintained activity against a panel of multidrug resistant HIV-1 variants. A high-resolution X-ray crystal structure of 16a-bound HIV-1 protease revealed important molecular insights into the ligand-binding site interactions which may account for the inhibitor’s potent antiviral activity and excellent resistance profiles.

HIV-1 protease is an important target for the development of antiviral inhibitors to treat AIDS. A room-temperature joint X-ray/neutron structure of the protease in complex with clinical drug amprenavir has been determined at 2.0 Å resolution. The structure provides direct determination of hydrogen atom positions in the enzyme active site. Analysis of the enzyme-drug interactions suggests that some hydrogen bonds may be weaker than deduced from the non-hydrogen interatomic distances. This information may be valuable for the design of improved protease inhibitors.

An enantioselective synthesis of (+)-lithospermic acid, a potent anti-HIV agent, has been accomplished in a convergent manner in nine steps. The synthesis features an enantioselective intramolecular oxa-Michael addition catalyzed by a quinidine derivative, a hypervalent iodine-mediated rearrangement of chromanone to dihydrobenzofuran, an enantioselective α-oxyamination, and an intermolecular C-H olefination.

We report the design, synthesis, biological evaluation, and the X-ray crystal structure of a novel inhibitor bound to the HIV-1 protease. Various C3-functionalized cyclopentanyltetrahydrofurans (Cp-THF) were designed to interact with the flap Gly48 carbonyl or amide NH in the S2-subsite of the HIV-1 protease. We investigated the potential of those functionalized ligands in combination with hydroxyethylsulfonamide isosteres. Inhibitor 26 containing a 3-(R)-hydroxyl group on the Cp-THF core displayed the most potent enzyme inhibitory and antiviral activity. Our studies revealed a preference for the 3-(R)-configuration over the corresponding 3-(S)-derivative. Inhibitor 26 exhibited potent activity against a panel of multidrug-resistant HIV-1 variants. A high resolution X-ray structure of 26-bound HIV-1 protease revealed important molecular insight into the ligand-binding site interactions.

An enantioselective total synthesis of zampanolide has been accomplished using a novel DDQ/Brønsted acid promoted cyclization as the key reaction. The synthesis features cross-metathesis to construct the trisubstituted olefin and a ring-closing metathesis to form the macrolactone. The final N-acyl aminal formation was stereoselectively accomplished by an organocatalytic reaction.

We investigated substituted bis-THF-derived HIV-1 protease inhibitors in order to enhance ligand-binding site interactions in the HIV-1 protease active site. In this context, we have carried out convenient syntheses of optically active bis-THF and C4-substituted bis-THF ligands using a [2,3]-sigmatropic rearrangement as the key step. The synthesis provided convenient access to a number of substituted bis-THF derivatives. Incorporation of these ligands led to a series of potent HIV-1 protease inhibitors. Inhibitor 23c turned out to be the most potent (Ki = 2.9 pM; IC50 = 2.4 nM) among the inhibitors. An X-ray structure of 23c-bound HIV-1 protease showed extensive interactions of the inhibitor with the protease active site, including a unique water-mediated hydrogen bond to the Gly-48 amide NH in the S2 site.

We used synthetic peloruside A for the commercial preparation of [³H]peloruside A. The radiolabeled compound bound to preformed tubulin polymer in amounts stoichiometric with the polymer's tubulin content, with an apparent K(d) value of 0.35 μM. A less active peloruside A analogue, (11-R)-peloruside A and laulimalide acted as competitive inhibitors of the binding of the [³H]peloruside A, with apparent K(i) values of 9.3 and 0.25 μM, respectively. Paclitaxel, epothilone B, and discodermolide had essentially no ability to inhibit [³H]peloruside A binding, confirming that these compounds bind to a different site on tubulin polymer. We modeled both laulimalide and peloruside A into the binding site on β-tubulin that was identified by Huzil et al. (J. Mol. Biol. 2008, 378, 1016-1030), but our model provides a more reasonable structural basis for the protein-ligand interaction. There is a more complete desolvation of the peloruside A ligand and a greater array of favorable hydrophobic and electrostatic interactions exhibited by peloruside A at its β-tubulin binding site. In addition, the protein architecture in our peloruside A binding model was suitable for binding laulimalide. With the generation of both laulimalide and peloruside A binding models, it was possible to delineate the structural basis for the greater activity of laulimalide relative to peloruside A and to rationalize the known structure-activity relationship data for both compounds.

