Biological Effects of Modifications of the Englerin A Glycolate.

Modifications at the glycolate moiety of englerin A were made to explore variations at the most sensitive site on the molecule for activity in the NCI 60 screen, wherein englerin A is highly potent and selective for renal cancer cells. Replacement of the glycolate by other functionalities as well as esterification of the glycolate hydroxyl yielded compounds which displayed excellent selectivity and potency compared with the natural product. TRPC4/5 ion channel experiments with five compounds showed delayed or reduced agonism with TRPC5, at much higher concentrations than englerin A. With TRPC4, these compounds all had no effect at 10 µM. The same compounds were not detectable in mouse serum after a single oral dose of 12.5 mg/kg. At 100 mg/kg p.o., no toxicity was observed, and blood levels were barely detectable. Intravenous administration led to toxicity but at substantially lower doses than for englerin A.

Importance of a 4-Alkyl Substituent for Activity in the Englerin Series.

The ring closing metathesis/transannular etherification approach to the englerin nucleus was adapted to provide two key intermediates for analogue synthesis: the 4-desmethyl Δ5,6 tricycle and the 4-oxo Δ5,6 tricycle. The former was elaborated to 4-desmethyl englerin A and the latter served as a common precursor for englerin A, 4-ethyl englerin A, and 4-isopropyl englerin A. 4-Desmethyl englerin A was less active than the natural product by an order of magnitude, but the 4-ethyl and 4-isopropyl analogues were comparable in activity to englerin A. These results are consistent with the premise that the 4-alkyl group enforces the binding conformation of the cinnamoyl ester substituent. Furthermore, they suggest that 4-alkyl englerin structures may prove to be useful tool compounds.

Precision Synthesis of Alternating Copolymers via Ring-Opening Polymerization of 1-Substituted Cyclobutenes.

Investigation of complex molecular systems depends on our ability to correlate physical measurements with molecular structure. Interpretation of studies that rely on synthetic polymers is generally limited by their heterogeneity; i.e., there is variation in the number and arrangement of the monomeric building blocks that have been incorporated. Superior physics and biology can be performed with materials and tools that exert precise control over the sequence and spacing of functional groups. An interest in functional ligands combined with a desire to control the orientation and stereochemistry of monomer incorporation led to the design of new substrates for ruthenium-catalyzed ring-opening metathesis polymerization (ROMP). We discovered that ROMP of cyclobutene-1-carboxamides provides uniform and translationally invariant polymers. In contrast, cyclobutene-1-carboxylate esters ring open upon treatment with ruthenium catalyst, but they are stable to homopolymerization. However, in the presence of cyclohexene monomers, they undergo alternating ROMP (AROMP or alt-ROMP) to give copolymers with a precisely controlled sequence. The alternating cyclobutene ester/cyclohexene pair provides access to functional group spacing larger than is possible with homopolymers. This can be desirable; for example, polymers with a regular 8-10 Å backbone spacing of cationic charge and with between four and eight cationic groups were the most effective antibacterial agents and had low cytotoxicity. Moreover, the AROMP chemistry allows alternation of two functional moieties: one associated with the cyclohexene and one attached to the cyclobutene. In the case of antibacterial copolymers, the alternating chemistry allowed variation of hydrophobicity via the cyclohexene while maintaining a constant cation spacing through the cyclobutene. In the case of copolymers that bear donor and acceptor groups, strict alternation of the groups increased intrachain charge transfer. Like cyclobutene-1-carboxylate esters, bicyclo[4.2.0]oct-7-ene-7-carboxylate esters ring open upon treatment with ruthenium catalyst and undergo ring opening cross-metathesis with cyclohexene to form alternating copolymers. The corresponding bicyclo[4.2.0]oct-7-ene-7-carboxyamides isomerize to the bicyclo[4.2.0]oct-1(8)-ene-8-carboxamides before they can ring open. However, the isomerized amides undergo ruthenium-catalyzed ring opening metathesis and rapidly AROMP with cyclohexene. Our alternating copolymer systems allow functionality to be placed along a polymer chain with larger than typical spacing. We have used both homopolymers and alternating copolymers for defining the functional group density required for targeting a cell surface and for the exploration of functional group positioning within a polymer chain. These polymer systems provide access to new materials with previously inaccessible types of nanoscale structures.

