Computer-guided design, synthesis, and protein kinase C affinity of a new salicylate-based class of bryostatin analogs

CXCR4 Targeting Nanoplatform for Transcriptional Activation of Latent HIV-1 Infected T Cells

Antiretroviral drugs are limited in their ability to target latent retroviral reservoirs in CD4+ T cells, highlighting the need for a T cell-targeted drug delivery system that activates the transcription of inactivated viral DNA in infected cells. Histone deacetylase inhibitors (HDACi) disrupt chromatin-mediated silencing of the viral genome and are explored in HIV latency reversal. But single drug formulations of HDACi are insufficient to elicit therapeutic efficacy, warranting combination therapy. Furthermore, protein kinase C activators (PKC) have shown latency reversal activity in HIV by activating the NF-κB signaling pathway. Combining HDACi (SAHA) with PKC (PMA) activators enhances HIV reservoir activation by promoting chromatin decondensation and subsequent transcriptional activation. In this study, we developed a mixed nanomicelle (PD-CR4) drug delivery system for simultaneous targeting of HIV-infected CD4+ T cells with two drugs, suberoylanilide hydroxamic acid (SAHA) and phorbol 12-myristate 13-acetate (PMA). SAHA is a HDACi that promotes chromatin decondensation, while PMA is a PKC agonist that enhances transcriptional activation. The physicochemical properties of the formulated PD-CR4 nanoparticles were characterized by NMR, CMC, DLS, and TEM analyses. Further, we investigated in vitro safety profiles, targeting efficacy, and transcriptional activation of inactivated HIV reservoir cells. Our results suggest that we successfully prepared a targeted PD system with dual drug loading. We have compared latency reversal efficacy of a single drug nanoformulation and combination drug nanoformulation. Final PD-SP-CR4 successfully activated infected CD4+ T cell reservoirs and showed enhanced antigen release from HIV reservoir T cells, compared with the single drug treatment group as expected. To summarize, our data shows PD-SP-CR4 has potential T cell targeting efficiency and efficiently activated dormant CD4+ T cells. Our data indicate that a dual drug-loaded particle has better therapeutic efficacy than a single loaded particle as expected. Hence, PD-CR4 can be further explored for HIV therapeutic drug delivery studies.

Function-Oriented Synthesis: Design, Synthesis, and Evaluation of Highly Simplified Bryostatin Analogues

Using a function-oriented synthesis strategy, we designed, synthesized, and evaluated the simplest bryostatin 1 analogues reported to date, in which bryostatin's A- and B-rings are replaced by a glutarate linker. These analogues, one without and one with a C26-methyl group, exhibit remarkably different protein kinase C (PKC) isoform affinities. The former exhibited bryostatin-like binding to several PKC isoforms with K_i's < 5 nM, while the latter exhibited PKC affinities that were up to ∼180-fold less potent. The analogue with bryostatin-like PKC affinities also exhibited bryostatin-like PKC translocation kinetics in vitro, indicating rapid cell permeation and engagement of its PKC target. This study exemplifies the power of function-oriented synthesis in reducing structural complexity by activity-informed design, thus enhancing synthetic accessibility, while still maintaining function (biological activity), collectively providing new leads for addressing the growing list of therapeutic indications exhibited by PKC modulators.

Bryostatin 1 Promotes Synaptogenesis and Reduces Dendritic Spine Density in Cortical Cultures through a PKC-Dependent Mechanism

The marine natural product bryostatin 1 has demonstrated procognitive and antidepressant effects in animals and has been entered into human clinical trials for treating Alzheimer's disease (AD). The ability of bryostatin 1 to enhance learning and memory has largely been attributed to its effects on the structure and function of hippocampal neurons. However, relatively little is known about how bryostatin 1 influences the morphology of cortical neurons, key cells that also support learning and memory processes and are negatively impacted in AD. Here, we use a combination of carefully designed chemical probes and pharmacological inhibitors to establish that bryostatin 1 increases cortical synaptogenesis while decreasing dendritic spine density in a protein kinase C (PKC)-dependent manner. The effects of bryostatin 1 on cortical neurons are distinct from those induced by neural plasticity-promoting psychoplastogens such as ketamine. Compounds capable of increasing synaptic density with concomitant loss of immature dendritic spines may represent a unique pharmacological strategy for enhancing memory by improving signal-to-noise ratio in the central nervous system.