The structure-based design, synthesis, and biological evaluation of a series of nonpeptidic macrocyclic HIV protease inhibitors are described. The inhibitors are designed to effectively fill in the hydrophobic pocket in the S1'-S2' subsites and retain all major hydrogen bonding interactions with the protein backbone similar to darunavir (1) or inhibitor 2. The ring size, the effect of methyl substitution, and unsaturation within the macrocyclic ring structure were assessed. In general, cyclic inhibitors were significantly more potent than their acyclic homologues, saturated rings were less active than their unsaturated analogues and a preference for 10- and 13-membered macrocylic rings was revealed. The addition of methyl substituents resulted in a reduction of potency. Both inhibitors 14b and 14c exhibited marked enzyme inhibitory and antiviral activity, and they exerted potent activity against multidrug-resistant HIV-1 variants. Protein-ligand X-ray structures of inhibitors 2 and 14c provided critical molecular insights into the ligand-binding site interactions.

Structure-based design, synthesis, and biological evaluation of a series of novel HIV-1 protease inhibitors are described. In an effort to enhance interactions with protease backbone atoms, we have incorporated stereochemically defined methyl-2-pyrrolidinone and methyl oxazolidinone as the P1'-ligands. These ligands are designed to interact with Gly-27' carbonyl and Arg-8 side chain in the S1'-subsite of the HIV protease. We have investigated the potential of these ligands in combination with our previously developed bis-tetrahydrofuran (bis-THF) and cyclopentanyltetrahydrofuran (Cp-THF) as the P2-ligands. Inhibitor 19b with a (R)-aminomethyl-2-pyrrolidinone and a Cp-THF was shown to be the most potent compound. This inhibitor maintained near full potency against multi-PI-resistant clinical HIV-1 variants. A high resolution protein-ligand X-ray crystal structure of 19b-bound HIV-1 protease revealed that the P1'-pyrrolidinone heterocycle and the P2-Cp-ligand are involved in several critical interactions with the backbone atoms in the S1' and S2 subsites of HIV-1 protease.

Historically, nature has provided an incredible variety of structurally complex and biologically important molecules. Indeed, a great many of today’s clinical medicines are obtained directly from natural products or from their derivatives.1 Natural products continue to be a very important source for modern drug discovery and development.2 The seemingly limitless structural features coupled with their novel biological properties have provided enormous inspiration for the development of new reactions and methodologies en route to natural product synthesis and structural variations for drug-discovery.3 Beginning in the fall of 1994, we initiated a research program aimed at synthesizing bioactive natural products with interesting structural features. This endeavor resulted in the total synthesis of numerous targets, covering over two dozen or so different structural families. Some notable examples of our accomplished bioactive targets include novel and exceedingly potent microtubule stabilizing agents, laulimalide (1)4 and peloruside A (2)5 a potent microtubule destabilizing agent cryptophycin 52 (3)6; anticancer agents amphidinolide T (4)7, amphidinolide W (5)8, and lasonolide A (6)9; antibiotic agent, madumycin II (7)10; pancreatic lipase inhibitor, tetrahydrolipstatin (8)11; novel actin inhibitory agents doliculide (9)12 and jasplakinolide (10)13; novel antibacterial agent platensimycin (11)14; and the histone deacytelase inhibitor, largazole (12)15 (see Figure 1). The unique structural features of these natural products required the development of new synthetic tools and methodologies for their synthesis. In the context of our synthesis of various bioactive targets, we have developed a variety of new and practical asymmetric reactions based upon intermolecular and intramolecular metal chelation.16 Notable carbon-carbon bond forming synthetic methodologies include highly diastereoselective syn-and anti-aldol reactions17; asymmetric inter and intramolecular Diels-Alder and hetero Diels-Alder reactions18, and asymmetric multicomponent reactions.19 The scope and utility of these methodologies have been demonstrated through the synthesis of a variety of bioactive molecules. Open in a separate window Figure 1 Structures of Recent Bioactive Targets

Synthesis of a novel, stable reagent (1,3-benzothiazol-2-ylsulfonyl)fluoroacetonitrile from readily synthesized ethyl alpha-(1,3-benzothiazol-2-ylsulfanyl)-alpha-fluoroacetate is reported. Aldehydes undergo condensations with (1,3-benzothiazol-2-ylsulfonyl)fluoroacetonitrile in the presence of DBU leading to alpha-fluoro acrylonitriles in high yields and with good Z-stereoselectivity. Lowering of reaction temperature increases the Z selectivity.

