Synthesis and biological activities of simplified aplysiatoxin analogs focused on the CH/π interaction.

Debromoaplysiatoxin (DAT) is a potent protein kinase C (PKC) activator with tumor-promoting and pro-inflammatory activities. Irie and colleagues have found that 10-methyl-aplog-1 (1), a simplified analog of DAT, has strong anti-proliferative activity against several cancer cell lines with few adverse effects. Therefore, 1 is a potential lead compound for cancer therapy. We synthesized a new derivative 2 which has a naphthalene ring at the side chain terminal position instead of a benzene ring, to increase CH/π interactions with Pro-241 of the PKCδ-C1B domain. Based on the synthetic route of 1, 2 was convergently synthesized in 26 linear steps from 6-hydroxy-1-naphthoic acid with an overall yield of 0.18%. Although the anti-proliferative activity of 2 was more potent than that of 1, the binding potency of 2 to the PKCδ-C1B domain did not exceed that of 1. Molecular dynamics simulation indicated the capability of 2 to simultaneously form hydrogen bonds and CH/π interactions with the PKCδ-C1B domain. Focusing on the hydrogen bonds, their geometry in the binding modes involving the CH/π interactions seemed to be sub-optimal, which may explain the slightly lower affinity of 2 compared to 1. This study could be of help in optimizing such interactions and synthesizing a promising lead cancer compound.

11-Step Total Synthesis of Teleocidins B-1-B-4.

A unified and modular approach to the teleocidin B family of natural products is presented that proceeds in 11 steps and features an array of interesting strategies and methods. Indolactam V, the known biosynthetic precursor to this family, was accessed through electrochemical amination, Cu-mediated aziridine opening, and a remarkable base-induced macrolactamization. Guided by a desire to minimize concession steps, the tactical combination of C-H borylation and a Sigman-Heck transform enabled the convergent, stereocontrolled synthesis of the teleocidins.

Nhatrangin A: Total Syntheses of the Proposed Structure and Six of Its Diastereoisomers.

A total synthesis of the proposed structure of nhatrangin A is described. This strategy relies on two aldol reactions to install the chiral centers at C3/C4 and C3'/C4', a lithium-mediated coupling between an advanced intermediate alkyne and a Weinreb amide to complete the C1-C13 alkyl scaffold, and a Yamaguchi esterification to set the side chain. Discrepancies in the spectroscopic data between synthetic and natural nhatrangins led us to synthesize six more diastereoisomers of the proposed structure of nhatrangin A.

Structural optimization of 10-methyl-aplog-1, a simplified analog of debromoaplysiatoxin, as an anticancer lead.

Aplog-1 is a simplified analog of debromoaplysiatoxin (DAT) with potent tumor-promoting and proinflammatory activities. Aplog-1 and DAT exhibited anti-proliferative activities against several human cancer cell lines, whereas aplog-1 did not have tumor-promoting nor proinflammatory activities. We have recently found 10-methyl-aplog-1 (1) to have strong anti-proliferative activity compared with aplog-1. To further investigate the structural factors involved in the tumor-promoting, proinflammatory, and anti-proliferative activities, two dimethyl derivatives of aplog-1 (2, 3) were synthesized, where two methyl groups were installed at positions 4 and 10 or 10 and 12. 10,12-Dimethyl-aplog-1 (2) had stronger inhibitory effects on the growth of several human cancer cell lines than 1 and DAT, but exhibited no tumor-promoting and proinflammatory activities. In contrast, 4,10-dimethyl-aplog-1 (3) displayed weak tumor-promoting and proinflammatory activities along with anti-proliferative activity similar to that of 1 and DAT. Compound 2 would be the optimized seed for anticancer drugs among the simplified analogs of DAT.

Enantioselective total synthesis of (+)-lyngbyabellin M.

Lyngbyabellin M is a non-ribosomal peptide synthetase/polyketide synthase derived metabolite isolated from the cyanobacterium M. bouillonii displaying thiazole rings and a distinct chlorinated octanoic acid chain. Its absolute configuration was proposed based on the comparison of its spectroscopic data with those of other representatives of this family of marine natural products, as well as degradation and derivatization studies. Here the first total synthesis of (+)-lyngbyabellin M is described based on the coupling of three key intermediates: two chiral thiazole moieties and an anti hydroxycarboxylic acid prepared stereoselectively via a boron enolate mediated aldol reaction directed by Masamune's chiral auxiliary.

Studies toward the total synthesis of Itralamide B and biological evaluation of its structural analogs.

