Convergent assembly of highly potent analogues of bryostatin 1 via pyran annulation: bryostatin look-alikes that mimic phorbol ester function

Highly potent bryostatin analogues which contain the complete bryostatin core structure have been synthesized using a pyran annulation approach as a key strategic element. The A ring pyran was assembled using a pyran annulation reaction between a C1-C8 hydroxy allylsilane and an aldehyde comprising C9-C13. This pyran was transformed to a new hydroxy allylsilane and then coupled with a preformed C ring aldehyde subunit in a second pyran annulation, with concomitant formation of the B ring. This tricyclic intermediate was elaborated to bryostatin analogues which displayed nanomolar to subnanomolar affinity for PKC, but displayed properties indistinguishable from a phorbol ester in a proliferation/attachment assay.     

Natural Product Synthesis on Polymeric Supports—Synthesis and Biological Evaluation of an Indolactam Library

Potent activators of protein kinase C in fibroblasts: This property was determined for several indolactam V analogues ( 1 ) with a new cell-based assay system. This tumor-promoting indole alkaloid and analogues thereof can be synthesized efficiently on the solid phase. The key steps of the combinatorial approach are a regioselective amination of the indole ring and an enantioselective enzymatic reaction.     

Synthesis,biological evaluation and molecular docking study of dihydropyrimidine derivatives as potential anticancer agents

A series of dihydropyrimidine analogues were prepared via one-pot Biginelli three-component condensation reaction and characterized by NMR, FT-IR, MS spectra, and element analysis. Subsequently, they were screened for in vitro anticancer effect. These analogues revealed good cytotoxic activity against three human cancer cell lines including MCF-7, HepG-2, and A549. Among these analogues, compounds 4d and 4h were the most potent against three cell lines. Cell viability assays indicated 4a and 4c had lower cytotoxicity. In vitro cytotoxicity study on all synthesized compounds demonstrated that introduction of electron withdrawing substituents on C4 position of phenyl ring of dihydropyrimidine contributed to the antiproliferative potency. Moreover, in silico molecular docking results stipulated a sign of good correlation between experimental activity and calculated binding affinity. It proved 4d and 4h as the strongest compounds. Binding modes of analogues proposed the involvement of hydrophobic interactions and hydrogen bonds with Eg5 active site. Structure activity relationship studies indicated that incorporating electron withdrawing substituents on C4 position of phenyl ring of dihydropyrimidine are important for this biological activity.     

Synthesis of new pyridine intermediates as precursors for the elaboration of streptonigrin analogues by the metalation-cross-coupling strategy

Starting from available 2-amino-6-chloro-3-nitropyridine, synthesis of 3-amino-6-pyridinecarboxylic acid derivatives precursor for pyridine ring C of streptonigrin analogues is described.     

Design and applications of biomimetic anthraquinone dyes. III. Anthraquinone-immobilised C.I. reactive blue 2 analogues and their interaction with horse liver alcohol dehydrogenase and other adenine nucleotide-binding proteins

C.I. Reactive Blue 2 analogues were bonded onto an agarose support matrix by a novel method which entailed immobilisation by the anthraquinone ring 1-amino group as opposed to the usual triazine ring coupling methods. Dyes with spacer arms attached to the anthraquinone ring 1-amino group were synthesised by reacting methoxytriazine analogues of C.I. Reactive Blue 2 with chloroacetyl chloride and ethylenediamine. Unlike the blue parent dyes, all C.I. Reactive Blue 2 analogues with derivatised anthraquinone ring 1-amino groups were of a characteristic red colour. This change of chromaticity was entirely expected since the anthraquinone ring 1-amino group is an important component of the C.I. Reactive Blue 2 chromophore. Chromatographic studies indicated that, in comparison to adsorbents comprising triazine ring-immobilised dyes, adsorbents formed from C.I. Reactive Blue 2 analogues immobilised by the anthraquinone ring were better suited to the isolation of horse liver alcohol dehydrogenase and other adenine nucleotide-requiring enzymes. Similarities between C.I. Reactive Blue 2 analogues immobilised by the anthraquinone ring and N6-(6-aminohexyl)adenine nucleotide derivatives could be identified which may account for these observations. These studies confirm that highly effective affinity ligands based on synthetic textile dyes can be designed in a rational manner.     