An enantioselective total synthesis of (+)-peloruside A (1) is described. Peloruside A (1) is a potent microtubule stabilizer with significant clinical potential. The synthesis is convergent and involves the assembly of C1-C10 segment 2 and C11-C24 segment 3 by a novel aldol protocol followed by Yamaguchi macrolactonization of the resulting seco-acid, selective methylation of hemi-ketal and removal of the protecting groups to peloruside A.

An enantioselective route to the oxatetracyclic core of (-)-platensimycin (1) has been investigated by using an intramolecular Diels-Alder reaction as the key step. The thermal reaction of E/Z mixture (1:1) provided oxatetracyclic core 2 from the E-diene and the Z-diene was recovered unchanged. The Diels-Alder substrate was conveniently assembled in optically active form with use of (S)-carvone as the starting material.

[structure: see text] An enantioselective total synthesis of (-)-lasonolide A is described. The upper tetrahydropyran ring was constructed stereoselectively by an intramolecular 1,3-dipolar cycloaddition reaction. The bicyclic isooxazoline led to the tetrahydropyran ring as well as the quaternary stereocenter present in the molecule. The lower tetrahydropyran ring was assembled by a catalytic asymmetric hetero-Diels-Alder reaction as the key step. Three stereocenters were enantioselectively installed in this single step reaction.

The potent new antiviral inhibitor TMC-114 (UIC-94017) of HIV-1 protease (PR) has been studied with three PR variants containing single mutations D30N, I50V, and L90M, which provide resistance to the major clinical inhibitors. The inhibition constants (K(i)) of TMC-114 for mutants PR(D30N), PR(I50V), and PR(L90M) were 30-, 9-, and 0.14-fold, respectively, relative to wild-type PR. The molecular basis for the inhibition was analyzed using high-resolution (1.22-1.45 A) crystal structures of PR mutant complexes with TMC-114. In PR(D30N), the inhibitor has a water-mediated interaction with the side chain of Asn30 rather than the direct interaction observed in PR, which is consistent with the relative inhibition. Similarly, in PR(I50V) the inhibitor loses favorable hydrophobic interactions with the side chain of Val50. TMC-114 has additional van der Waals contacts in PR(L90M) structure compared to the PR structure, leading to a tighter binding of the inhibitor. The observed changes in PR structure and activity are discussed in relation to the potential for development of resistant mutants on exposure to TMC-114.

Design, synthesis, and biological evaluation of peptidomimetic severe acute respiratory syndrome chymotrypsin-like protease (SARS-3CLpro) inhibitors for severe acute respiratory syndrome coronavirus (SARS-CoV) are described. These inhibitors exhibited antiviral activity against SARS-CoV in infected cells in the micromolar range. An X-ray crystal structure of our lead inhibitor (4) bound to SARS-3CLpro provided important drug-design templates for the design of small-molecule inhibitors.

TiCl(4)-promoted multicomponent reactions involving N-tosyl imino ester, cyclic enol ether, and silane reagents in a single one-pot operation provide functionalized alpha-amino acids with multiple stereogenic centers in good to excellent yields. Cis/trans selectivities with optically active substituted dihydrofurans have been investigated.

Aldol reaction of the Ti-enolate derived from cis-1-tosylamido-2-indanyl chloroacetate with representative aldehydes proceeded in excellent yield and high diastereoselectivities. Removal of chlorine provided alternative access to highly diastereoselective acetate aldol equivalents or the corresponding glycidic ester condensation products. [reaction: see text]

[structure: see text] An enantioselective total synthesis of (-)-spongidepsin (2) and elucidation of the absolute stereochemistry of its four stereocenters are described. Spongidepsin (2), a 13-membered depsipeptide isolated from the Vanuatu marine sponge Spongia sp., has shown potent antitumor properties against a variety of NCI tumor cell lines. Our synthesis is convergent, and the absolute stereochemistry of four of the five chiral centers was assigned through synthesis.