Five Easy Pieces. The Total Synthesis of Phosphoiodyn A (and Placotylene A).

The convergent total synthesis of the marine natural product phosphoiodyn A, a nanomolar agonist of human peroxisome proliferator-activated receptor delta (hPPARδ), was achieved in five steps total from commercially available and inexpensive starting materials. The synthesis relies on the unprecedented regioselective hydrozirconation of a terminal acetylene in the presence of a conjugated 1,3-diyne and on ammonolysis of a ß-chlorophosphonic acid monoester. The scheme also provides the newly isolated placotylene A, an inhibitor of bone marrow-derived macrophage (BMM) differentiation.

Ru-Catalyzed Isomerization Provides Access to Alternating Copolymers via Ring-Opening Metathesis Polymerization.

We describe an isomerization-alternating ROMP protocol that gives linear copolymers with rigorous sequence alternation. Bicyclo[4.2.0]oct-7-ene-7-carboxamides of primary amines are isomerized in the presence of (3-BrPyr)2Cl2(H2IMes)Ru=CHPh to the corresponding bicyclo[4.2.0]oct-1(8)-ene-8-carboxamides in which the olefinic bond is tetrasubstituted. The isomerized amides undergo alternating ring-opening metathesis polymerization with cyclohexene to provide soluble and linear copolymers with molecular weights up to ∼130 kDa. This process provides efficient entry to strictly alternating copolymers that can display diverse functional groups.

A Bicyclo[4.2.0]octene-Derived Monomer Provides Completely Linear Alternating Copolymers via Alternating Ring-Opening Metathesis Polymerization (AROMP).

Strained bicyclic carbomethoxy olefins were utilized as substrates in alternating ring-opening metathesis polymerization and found to provide low-dispersity polymers with novel backbones. The polymerization of methyl bicyclo[4.2.0]oct-7-ene-7-carboxylate with cyclohexene in the presence of the fast-initiating Grubbs catalyst (H2IMes)(3-Br-Pyr)2Cl2Ru=CHPh leads to a completely linear as well as alternating copolymer, as demonstrated by NMR spectroscopy, isotopic labeling, and gel permeation chromatography. In contrast, intramolecular chain-transfer reactions were observed with [5.2.0] and [3.2.0] bicyclic carbomethoxy olefins, although to a lesser extent than with the previously reported monocyclic cyclobutenecarboxylic ester monomers [Song A.; Parker K. A.; Sampson N. S.J. Am. Chem. Soc.2009, 131, 3444]. Inclusion of cyclohexyl rings fused to the copolymer backbone minimizes intramolecular chain-transfer reactions and provides a framework for creating alternating functionality in a one-step polymerization.

Intermolecular radical cation Diels-Alder (RCDA) reaction of bicyclooctadienes: biomimetic formal total synthesis of kingianin A and total syntheses of kingianins D, F, H, and J.

Three endo bicyclooctadienol dimers corresponding to kingianins A and H, D, and F and J were obtained by the intermolecular radical cation Diels-Alder (RCDA) reaction. Each isomer was cleanly isolated without the aid of preparative HPLC. Kingianins D, F, H, and J were prepared by way of these intermediates from commercially available materials in 10, 13, 9, and 17 steps, respectively. Kingianin A has already been prepared from one of these compounds. Completion of the synthesis of kingianin H relied on Manchand's one-step, three-carbon homologation.

Bifunctional inhibition of HIV-1 reverse transcriptase: a first step in designing a bifunctional triphosphate.

The onset of resistance to approved anti-AIDS drugs by HIV necessitates the search for novel inhibitors of HIV-1 reverse transcriptase (RT). Developing single molecular agents concurrently occupying the nucleoside and nonnucleoside binding sites in RT is an intriguing idea but the proof of concept has so far been elusive. As a first step, we describe molecular modeling to guide focused chemical syntheses of conjugates having nucleoside (d4T) and nonnucleoside (TIBO) moieties tethered by a flexible polyethylene glycol (PEG) linker. A triphosphate of d4T-6PEG-TIBO conjugate was successfully synthesized that is recognized as a substrate by HIV-1 RT and incorporated into a double-stranded DNA.

Total synthesis of kingianin A.