Unlocking the Drug Potential of the Bryostatin Family: Recent Advances in Product Synthesis and Biomedical Applications

Bryostatins are a class of naturally occurring macrocyclic lactones with a unique fast developing portfolio of clinical applications, including treatment of AIDS, Alzheimer's disease, and cancer. This comprehensive account summarizes the recent progress (2014-present) in the development of bryostatins, including their total synthesis and biomedical applications. An emphasis is placed on the discussion of bryostatin 1, the most-studied analogue to date. This review highlights the synthetic and biological challenges of bryostatins and provides an outlook on their future development.

Structural Simplification of Natural Products

Natural products (NPs) are important sources of clinical drugs due to their structural diversity and biological prevalidation. However, the structural complexity of NPs leads to synthetic difficulties, unfavorable pharmacokinetic profiles, and poor drug-likeness. Structural simplification by truncating unnecessary substructures is a powerful strategy for overcoming these limitations and improving the efficiency and success rate of NP-based drug development. Herein, we will provide a comprehensive review of the structural simplification of NPs with a focus on design strategies, case studies, and new technologies. In particular, a number of successful examples leading to marketed drugs or drug candidates will be discussed in detail to illustrate how structural simplification is applied in lead optimization of NPs.

Towards 20,20-difluorinated bryostatin: synthesis and biological evaluation of C17,C27-fragments

The syntheses of compounds corresponding to 20,20-difluorinated C17–C27 fragments of bryostatin are reported together with preliminary PKC binding data.

Molecular Targets of Active Anticancer Compounds Derived from Marine Sources

Over the past decades, a number of novel compounds, which are produced in the marine environment, have been found to exhibit the anticancer effects. This review focuses on molecular targets of marine-derived anticancer candidates in clinical and preclinical studies. They are kinases, transcription factors, histone deacetylase, the ubiquitin-proteasome system, and so on. Specific emphasis of this review paper is to provide information on the optimization of new target compounds for future research and development of anticancer drugs, based on the identification of structures of these target molecules and parallel compounds.

Synthetic approaches to the C11-C27 fragments of bryostatins

The modified Julia reaction and acyl carbanion chemistry, especially reactions of 2-lithiated dithianes, have been investigated for the synthesis of intermediates that are the synthetic equivalents of the C11-C27 fragments of bryostatins. The modified Julia reaction using 2-benzothiazolylsulfones was found to be more useful for the formation of the C16-C17 double-bond than the classical Julia reaction using phenylsulfones, and bulky sulfones gave very good (E)-stereoselectivity. The alkylation of a dithiane monoxide that corresponded to a C19-acyl carbanion using (E)-1-bromobut-2-ene was efficient but the use of a more complex allylic bromide corresponding to the C20-C27 fragment of the bryostatins was unsuccessful, possibly due to competing elimination reactions. This meant that the use of C19 dithianes for the synthesis of 20-deoxybryostatins would have to involve the stepwise assembly of the C20-C27 fragment from simpler precursors. However, lithiated C19 dithianes gave good yields of adducts with aldehydes and conditions were developed for the stereoselective conversion of the major adducts into methoxyacetals that corresponded to the C17-C27 fragment of 20-oxygenated bryostatins. A convergent synthesis of the C11-C27 fragment of a 20-deoxybryostatin was subsequently achieved using a 2-benzothiazolylsulfone corresponding to the intact C17-C27 fragment.

Total synthesis of 7-des-O-pivaloyl-7-O-benzylbryostatin 10

The first total synthesis of a derivative of a 20-deoxybryostatin, namely 7-des-O-pivaloyl-7-O-benzylbryostatin 10 is described. Preliminary studies demonstrated that the modified Julia reactions of 2-benzothiazolylsulfones corresponding to the C17-C27 fragment with aldehydes corresponding to the C1-C16 fragment, provided an efficient and stereoselective assembly of advanced intermediates with the (E)-16,17-double-bond. The synthesis of the C1-C16 fragment was then modified so that the C1 acid was present as its allyl ester before the Julia coupling. A more efficient synthesis of the C17-C27 sulfone was developed in which a key step was the bismuth mediated coupling of an allylic bromide with an aldehyde in the presence of an acrylate moiety in the allylic bromide. A scalable synthesis of an advanced macrolide was completed using the modified Julia reaction followed by selective deprotection and macrolactonisation. The final stages of the synthesis required selective hydroxyl deprotection and the introduction of the sensitive C19-C21 unsaturated keto-ester functionality. Unexpected selectivities were observed during studies of the hydroxyl group deprotections. In particular, cleavage of tri-isopropylsilyl ethers of the exocyclic primary allylic alcohols was observed in the presence of the triethylsilyl ether of the secondary alcohol at C19. Model studies helped in the design of the methods used to introduce the C19-C21 keto-ester functionality and led to the completion of a total synthesis of a close analogue of bryostatin 10 in which a benzyloxy group rather than the pivaloyloxy group was present at C7.