[reaction: see text] anti-Aldol reaction of Ti-enolate derived from cis-2-arylsulfonamido-1-acenaphthenyl propionate with representative aldehydes proceeded in excellent yield with high diastereoselectivity. Both enantiomers of cis-2-amino-1-acenaphthenol were synthesized employing lipase-catalyzed kinetic resolution as the key step.

Chelation-controlled reduction of chiral beta-alkoxy ketones containing a competing beta'-oxygen functionality has been investigated. Various syn-1,3-diols were prepared conveniently by reduction of beta-alkoxy ketones with LiI/LiAlH(4) (syn:anti selectivity up to >99:1). The corresponding beta-alkoxy ketones were derived from nitro-aldol reactions of chiral alkoxy aldehydes with a series of nitro compounds. This methodology is utilized in a short and efficient synthesis of the delta-lactone moiety of the HMG-CoA reductase inhibitors compactin and mevinolin.

Laulimalide is a cytotoxic natural product that stabilizes microtubules. The compound enhances tubulin assembly, and laulimalide is quantitatively comparable to paclitaxel in its effects on the reaction. Laulimalide is also active in P-glycoprotein overexpressing cells, while isolaulimalide, a congener without the drug's epoxide moiety, was reported to have negligible cytotoxic and biochemical activity [Mooberry et al. (1999) Cancer Res. 59, 653-660]. We report here that laulimalide binds at a site on tubulin polymer that is distinct from the taxoid site. We found that laulimalide, while as active as paclitaxel, epothilone A, and eleutherobin in promoting the assembly of cold-stable microtubules, was unable to inhibit the binding of radiolabeled paclitaxel or of 7-O-[N-(2,7-difluoro-4'-fluoresceincarbonyl)-L-alanyl]paclitaxel, a fluorescent paclitaxel derivative, to tubulin. Confirming this observation, we demonstrated that microtubules formed in the presence of both laulimalide and paclitaxel contained near-molar quantities, relative to tubulin, of both drugs. Laulimalide was active against cell lines resistant to paclitaxel or epothilones A and B on the basis of mutations in the M40 human beta-tubulin gene. We also report that a laulimalide analogue lacking the epoxide moiety, while less active than laulimalide in biochemical and cellular systems, is probably more active than isolaulimalide. Further exploration of the role of the epoxide in the interaction of laulimalide with tubulin is therefore justified.

An enantioselective first total synthesis of laulimalide (1) is described. Laulimalide, a remarkably potent antitumor macrolide, has been isolated from the Indonesian sponge Hyattella sp. and the Okinawan sponge Fasciospongia rimosa. Laulimalide represents a new class of antitumor agents with significant clinical potential. The synthesis is convergent and involved the assembly of C3−C16 segment 4 and C17−C28 segment 5 by Julia olefination. The sensitive C2−C3 cis-olefin functionality was installed by Yamaguchi macrolactonization of a hydroxy alkynic acid followed by hydrogenation of the resulting alkynoic lactone over Lindlar's catalyst. Initial attempts of intramolecular Still's variant of Horner−Emmons olefination between the C19-phosphonocetate and C3-aldehyde provided a 1:2 mixture of cis- and trans-macrolactones. The trans-isomer was photoisomerized to a mixture of cis- and trans-isomers. The other key steps involved ring-closing olefin metathesis to construct both dihydropyran units, stereoselective...

Memapsin 2 (beta-secretase) is one of two proteases that cleave the beta-amyloid precursor protein (APP) to produce the 40-42 residue amyloid-beta peptide (Abeta) in the human brain, a key event in the progression of Alzheimer's disease. On the basis of the X-ray crystal structure of our lead inhibitor (2, OM99-2 with eight residues) bound to memapsin, we have reduced the molecular weight and designed potent memapsin inhibitors. Structure-based design and preliminary structure-activity studies have been presented.

[structure: see text]. An efficient and highly stereoselective synthesis of the gastroprotective natural product AI-77-B is described. The stereocenters of the hydroxy amino acid moiety were generated by an ester-derived titanium-enolate-mediated syn-aldol reaction, a Curtius rearrangement, and application of Dondoni's aldehyde homologation. Condensation with the dihydroisocoumarin fragment and subsequent deprotecting transformations furnished optically active AI-77-B.