A 12-step synthesis of kingianin A, an inhibitor of the antiapoptotic protein Bcl-xL, is based on a radical cation Diels-Alder reaction (RCDA). This approach is thought to be biomimetic. The use of a tether in the key RCDA step controls the regiochemistry of the cycloaddition, leading to the desired core structure and a separable diastereomer.

Total synthesis of (±)-bisabosqual A.

The synthesis of the novel squalene synthase inhibitor, bisabosqual A, was completed in 14 steps (longest linear sequence) from commercially available starting materials. The doubly convergent route employs a tandem 5-exo, 6-exo radical cyclization as the key step. This reaction assembles the fully functionalized tetracyclic core and introduces three stereogenic centers. Other effective transformations are the regioselective deoxygenation of an advanced enone intermediate and the chemo- and diastereoselective addition of trimethylaluminum to a ketone in the presence of esters.

A formal synthesis of (-)-englerin A by relay ring closing metathesis and transannular etherification.

A bicyclization approach to englerin A has culminated in a formal asymmetric total synthesis. Key transformations in the 10-step sequence are a regiospecific epoxide opening and a relay ene-yne-ene metathesis that converts linear substrates specifically to Δ(4,6)-guaiadiene-9,10 diol derivatives. Regiospecific functionalization of the diene moiety installs the oxygen bridge required for the englerin tricyclic core.

UVA/B exposure promotes the biosynthesis of dehydroretinol in cultured human keratinocytes.

Retinol and its metabolites modulate epithelial differentiation and serve as cellular UV sensors through changes in retinoid status. Of note is the dehydroretinol family which may serve functions distinct from parental retinol. This study focuses on the metabolism of this family and its potential participation in the response of normal epidermal human keratinocytes to UV irradiation. There were three findings. First, keratinocytes contain two pools of dehydroretinyl esters, one of which is shielded from UVB-, but not from UVA-induced decomposition. Second, using a novel in vitro assay we demonstrated that both UVA and UVB promote dehydroretinol biosynthesis in keratinocytes, but only UVB exposure promotes retinoid ester accretion by enhancing the activity of at least one acyl transferase. Finally, dehydroretinol sufficiency reduces UVA/B driven apoptosis more effectively than retinol sufficiency. This may in part be due to differences in the expression of Fas ligand, which we found to be upregulated by retinoic acid, but not dehydroretinoic acid. These observations implicate a role of dehydroretinol and its metabolites in UVA/B adaptation. Thus, the keratinocyte response to UV is jointly shaped by both the retinoids and dehydroretinoids.

Asymmetric induction in 8π electrocyclizations. Design of a removable chiral auxiliary.

The pseudo C(2) symmetric trans diphenyl oxazoline group acts as an effective chiral auxiliary in the 8π, 6π tandem electrocyclization of a substituted tetraene 1-carboxylic acid. Assignment of absolute stereochemistry to the [4.2.0] bicyclooctadiene product supports a model in which both s-cis and s-trans conformations favor the transition states with the same helical twist. This assignment prefaces the development of analogs of SNF4435 C and D. These natural products demonstrate activity as androgen receptor antagonists and as multidrug resistance (mdr) reversal agents.

Total synthesis of the potent androgen receptor antagonist (-)-arabilin: a strategic, biomimetic [1,7]-hydrogen shift.

The first total synthesis of (-)-arabilin, a Streptomyces metabolite that inhibits hormone activation of the androgen receptor, has been completed. The key step, a [1,7]-hydrogen shift, establishes the enol ether-containing skipped-tetraene substructure. This nonenzymatic pericyclic reaction is considered to be biomimetic.

Antibacterial studies of cationic polymers with alternating, random, and uniform backbones.

Antibacterial polymers have potential as pharmaceuticals and as coatings for implantation devices. The design of these materials will be optimized when we have a complete understanding of the structural features that impart activity toward target organisms and those that are benign with respect to the mammalian host. In this work, four series of polymers in which cationic and hydrophobic groups were distributed along the backbone were tested against six different bacterial species (both Gram-positive and Gram-negative) and for host cytotoxicities (red blood cell lysis). The most effective of the polymers studied are regularly spaced, featuring a 6-8 carbon stretch along the backbone between side chains that present positively charged groups. They cause potassium efflux, disorder the bacterial cytoplasmic membrane, and disrupt the membrane potential. These polymers, available from alternating ring-opening metathesis polymerization (AROMP), offer proof of principle for the importance of regular spacing in antibacterial polymers and for the synthesis of additional functional materials based on regularly spaced scaffolds.