Triterpenoids from Ocimum labiatum Activates Latent HIV-1 Expression In Vitro: Potential for Use in Adjuvant Therapy

Latent HIV reservoirs in infected individuals prevent current treatment from eradicating infection. Treatment strategies against latency involve adjuvants for viral reactivation which exposes viral particles to antiretroviral drugs. In this study, the effect of novel triterpenoids isolated from Ocimum labiatum on HIV-1 expression was measured through HIV-1 p24 antigen capture in the U1 latency model of HIV-1 infection and in peripheral blood mononuclear cells (PBMCs) of infected patients on combination antiretroviral therapy (cART). The mechanism of viral reactivation was determined through the compound’s effect on cytokine production, histone deacetylase (HDAC) inhibition, and protein kinase C (PKC) activation. Cytotoxicity of the triterpenoids was determined using a tetrazolium dye and flow cytometry. The isolated triterpene isomers, 3-hydroxy-4,6a,6b,11,12,14b-hexamethyl-1,2,3,4,6,6a,6b,7,8,8a,9,10,11,12,12a,14,14a,14b-octadecahydropicene-4,8a-dicarboxylic acid (HHODC), significantly (p < 0.05) induced HIV-1 expression in a dose-dependent manner in U1 cells at non-cytotoxic concentrations. HHODC also induced viral expression in PBMCs of HIV-1 infected patients on cART. In addition, the compound up-regulated the production of interleukin (IL)-2, IL-6, tumour necrosis factor (TNF)-α, and interferon (IFN)-γ but had no effect on HDAC and PKC activity, suggesting cytokine upregulation as being involved in latency activation. The observed in vitro reactivation of HIV-1 introduces the adjuvant potential of HHODC for the first time here.

Transformation of the B Ring to the C Ring of Bryostatins by Csp3-H Amination and Z to E Isomerization

An interesting approach to transform the B ring of bryostatins to the C ring has been developed. The key tactics of the approach feature an intramolecular Csp³-H bond amination to form spirocyclic hemiaminal, which undergoes ring opening to afford the C ring found in bryostatin 17. The subsequent epoxidation/oxidation sequence results in Z to E isomerization of the exo-cyclic enoate, delivering the common precursor, which could be transformed into the C ring found in bryostatins 1, 2, 4-9, 12, 14, and 15.

The therapeutic landscape of HIV-1 via genome editing

Current treatment for HIV-1 largely relies on chemotherapy through the administration of antiretroviral drugs. While the search for anti-HIV-1 vaccine remain elusive, the use of highly active antiretroviral therapies (HAART) have been far-reaching and has changed HIV-1 into a manageable chronic infection. There is compelling evidence, including several side-effects of ARTs, suggesting that eradication of HIV-1 cannot depend solely on antiretrovirals. Gene therapy, an expanding treatment strategy, using RNA interference (RNAi) and programmable nucleases such as meganuclease, zinc finger nuclease (ZFN), transcription activator-like effector nuclease (TALEN), and clustered regularly interspaced short palindromic repeats/CRISPR-associated proteins (CRISPR-Cas9) are transforming the therapeutic landscape of HIV-1. TALENS and ZFNS are structurally similar modular systems, which consist of a FokI endonuclease fused to custom-designed effector proteins but have been largely limited, particularly ZFNs, due to their complexity and cost of protein engineering. However, the newly developed CRISPR-Cas9 system, consists of a single guide RNA (sgRNA), which directs a Cas9 endonuclease to complementary target sites, and serves as a superior alternative to the previous protein-based systems. The techniques have been successfully applied to the development of better HIV-1 models, generation of protective mutations in endogenous/host cells, disruption of HIV-1 genomes and even reactivating latent viruses for better detection and clearance by host immune response. Here, we focus on gene editing-based HIV-1 treatment and research in addition to providing  perspectives for refining these techniques.