Itralamides A and B were isolated from the lipophilic extract of Lyngbya majuscula collected from the eastern Caribbean. Itralamide B (1) showed cytotoxic activity towards human embryonic kidney cells (HEK293, IC50 = 6 µM). Preliminary studies disapproved the proposed stereochemistry of itralamide. In this paper, we will provide a full account of the total synthesis of four stereoisomers of itralamide B and the results derived from biological tests of these structural congeners.

Synthesis and biological activities of the amide derivative of aplog-1, a simplified analog of aplysiatoxin with anti-proliferative and cytotoxic activities.

Aplog-1 is a simplified analog of the tumor-promoting aplysiatoxin with anti-proliferative and cytotoxic activities against several cancer cell lines. Our recent findings have suggested that protein kinase Cδ (PKCδ) could be one of the target proteins of aplog-1. In this study, we synthesized amide-aplog-1 (3), in which the C-1 ester group was replaced with an amide group, to improve chemical stability in vivo. Unfortunately, 3 exhibited seventy-fold weaker binding affinity to the C1B domain of PKCδ than that of aplog-1, and negligible anti-proliferative and cytotoxic activities even at 10(-4) M. A conformational analysis and density functional theory calculations indicated that the stable conformation of 3 differed from that of aplog-1. Since 27-methyl and 27-methoxy derivatives (1, 2) without the ability to bind to PKC isozymes exhibited marked anti-proliferative and cytotoxic activities at 10(-4) M, 3 may be an inactive control to identify the target proteins of aplogs.

Synthesis and biological activities of simplified analogs of the natural PKC ligands, bryostatin-1 and aplysiatoxin.

Protein kinase C (PKC) isozymes play central roles in signal transduction on the cell surface and could serve as promising therapeutic targets of intractable diseases like cancer, Alzheimer's disease, and acquired immunodeficiency syndrome (AIDS). Although natural PKC ligands like phorbol esters, ingenol esters, and teleocidins have the potential to become therapeutic leads, most of them are potent tumor promoters in mouse skin. By contrast, bryostatin-1 (bryo-1) isolated from marine bryozoan is a potent PKC activator with little tumor-promoting activity. Numerous investigations have suggested bryo-1 to be a promising therapeutic candidate for the above intractable diseases. However, there is a supply problem of bryo-1 both from natural sources and by organic synthesis. Recent approaches on the synthesis of bryo-1 have focused on its simplification, without decreasing the ability to activate PKC isozymes, to develop new medicinal leads. Another approach is to use the skeleton of natural PKC ligands to develop bryo-1 surrogates. We have recently identified 10-methyl-aplog-1 (26), a simplified analog of tumor-promoting aplysiatoxin (ATX), as a possible therapeutic lead for cancer. This review summarizes recent investigations on the simplification of natural PKC ligands, bryo-1 and ATX, to develop potential medicinal leads.

Total synthesis of nhatrangin A.

A concise and stereoselective approach for the synthesis of key intermediates for aplysiatoxins, oscillatoxins, and nhatrangins and their utility for the total synthesis of nhatrangin A has been demonstrated. The advanced intermediates aromatic aldehyde 11 and dihydroxy acid 12 were synthesized in eight steps (44% overall yield) and three steps (55% overall yield), respectively. An asymmetric Michael addition, CBS reduction, and proline-catalyzed crossed-aldol reactions were utilized as key steps for the generation of all the chirality of main chain hydroxyaldehyde, while the appended side-chain-protected 3,4-dihydroxypentanoic acid was achieved in a shortest route, using Sharpless dihydroxylation, diol protection, and RuO4-catalyzed aromatic over-oxidation reactions. Synthesis of nhatrangin A was accomplished by coupling of dihydroxy acid 12 with ß-hydroxyallyl ester (obtained from 11) under Yamaguchi reaction conditions followed by a one-pot deprotection of all protecting groups.

The first total synthesis of nhatrangin A.

The first total synthesis of nhatrangin A has been achieved. Pivotal bond forming events in the synthesis include Brown crotylboration, olefin cross-metathesis, Sharpless asymmetric dihydroxylation and Yamaguchi esterification.

Artificial analogs of naturally occurring tumor promoters as biochemical tools and therapeutic leads.

Tumor promoters are non-carcinogenic chemicals that enhance tumor formation when administered repeatedly after a low dose of a carcinogen. Phorbol esters, teleocidins, and aplysiatoxins are typical examples of naturally occurring tumor promoters. All of them share the ability to bind and activate protein kinase C (PKC) despite the differences in their chemical structures. A variety of analogs with unique chemical and biological properties have been developed to analyze the molecular mechanism of tumor promotion through PKC activation. Moreover, coupled with the emerging significance of PKC in the pathological processes of Alzheimer's disease (AD) and acquired immune deficiency syndrome (AIDS) as well as cancer, several efforts have been made recently to generate analogs of tumor promoters with therapeutic potential. This review focuses on artificial analogs of phorbol esters, teleocidins, and aplysiatoxins, and discusses their potential as biochemical tools and therapeutic leads.