Convergent Synthesis of 2H-Chromenes - a Formal [3+3] Cycloaddition by a One-pot, Three-Step Cascade.

In cases in which the palladium-catalyzed coupling of a bromoquinone with a vinyl stannane affords a vinyl quinone that enolizes, the resulting ortho-quinone methide undergoes an oxa-6π electrocyclization. Enolization is promoted by the presence of a polar additive. The net conversion is a formal [3+3] cycloaddition that gives 2H-chromenes. Because the first two steps of the cascade are catalyzed, the overall conversion is an example of multicatalysis. Yields for the optimized, one-pot protocol are dramatically improved over the conventional stepwise process.

Cyclic alternating ring-opening metathesis polymerization (CAROMP). Rapid access to functionalized cyclic polymers.

Catalysis of alternating ROMP with (H(2)IMes)Cl(2)Ru=CHPh(OiPr), the second generation Hoveyda-Grubbs catalyst, provided an entirely cyclic alternating polymer. Conditions for the cyclic AROMP were used to prepare a polymer in which one of the repeat units bore a primary alkyl chloride that was used for further elaboration.

Scope of the ring-opening metathesis polymerization (ROMP) reaction of 1-substituted cyclobutenes.

The reactivities of a series of 1-substituted cyclobutene derivatives (carboxylate esters, carboxamides, and carbinol esters) were investigated as substrates for ring-opening metathesis polymerization (ROMP) with [(H(2)IMes)(3-Br-pyridine)(2)(Cl)(2)Ru=CHPh]. Both the secondary amides of 1-cyclobutenecarboxylic acid and the esters of 1-cyclobutene-1-methanol undergo polymerization. The secondary amides provide translationally invariant polymers (E-olefins). Although the carbinol esters yield stereo- and regiochemically heterogeneous polymers, the 1-cyclobutenecarboxylic acid esters and tertiary amides undergo ring-opening metathesis (ROM) but not ROMP. The regio- and stereochemical outcomes of these ROMP and ROM reactions were analyzed at the B3LYP/6-31G* and LANL2DZ levels of theory. Calculations suggest that the regiochemistry and stereochemistry of the addition to the propagating carbene to form the metallocyclobutane intermediate depend on both charge distribution and steric interactions.

Structures of the phosphorylated and VO(3)-bound 2H-phosphatase domain of Sts-2.

The C-terminal domain of the suppressor of T cell receptor (TCR) signaling 1 and 2 (Sts-1 and -2) proteins has homology to the 2H-phosphatase family of enzymes. The phosphatase activity of the correspondent Sts-1 domain, Sts-1(PGM), is key for its ability to negatively regulate the signaling of membrane-bound receptors including TCR and the epidermal growth factor receptor (EGFR). A nucleophilic histidine, which is transiently phosphorylated during the phosphatase reaction, is essential for the activity. Here, we present the crystal structure of Sts-2(PGM) in the phosphorylated active form and bound to VO(3), which represent structures of an intermediate and of a transition state analogue along the path of the dephosphorylation reaction. In the former structure, the proposed nucleophilic His366 is the only phoshorylated residue and is stabilized by several interactions with conserved basic residues within the active site. In the latter structure, the vanadium atom sits in the middle of a trigonal bipyramid formed by the three oxygen atoms of the VO(3) molecule, atom NE2 of His366, and an apical water molecule W(a). The V-NE2 bond length (2.25 A) suggests that VO(3) is not covalently attached to His366 and that the reaction mechanism is partially associative. The two structures also suggest a role for Glu476 in activating a uniquely positioned water molecule. In both structures, the conformation of the active site is remarkably similar to the one seen in apo-Sts-2(PGM) suggesting that the spatial arrangement of the catalytic residues does not change during the dephosphorylation reaction.

Functional group compatibility. Propargyl alcohol reduction in the presence of a vinyl iodide.

Vinyl iodides are stable to the reduction of propargyl alcohols to cis allylic alcohols by hydrogen over Pd/CaCO(3) in hexane. They are also stable to the reduction of propargyl alcohols to saturated alcohols by hydrogen over Crabtree's iridium catalyst in CH(2)Cl(2).