Endothelial Cell Autophagy Maintains Shear Stress-Induced Nitric Oxide Generation via Glycolysis-Dependent Purinergic Signaling to Endothelial Nitric Oxide Synthase

OBJECTIVE

Impaired endothelial cell (EC) autophagy compromises shear stress-induced nitric oxide (NO) generation. We determined the responsible mechanism.

APPROACH AND RESULTS

On autophagy compromise in bovine aortic ECs exposed to shear stress, a decrease in glucose uptake and EC glycolysis attenuated ATP production. We hypothesized that decreased glycolysis-dependent purinergic signaling via P2Y1 (P2Y purinoceptor 1) receptors, secondary to impaired autophagy in ECs, prevents shear-induced phosphorylation of eNOS (endothelial nitric oxide synthase) at its positive regulatory site S1117 (p-eNOS^S1177) and NO generation. Maneuvers that restore glucose transport and glycolysis (eg, overexpression of GLUT1 [glucose transporter 1]) or purinergic signaling (eg, addition of exogenous ADP) rescue shear-induced p-eNOS^S1177 and NO production in ECs with impaired autophagy. Conversely, inhibiting glucose transport via GLUT1 small interfering RNA, blocking purinergic signaling via ectonucleotidase-mediated ATP/ADP degradation (eg, apyrase), or inhibiting P2Y1 receptors using pharmacological (eg, MRS2179 [2'-deoxy-N⁶-methyladenosine 3',5'-bisphosphate tetrasodium salt]) or genetic (eg, P2Y1-receptor small interfering RNA) procedures inhibit shear-induced p-eNOS^S1177 and NO generation in ECs with intact autophagy. Supporting a central role for PKCδ^T505 (protein kinase C delta T505) in relaying the autophagy-dependent purinergic-mediated signal to eNOS, we find that (1) shear stress-induced activating phosphorylation of PKCδ^T505 is negated by inhibiting autophagy, (2) shear-induced p-eNOS^S1177 and NO generation are restored in autophagy-impaired ECs via pharmacological (eg, bryostatin) or genetic (eg, constitutively active PKCδ) activation of PKCδ^T505, and (3) pharmacological (eg, rottlerin) and genetic (eg, PKCδ small interfering RNA) PKCδ inhibition prevents shear-induced p-eNOS^S1177 and NO generation in ECs with intact autophagy. Key nodes of dysregulation in this pathway on autophagy compromise were revealed in human arterial ECs.

CONCLUSIONS

Targeted reactivation of purinergic signaling and PKCδ has strategic potential to restore compromised NO generation in pathologies associated with suppressed EC autophagy.

Syntheses of C17–C27 fragments of 20-deoxybryostatins for assembly using Julia and metathesis reactions

Two approaches to the synthesis of compounds corresponding to the C17–C27 fragment of the 20-deoxybryostatins are described.

Some limitations of an approach to the assembly of bryostatins by ring-closing metathesis

Preliminary studies into the use of ring-closing metathesis (RCM) in a convergent approach for the total synthesis of bryostatins are described.

Evaluation of Chromane-Based Bryostatin Analogues Prepared via Hydrogen-Mediated C-C Bond Formation: Potency Does Not Confer Bryostatin-like Biology

The synthesis and biological evaluation of chromane-containing bryostatin analogues WN-2-WN-7 and the previously reported salicylate-based analogue WN-8 are described. Analogues WN-2-WN-7 are prepared through convergent assembly of the chromane-containing fragment B-I with the "binding domain" fragment A-I or its C26-des-methyl congener, fragment A-II. The synthesis of fragment B-I features enantioselective double C-H allylation of 1,3-propanediol to form the C2-symmetric diol 3 and Heck cyclization of bromo-diene 5 to form the chromane core. The synthesis of salicylate WN-8 is accomplished through the union of fragments A-III and B-II. The highest binding affinities for PKCα are observed for the C26-des-methyl analogues WN-3 (Ki = 63.9 nM) and WN-7 (Ki = 63.1 nM). All analogues, WN-2-WN-8, inhibited growth of Toledo cells, with the most potent analogue being WN-7. This response, however, does not distinguish between phorbol ester-like and bryostatin-like behavior. In contrast, while many of the analogues contain a conserved C-ring in the binding domain and other features common to analogues with bryostatin-like properties, all analogues evaluated in the U937 proliferation and cell attachment assays displayed phorbol ester-like and/or toxic behavior, including WN-8, for which "bryostatin-like PKC modulatory activities" previously was suggested solely on the basis of PKC binding. These results underscore the importance of considering downstream biological effects, as tumor suppression cannot be inferred from potent PKC binding.