Structure-activity studies on the spiroketal moiety of a simplified analogue of debromoaplysiatoxin with antiproliferative activity.

Aplog-1, a simplified analogue of tumor-promoting debromoaplysiatoxin, is antiproliferative but not tumor-promoting. Our recent study has suggested that local hydrophobicity around the spiroketal moiety is a crucial determinant for antiproliferative activity. To further clarify the structural features relevant to the activity, we synthesized two methyl derivatives of aplog-1, where a methyl group was installed at position 4 or 10 of the spiroketal moiety. 10-Methyl-aplog-1 (5) bound to the C1B domains of novel PKCs (δ, η, and θ) with subnanomolar K(i) values, approximately 10-20 times stronger than aplog-1, and markedly inhibited the growth of many human cancer cell lines, while 4-methyl-aplog-1 (4) had levels of activity similar to those of aplog-1. Interestingly, 5 showed little tumor-promoting activity unlike the tumor promoter debromoaplysiatoxin. These results suggest that 5 is a potent PKC ligand without tumor-promoting activity and could be a therapeutic lead for the treatment of cancer, like bryostatins.

Role of the phenolic hydroxyl group in the biological activities of simplified analogue of aplysiatoxin with antiproliferative activity.

The 18-deoxy derivative (3) of a simplified analogue (1) of aplysiatoxin with antiproliferative activity was synthesized to examine the role of the phenolic hydroxyl group at position 18 in the biological activities of 1. Compound 3 as well as 1 showed significant affinity for protein kinase Cδ (PKCδ), and the antiproliferative activity of 3 was slightly higher than that of 1. However, the anti-tumor-promoting activity of 3 was less than that of 1 in vitro, suggesting that the phenolic hydroxyl group of 1 is necessary for the anti-tumor-promoting activity but not for the binding of PKCδ and antiproliferative activity. Moreover, PKC isozyme selectivity of 3 was similar to that of 1, suggesting non-PKC receptors for these compounds to play some roles in the anti-tumor-promoting activity of 1.

Synthesis of (S)-jamaicamide C carboxylic acid.

The jamaicamides are natural product sodium channel blockers derived from the cyanobacterium Lyngbya majuscula. The carboxylic acid fragment of jamaicamide C contains a methyl stereocenter and a trisubstituted E chloroolefin. Herein, we present the nonracemic synthesis of the aliphatic chain of jamaicamide C. The methyl stereocenter was installed using Evans' oxazolidinone methodology, and the trisubstituted chloroolefin was set by silylstannylation of a triple bond.

A simple analogue of tumor-promoting aplysiatoxin is an antineoplastic agent rather than a tumor promoter: development of a synthetically accessible protein kinase C activator with bryostatin-like activity.

Protein kinase C (PKC) is widely recognized as a therapeutic target in intractable diseases such as cancer, Alzheimer's disease (AD), and acquired immune deficiency syndrome (AIDS). While inhibition of PKC is a general therapeutic strategy for the treatment of cancer, PKC activators are potential therapeutic agents for AD and AIDS. However, concerns have been raised about their therapeutic use since PKC activators such as phorbol esters exhibit potent tumor-promoting activities. Naturally occurring bryostatin 1 (bryo-1), prostratin, and 12-deoxyphorbol 13-phenylacetate (DPP) are fascinating PKC activators without tumor-promoting activities. Bryo-1 is currently in clinical trials for the treatment of cancer and is also effective against AD. Prostratin and DPP are attractive candidates for the adjunctive treatment of human immunodeficiency virus (HIV) infection. However, their limited availability from natural sources and synthetic complexity have hampered further development as therapeutic agents. We report here easy access (22 steps) to a simple analogue (1) of the tumor-promoting aplysiatoxin (ATX) as a novel PKC activator with anticancer and anti-tumor-promoting activities. Anticancer activities of 1 against several human cancer cell lines were comparable to those of bryo-1. Moreover, 1 as well as bryo-1 significantly inhibited the Epstein-Barr virus early antigen (EBV-EA) induction by the tumor promoter 12-O-tetradecanoylphorbol 13-acetate (TPA), whereas ATX strongly induced EBV-EA. This inhibitory effect is characteristic of antitumor promoters. Compound 1 as well as bryo-1 displayed significant binding and activation of PKCdelta and induced its translocation to the nuclear membrane in CHO-K1 cells. This study provides a synthetically accessible PKC activator with bryo-1-like activities, which could be another therapeutic lead for cancer, AD, and AIDS.