A facile two synthon approach to the camptothecin skeleton

A synthesis of deethyldesoxycamptothecin via the reaction of two readily accessible synthons is described. One of the synthons constitutes the ABC ring system of camptothecin, while the second provides all the C atoms of the rings D and E. The synthetic approach is suited for the total synthesis of camptothecin analogues.     

Synthesis and conformational analysis of 17alpha,21-cyclo-22-unsaturated analogues of calcitriol

Six new calcitriol analogues, conformationally restricted at their side chain by the introduction of both a cyclopropane ring at C17-C20 and a double or triple bond at C22, were synthesized using the Wittig-Horner approach to construct the triene system. The six CD-ring and side-chain bearing fragments were prepared from ketone 14 by a divergent route to generate both series of epimers at C20, followed by stereoselective cyclopropanation. The (E)-alkenyl side chain was synthesized by means of a Wittig reaction. The alkynyl side chain was prepared by Corey-Fuchs homologation, followed by alkylation. The (Z)-alkenyl side chain was prepared from the previous alkyne by partial hydrogenation. The 20-epi analogues bind more strongly to VDR than the corresponding analogues with the C20 natural stereochemistry. These results can be reasoned by conformational analysis and hydrophobic interactions with the VDR ligand-binding domain.     

Synthesis of Tetracyclic Analogues of Calcitriol (1α,25‐Dihydroxyvitamin D3) with Side‐Chain‐Locked Spatial Orientations at C(20)

We present our first results on the synthesis of a new class of conformationally restricted vitamin D analogues bearing an extra five‐membered ring formed by linking C(18) and C(21). Two analogues of calcitriol ( 1 ) with unsaturations at the extra ring and the lateral chain were prepared. The triene system was introduced by the convergent Wittig Horner approach developed by Lythgoe [8] and F. Hoffmann‐La Roche [9]. The key steps in the preparation of the requisite fragments were: i) the long‐distance functionalization of ketal 11 at C(18), ii) the ring closure on 15 through an intramolecular aldol condensation to give the α,β‐unsaturated ketone 10 , and iii) the Pd‐catalyzed installation of the side chains.     

A Nozaki-Hiyama-Kishi Ni(II)/Cr(II) coupling approach to the phomactins

[reaction: see text] Through a unified synthetic strategy, appropriately functionalized bicyclic starting materials can be elaborated via Ni(II)/Cr(II) macrocylization to [9.3.1] bicycles. Elaboration of these core structures allows access to phomactin C/D analogues and establishes the first synthetic approach to phomactin A affording an intact octahydrochromene/macrocyclic ring system.     

A new class of simplified phorbol ester analogues: synthesis and binding to PKC and eta PKC-C1B (eta PKC-CRD2)

[formula: see text] A unique class of simplified phorbol ester analogues is described for the first time. A highly efficient retro-annelation sequence was developed in order to remove the five-membered ring from the phorbol diterpene core, allowing access to BCD ring analogues of the phorbol esters. The binding of these analogues to protein kinase C (PKC) and the truncated peptide eta PKC-C1B (eta PKC-CRD2) is also reported.     

The reactivity of a silacyclopentadienylidene towards aldehydes: silole ring expansion and the formation of base-stabilized silacyclohexadienones

Tossing aldehydes into the ring: The reaction of a silacyclopentadienylidene with aldehydes leads to C=O bond cleavage with the formation of base-stabilized silanones and cyclopropanation of the adjacent C=C bond, followed by silole ring expansion to give silicon analogues of cyclohexadienones.     