Comparative analysis of the anti-chikungunya virus activity of novel bryostatin analogs confirms the existence of a PKC-independent mechanism

Previously, we reported that salicylate-based analogs of bryostatin protect cells from chikungunya virus (CHIKV)-induced cell death. Interestingly, 'capping' the hydroxyl group at C26 of a lead bryostatin analog, a position known to be crucial for binding to and modulation of protein kinase C (PKC), did not abrogate the anti-CHIKV activity of the scaffold, putatively indicating the involvement of a pathway independent of PKC. The work detailed in this study demonstrates that salicylate-derived analog 1 and two capped analogs (2 and 3) are not merely cytoprotective compounds, but act as selective and specific inhibitors of CHIKV replication. Further, a detailed comparative analysis of the effect of the non-capped versus the two capped analogs revealed that compound 1 acts both at early and late stages in the chikungunya virus replication cycle, while the capped analogs only interfere with a later stage process. Co-dosing with the PKC inhibitors sotrastaurin and Gö6976 counteracts the antiviral activity of compound 1 without affecting that of capped analogs 2 and 3, providing further evidence that the latter elicit their anti-CHIKV activity independently of PKC. Remarkably, treatment of CHIKV-infected cells with a combination of compound 1 and a capped analog resulted in a pronounced synergistic antiviral effect. Thus, these salicylate-based bryostatin analogs can inhibit CHIKV replication through a novel, yet still elusive, non-PKC dependent pathway.

Targeting the latent reservoir to achieve functional HIV cure

While highly active anti-retroviral therapy has greatly improved the lives of HIV-infected individuals, current treatments are unable to completely eradicate the virus. This is due to the presence of HIV latently infected cells which harbor transcriptionally silent HIV. Latent HIV does not replicate or produce viral proteins, thereby preventing efficient targeting by anti-retroviral drugs. Strategies to target the HIV latent reservoir include viral reactivation, enhancing host defense mechanisms, keeping latent HIV silent, and using gene therapy techniques to knock out or reactivate latent HIV. While research into each of these areas has yielded promising results, currently no one mechanism eradicates latent HIV. Instead, combinations of these approaches should be considered for a potential HIV functional cure.

Inhibition of Chikungunya Virus-Induced Cell Death by Salicylate-Derived Bryostatin Analogues Provides Additional Evidence for a PKC-Independent Pathway

Chikungunya virus (CHIKV) has been spreading rapidly, with over one million confirmed or suspected cases in the Americas since late 2013. Infection with CHIKV causes devastating arthritic and arthralgic symptoms. Currently, there is no therapy to treat this disease, and the only medications focus on relief of symptoms. Recently, protein kinase C (PKC) modulators have been reported to inhibit CHIKV-induced cell death in cell assays. The salicylate-derived bryostatin analogues described here are structurally simplified PKC modulators that are more synthetically accessible than the natural product bryostatin 1, a PKC modulator and clinical lead for the treatment of cancer, Alzheimer's disease, and HIV eradication. Evaluation of the anti-CHIKV activity of these salicylate-derived bryostatin analogues in cell culture indicates that they are among the most potent cell-protective agents reported to date. Given that they are more accessible and significantly more active than the parent natural product, they represent new therapeutic leads for controlling CHIKV infection. Significantly, these analogues also provide evidence for the involvement of a PKC-independent pathway. This adds a fundamentally distinct aspect to the importance or involvement of PKC modulation in inhibition of chikungunya virus replication, a topic of recent and growing interest.