Efficient and highly enantioselective formation of the all-carbon quaternary stereocentre of lyngbyatoxin A.

Indole 25, an advanced intermediate in a projected enantioselective total synthesis of lyngbyatoxin A 1, was prepared from allylic alcohol 11 in 9 steps and >95% ee, key transformations being the enantiospecific rearrangement of vinyl epoxide 14 and the Hemetsberger-Knittel reaction of azide 24.

Studies of the formation of all-carbon quaternary centres, en route to lyngbyatoxin A. A comparison of phenyl and 7-substituted indole systems.

Copper mediated allylic substitutions and conjugate additions to geranyl, cinnamyl and allylic indole compounds have been investigated with the aim of finding a method for the creation of the all-carbon quaternary centre present in the natural product lyngbyatoxin A. Reaction conditions have been found giving a 68% SN2' selectivity in the copper mediated addition of PhMgBr to geranyl chloride, as well as 99% and 95% SN2' selectivity in the copper catalysed addition of EtMgBr to cinnamyl chloride and acetate, respectively. When the optimised reaction conditions were applied to the corresponding allylic compounds containing a 7-substituted indole moiety, the regioselectivity was reversed giving only the SN2 product. The allylic indole-containing substrates were also found to be unproductive in Pd- or Mo-catalysed SN2'-type substitution reactions. In related studies, copper catalysed conjugate addition of EtMgBr to the tricyclic lactam 6-methyl-pyrrolo[3,2,1-ij]quinolin-4-one gave a maximum of 20% of the 1,4-addition product.

C-C bond formation via C-H bond activation: synthesis of the core of teleocidin B4.

The core of teleocidin B4, a complex fragment of a natural product containing two quaternary stereocenters and a penta-substituted benzene ring, was synthesized in four C-C bond-forming steps starting from tert-butyl derivative 1. The first step involved alkenylation of the tert-butyl group with a vinyl boronic acid, followed by the successful annulation of the cyclohexane ring to the benzene nucleus via an intramolecular Friedel-Crafts reaction. The third step required a diastereoselective oxidative carbonylation of the geminal dimethyl group, followed at last by indole assembly via the alkenylation of the phenol nucleus, to afford the teleocidin B4 core. Noteworthy is the fact that steps 1 and 3 critically depended on the directing role of the aniline nitrogen (directed C-H bond functionalization).

Solid-phase synthesis and biological evaluation of a teleocidin library--discovery of a selective PKCdelta down regulator.

Protein kinaseC (PKC) is linked to the signal-induced modulation of a wide variety of cellular processes, such as growth, differentiation, secretion, apoptosis, and tumor development. The design and synthesis of small molecules that regulate these different cellular signaling systems is at the forefront of modern drug design. Herein we report a) an efficient method for the synthesis of indolactamV (6), a PKC activator, and its N13-des(methyl) analogues (19) using a regioselective organometallic transformation, a convenient aminomalonate derivative (10) to introduce the appropriate functionality and an enantiospecific enzymic hydrolysis as key steps; b) the use of this method in the first solid-phase synthesis of a teleocidin library modifying the N-13, C-12 and C-7 alkyl chains, and, therefore, producing a library of potential activators and/or inhibitors of PKC of the general structure (32); c) the activation of PKC by selected members of the library using a MARCKS translocation in vivo assay system; d) the observation that some of these analogues are nearly as effective as the natural PKC activators phorbol dibutyrate and (-)-indolactam V (6), and e) the observation that some of these analogues have different potential to induce down-regulation of members of the PKC gene family after chronic stimulation.

[The active conformation of teleocidins: design and synthesis of new active molecules].

This review deals with structure-activity relationships, conformational analysis of teleocidins and creation of new active compounds for the determination of active conformation of teleocidins. Phorbol esters containing 12-O-tetradecanoylphorbol-13-acetate (TPA) and teleocidins which are classified as TPA-type tumor promoters exhibit potent tumor-promoting activity as well as many important biological activities connected with cell prolification and cell differentiation. Teleocidins are known to exist in an equilibrium between two conformational states in solution, the twist and the sofa forms. The low energy barrier between the two conformers makes it difficult to identify the mode of interaction of these promoters with common macromolecular targets. Design and synthesis of molecules having a new skeleton (benzolactams) and producing two conformations of teleocidins solve the problem. Benzolactams become the simplest molecule reproducing the conformation and activity of teleocidin, and will be a useful tool for the study of tumor-promotion and cell differentiation.