Rational design of outer‐expanded purine analogues as building blocks of DNA‐based nanowires with enhanced electronic properties

Due to the fact that natural DNA may lack sufficient conductance for direct application in molecular electronics, a novel design of outer‐expanded purine analogues was proposed by incorporating an aromatic ring at the N7‐C8 site into natural G and A bases from the outside. The effect of the outer‐expansion modification on electronic properties of DNA was investigated by density functional theory and molecular dynamics. The analyses revealed that these purine analogues not only preserve the same sizes of natural purine bases, thus avoiding distortions of DNA skeleton induced by the normal ring‐inner‐expansion modification, but also keep the selectivity of pairing with their natural counterpart C and T bases. More importantly, their electronic properties are enhanced, indicated by the narrowed HOMO–LUMO gaps, the lowered ionization potentials and the improved ultraviolet absorption spectra. This work may provide helpful information for designing of artificial bases as promising building blocks of biomolecular nanowires. © 2014 Wiley Periodicals, Inc.     

Gram scale synthesis of the C(18)-C(34) fragment of amphidinolide C

The synthesis of the C(18)-C(34) fragment of amphidinolide C has been achieved via two routes, culminating in both the shortest (11 steps) and highest yielding (26% overall yield) approaches to this segment. The highly convergent approach will facilitate the synthesis of analogues, including the C(18)-C(29) fragment of amphidinolide F. Synthetic highlights include the selective methylation of a diyne, and the highly efficient use of a second generation cobalt catalyst in the Mukaiyama oxidative cyclization to form the trans-THF ring.     

Total syntheses of bryostatins: synthesis of two ring-expanded bryostatin analogues and the development of a new-generation strategy to access the C7-C27 fragment

Herein, we report the synthesis of novel ring-expanded bryostatin analogues. By carefully modifying the substrate, a selective and high-yielding Ru-catalyzed tandem enyne coupling/Michael addition was employed to construct the northern fragment. Ring-closing metathesis was utilized to form the 31-membered ring macrocycle of the analogue. These ring-expanded bryostatin analogues possess anticancer activity against several cancer cell lines. Given the difficulty in forming the C16-C17 olefin at a late stage, we also describe our development of a new-generation strategy to access the C7-C27 fragment, containing both the ring B and C subunits.     

Stereoselective Synthesis of Tropanes via a 6π‐Electrocyclic Ring‐Opening/ Huisgen [3+2]‐Cycloaddition Cascade of Monocyclopropanated Heterocycles

The synthesis of tropanes via a microwave‐assisted, stereoselective 6π‐electrocyclic ring‐opening/ Huisgen [3+2]‐cycloaddition cascade of cyclopropanated pyrrole and furan derivatives with electron‐deficient dipolarophiles is demonstrated. Starting from furans or pyrroles, 8‐aza‐ and 8‐oxabicyclo[3.2.1]octanes are accessible in two steps in dia‐ and enantioselective pure form, being versatile building blocks for the synthesis of pharmaceutically relevant targets, especially for new cocaine analogues bearing various substituents at the C‐6/C‐7 positions of the tropane ring system. Moreover, the 2‐azabicyclo[2.2.2]octane core (isoquinuclidines), being prominently represented in many natural and pharmaceutical products, is accessible via this approach.     

Synthesis and Anticholinesterase Activity of (-)-Physostigmine Analogues with Modifications at C3a and C5

A new series of physostigmine analogues 3a―3j with modifications at the C3a and C5 positions was designed and synthesized. Bioassay of the synthetic analogues 3a―3j, along with the previous synthesized C3a-ethyl-C5-triazole physostigmine analogues 1a―1g and 2a―2j was performed, which indicates that the replacement of the carbamoyl moiety of C3a-ethyl-C5-triazole analogues 1 and 2 with a triazole moiety decreased acetylcholinesterase(AchE) inhibitory activity, whereas the introduction of heterocycles into the triazole ring increased both AChE and butyrylcholinesterase(BchE) inhibitory activities. Structure-activity relationship(SAR) studies of C3a-methyl-C5-triazole analogues 3 reveal the C3a-methyl substituent is important for AChE and BChE inhibition and the introduction of a second ionizable N center improved the binding of the synthetic analogues to both AChE and BChE.     