Stereoselective total synthesis of (+)-polyoxin J is described. The synthesis was achieved in a convergent manner by coupling protected thymine polyoxin C (19) and 5-O-carbamoyl polyoxamic acid 27 and subsequent removal of the protecting groups. The key steps of the synthesis of protected thymine polyoxin C involved the stereoselective electrophilic epoxidation of E-allyl alcohol 7 derived from isopropylidene D-ribose derivative 5, followed by regioselective epoxide opening of 8 and conversion of resulting azido diol 9 to protected thymine polyoxin C (19). Protected polyoxamic acid 27 was synthesized stereoselectively by utilizing Sharpless epoxidation of tartrate-derived allylic alcohol 20 followed by a regioselective epoxide ring opening with diisopropoxytitanium diazide.

Sinefungin (1) a nucleoside antibiotic isolated from Streptomyces has been synthesized from D-ribose. Both the C-6 and C-9 stereogenic centers were constructed by efficient asymmetric syntheses. The C-6 amine stereochemistry was set by a highly diastereoselective allylation (99% de) of a (1S,2R)-1-amino-2-indanol-derived oxazolidinone 9 followed by a Curtius rearrangement of 11 to 12. The C-9 amino acid stereochemistry of sinefungin (1) was established by a rhodium chiral bisphosphine-catalyzed asymmetric hydrogenation of an alpha-(acylamino)acrylate derivative. The anomeric adenosylation of the mixture of anomeric acetates 20 in the presence of C-6 urethane NH was found to be extremely difficult. Conversion of the C-6 urethane NH as its N-benzyl derivative 21 was necessary prior to the adenosylation reaction. Successful adenosylation was effectively carried out by Vorbrüggen's protocol utilizing persilylated N(6)-benzoyladenine and trimethylsilyl triflate.

The synthesis of analogs of arthrobactin (5), a microbial iron chelator, and its imide 8 are described. The differentially protected citric acid residue 31 served as the key intermediate in making conjugates having a generalized structure 14 with two representative carbacephalosporin units, 15 and 16. Both conjugates, 49 and 51, showed antibiotic activity, while conjugate 51 obtained from β-lactam 16 bearing a phenylglycyl side chain was shown to be more effective

A series of protease inhibitors bearing constrained unnatural amino acids at the P2-position and novel heterocycles at the P3-position of compound 1 (Ro 31-8959) were synthesized, and their in vitro enzyme inhibitory and antiviral activities were evaluated. Replacement of P2-asparagine of compound 1 with (2S,3'R)-tetrahydrofuranylglycine resulted in improvement in enzyme inhibitory as well as antiviral potencies (compound 23). Interestingly, incorporation of (2S,3'S)-tetrahydrofuranylglycine at the P2-position proved to be less effective. The resulting compound 24 was 100-fold less potent than the 2S,3R-isomer (compound 23). This stereochemical preference indicated a hydrogen-bonding interaction between the tetrahydrofuranyl oxygen and the residues of the S2-region of the enzyme active site. Furthermore, replacement of P3-quinolinoyl ligand of 1 with various novel heterocycles resulted in potent inhibitors of HIV proteases. Of particular interest, compound 2 with (2S,3'R)-tetrahydrofuranylglycine at P2 and pyrazine derivative at P3 is one of the most potent inhibitors of HIV-1 (IC50 value 0.07 nM) and HIV-2 (IC50 value 0.18 nM) proteases. Another important result in this series is the identification of compound 27 in which the P2-P3-amide carbonyl has been removed. The resulting compound 27 has exhibited improvement in antiviral potency while retaining the enzyme inhibitory potency similar to compound 1.

A general procedure for the conversion of α-oximido-β-keto esteers into vicinal tricarbonyl moieties by TiCl 3 in aqueous, buffered (pH 5) conditions with acetone cosolvent was demonstrated. As applied to N-hydroxy β-lactam 17, which contained an α-oximido-β-keto ester side chain, these combined reductive and hydrolytic conditions effected simultaneous tricarbonyl formation and β-lactam N-O bond reduction

A short, stereocontrolled, high-yield synthesis of debromoaplysin (1) and aplysin (2) from 2,3,7 -trimethylchromone is described.