Capturing Biological Activity in Natural Product Fragments by Chemical Synthesis

Natural products have had an immense influence on science and have directly led to the introduction of many drugs. Organic chemistry, and its unique ability to tailor natural products through synthesis, provides an extraordinary approach to unlock the full potential of natural products. In this Review, an approach based on natural product derived fragments is presented that can successfully address some of the current challenges in drug discovery. These fragments often display significantly reduced molecular weights, reduced structural complexity, a reduced number of synthetic steps, while retaining or even improving key biological parameters such as potency or selectivity. Examples from various stages of the drug development process up to the clinic are presented. In addition, this process can be leveraged by recent developments such as genome mining, antibody–drug conjugates, and computational approaches. All these concepts have the potential to identify the next generation of drug candidates inspired by natural products.

The evolution of a stereoselective synthesis of the C1–C16 fragment of bryostatins

Scaleable syntheses of the C1–C16 fragment of bryostatins are described.

Conformation-activity relationships of polyketide natural products

Polyketides represent an important class of secondary metabolites that interact with biological targets connected to a variety of disease-associated pathways. Remarkably, nature's assembly lines, polyketide synthases, manufacture these privileged structures through a combinatorial mixture of just a few structural units. This review highlights the role of these structural elements in shaping a polyketide's conformational preferences, the use of computer-based molecular modeling and solution NMR studies in the identification of low-energy conformers, and the importance of conformational analogues in probing the bound conformation. In particular, this review covers several examples wherein conformational analysis complements classic structure-activity relationships in the design of biologically active natural product analogues.

Function through synthesis-informed design

In 1996, a snapshot of the field of synthesis was provided by many of its thought leaders in a Chemical Reviews thematic issue on “Frontiers in Organic Synthesis”. This Accounts of Chemical Research thematic issue on “Synthesis, Design, and Molecular Function” is intended to provide further perspective now from well into the 21st century. Much has happened in the past few decades. The targets, methods, strategies, reagents, procedures, goals, funding, practices, and practitioners of synthesis have changed, some in dramatic ways as documented in impressive contributions to this issue. However, a constant for most synthesis studies continues to be the goal of achieving function with synthetic economy. Whether in the form of new catalysts, reagents, therapeutic leads, diagnostics, drug delivery systems, imaging agents, sensors, materials, energy generation and storage systems, bioremediation strategies, or molecules that challenge old theories or test new ones, the function of a target has been and continues to be a major and compelling justification for its synthesis. While the targets of synthesis have historically been heavily represented by natural products, increasingly design, often inspired by natural structures, is providing a new source of target structures exhibiting new or natural functions and new or natural synthetic challenges. Complementing isolation and screening approaches to new target identification, design enables one to create targets de novo with an emphasis on sought-after function and synthetic innovation with step-economy. Design provides choice. It allows one to determine how close a synthesis will come to the ideal synthesis and how close a structure will come to the ideal function.

In this Account, we address studies in our laboratory on function-oriented synthesis (FOS), a strategy to achieve function by design and with synthetic economy. By starting with function rather than structure, FOS places an initial emphasis on target design, thereby harnessing the power of chemists and computers to create new structures with desired functions that could be prepared in a simple, safe, economical, and green, if not ideal, fashion. Reported herein are examples of FOS associated with (a) molecular recognition, leading to the first designed phorbol-inspired protein kinase C regulatory ligands, the first designed bryostatin analogs, the newest bryologs, and a new family of designed kinase inhibitors, (b) target modification, leading to highly simplified but functionally competent photonucleases—molecules that cleave DNA upon photoactivation, (c) drug delivery, leading to cell penetrating molecular transporters, molecules that ferry other attached or complexed molecules across biological barriers, and (d) new reactivity-regenerating reagents in the form of functional equivalents of butatrienes, reagents that allow for back-to-back three-component cycloaddition reactions, thus achieving structural complexity and value with step-economy. While retrosynthetic analysis seeks to identify the best way to make a target, retrofunction analysis seeks to identify the best targets to make. In essence, form (structure) follows function.

Improved protein kinase C affinity through final step diversification of a simplified salicylate-derived bryostatin analog scaffold

Bryostatin 1, in clinical trials or preclinical development for cancer, Alzheimer’s disease, and a first-of-its-kind strategy for HIV/AIDS eradication, is neither readily available nor optimally suited for clinical use. In preceding work, we disclosed a new class of simplified bryostatin analogs designed for ease of access and tunable activity. Here we describe a final step diversification strategy that provides, in only 25 synthetic steps, simplified and tunable analogs with bryostatin-like PKC modulatory activities.