Total synthesis and biological evaluation of neodysiherbaine A and analogues

Dysiherbaine (1) and its congener neodysiherbaine A (2) are naturally occurring excitatory amino acids with selective and potent agonistic activity for ionotropic glutamate receptors. We describe herein the total synthesis of 2 and its structural analogues 3-8. Advanced key intermediate 16 was employed as a branching point to assemble a series of these analogues 3-8 with respect to the C8 and C9 functionalities, which would not have been accessible through manipulations of the natural product itself. The synthesis of key intermediate 16 features (i) stereocontrolled C-glycosylation to set the C6 stereocenter, (ii) concise synthesis of the bicyclic ether skeleton through chemo- and stereoselective dihydroxylation of the exo-olefin and stereoselective epoxidation of the endo-olefin, followed by epoxide ring opening/5-exo ring closure, and (iii) catalytic asymmetric hydrogenation of enamide ester to construct the amino acid appendage. A preliminary biological evaluation of analogues for their in vivo toxicity against mice and binding affinity for glutamate receptors showed that both the type and stereochemistry of the C8 and C9 functional groups affected the subtype selectivity of dysiherbaine analogues for members of the kainic acid receptor family.     

Synthesis and Structure–Activity Relationship of Vicenistatin,a Cytotoxic 20‐Membered Macrolactam Glycoside

We have developed two syntheses of vicenistatin and its analogues. Our first‐generation strategy included the rapid and sequential assembly of the macrocyclic lactam by using an intermolecular Horner–Wadsworth–Emmons reaction between the C3–C13 fragment and the C1–C2, C14–C19 fragment, followed by an intramolecular Stille coupling reaction. The second‐generation strategy utilized a ring‐closing metathesis of a hexaene intermediate to generate the desired 20‐membered macrolactam. This second‐generation strategy made it possible to prepare synthetic analogues of vicenistatin, including the C20‐ and/or C23‐demethyl analogues. Evaluation of the cytotoxic effect of these analogues indicated the importance of the fixed conformation of aglycon for determining the biological activity of the vicenistatins.     

Diels-Alder reaction-aromatization approach toward functionalized ring C allocolchicinoids. Enantioselective total synthesis of (-)-7S-allocolchicine

Allocolchicinoids are analogues of the important antimitotic compound (-)-colchicine 1. A strategy is reported for the synthesis of ring C functionalized allocolchicinoids, which is based on a Diels-Alder reaction-aromatization sequence. This route is complementary to the previously disclosed benzannulation approach involving Fischer carbene complexes and alkynes. Dienes 12 and 14 incorporate the natural substitution pattern on ring A and undergo Diels-Alder reactions with various dienophiles. Subsequent aromatization affords the set of differently functionalized ring C allocolchicinoids 15-19, 23, and 25, with high regioselectively and in moderate to good yields. An intramolecular Diels-Alder reaction-aromatization sequence allows for access to allocolchicinoids with reversed regiochemical introduction of ring C substituents. The equilibria of the atropisomers of 15 and 19 are studied in three NMR solvents. Reactions of the dienes 12 and 14 with DMAD lead to the corresponding cycloadducts, but the subsequent aromatization is complicated. A regioselective Diels-Alder reaction-aromatization sequence is utilized as the key step in the first stereoselective total synthesis of (-)-allocolchicine 2. Asymmetric introduction of hydroxy group at C7 is achieved by the enantioselective reduction of ketone 29. The correct stereochemistry is then established by Mitsunobu inversion reaction using Zn(N(3))(2)-2Py.     

General diastereoselective synthetic approach toward isospongian diterpenes. Synthesis of (-)-marginatafuran, (-)-marginatone, and (-)-20-acetoxymarginatone

This work describes a synthetic approach to the carbocyclic skeleton of isospongian diterpenes that uses the commercially available monoterpene (S)-carvone as a C-ring synthon, which is incorporated into the tetracyclic isospongian framework via a C→ABC→ABCD ring annulation strategy using intramolecular Diels-Alder and ring-closing metathesis reactions. This approach has been successfully used to prepare both the title natural isospongians and several nonnatural oxygenated analogues. A preliminary evaluation of the inhibitory activity of the small collection of synthesized isospongians on the mammalian mitochondrial respiratory chain revealed that most were able to inhibit the integrated electron transfer chain (NADH oxidase activity) in the micromolar range.