Total synthesis of bryostatin 9

Tandem Macrolactone Synthesis: Total Synthesis of (-)-Exiguolide by a Macrocyclization/Transannular Pyran Cyclization Strategy

Tetrahydropyran-containing macrolactones were synthesized by integrating Meyer-Schuster rearrangement, macrocyclic ring-closing metathesis, and transannular oxa-Michael addition under gold and ruthenium catalysis. Single-step access to a variety of 14- to 20-membered macrolactones containing a tetrahydropyran ring was possible from readily available linear precursors in good yields and with moderate to excellent diastereoselectivity. A 13-step synthesis of (-)-exiguolide, an anticancer marine macrolide, showcased the feasibility of our tandem reaction sequence for macrolactone synthesis and also demonstrated the power of transannular reactions for rapid assembly of the tetrahydropyran rings of the target natural product.

Amphidinolides F and C2: An Odyssey in Total Synthesis

Amphidinolides F, C, C2, and C3 are marine natural products isolated from dinoflagellates Amphidinium species. They share the same macrolactone core, with the difference between them residing at the side chain level. A predominant feature of these amphidinolides is the presence of two trans-THF rings inside the macrolactone core, which is thought to be built by C-glycosylation with titanium enolate of N-acetyl oxazolinethiones. Thus, the original strategy for their total synthesis was based on the assembly of three main fragments corresponding to C₁-C₉, C₁₀-C₁₉, and C₂₀-C₂₉ or C₂₀-C₃₄ disconnections. Whereas synthesis of all fragments was successful, the C-glycosylation reaction between C₁₉ and C₂₀ turned out to be an issue. Therefore, a second route was designed. The new disconnection between C₁₇ and C₁₈ was based on a sulfone addition and a desulfonylation sequence. Our convergent strategy allowed the total synthesis of amphidinolide F and enabled a new unifying route toward the synthesis of amphidinolides C, C2, and C3 using a late-stage divergent approach. Although there were unsatisfying yields at some critical steps, our work culminated into the first total synthesis of amphidinolide C2.

Preclinical and Clinical Studies on Bryostatins, A Class of Marine-Derived Protein Kinase C Modulators: A Mini-Review

Bryostatins are complex macrolactones isolated from marine organisms Bryozoan Bugula neritina. They are potent modulators of protein kinase C isozymes (PKCα: ki = 1.3-188 nM), and are one of the most extensively investigated marine natural products in clinical trials. Although ~21 natural bryostatins have been isolated, however only bryostatin-1 (1) has received much interest among medicinal chemists and clinicians. The structure-activity relationship of bryostatins has been well established, with the identification of key pharmacophoric features important for PKC modulation. The low natural abundance and the long synthetic route have prompted medicinal chemists to come-up with simplified analogs. Bryostatin skeleton comprises three pyran rings connected to each other to form a macrocyclic lactone. The simplest analog 27 contains only one pyran, which is also able to modulate the PKCα activity; however, the cyclic framework appears to be essential for the desired level of potency. Another simplified analog 17 ("picolog") exhibited potent and in-vivo efficacy against lymphoma. Bryostatin-1 (1) has shown an acceptable intravenous pharmacokinetic profile in mice and displayed promising in-vivo efficacy in mice models of various cancers and Alzheimer's disease. Bryostatin-1 was investigated in numerous Phase I/II oncology clinical trials; it has shown minimal effect as a single agent, however, provided encouraging results in combination with other chemotherapy agents. FDA has granted orphan drug status to bryostatin-1 in combination with paclitaxel for esophageal cancer. Bryostatin-1 has also received orphan drug status for fragile X syndrome. Bryostatin-1 was also investigated in clinical studies for Alzheimer's disease and HIV infection. In a nutshell, the natural as well as synthetic bryostatins have generated a strong hope to emerge as treatment for cancer along with many other diseases.

Structural insights into C1-ligand interactions: Filling the gaps by in silico methods

Protein Kinase C isoenzymes (PKCs) are the key mediators of the phosphoinositide signaling pathway, which involves regulated hydrolysis of phosphatidylinositol (4,5)-bisphosphate to diacylglycerol (DAG) and inositol-1,4,5-trisphosphate. Dysregulation of PKCs is implicated in many human diseases making this class of enzymes an important therapeutic target. Specifically, the DAG-sensing cysteine-rich conserved homology-1 (C1) domains of PKCs have emerged as promising targets for pharmaceutical modulation. Despite significant progress, the rational design of the C1 modulators remains challenging due to difficulties associated with structure determination of the C1-ligand complexes. Given the dearth of experimental structural data, computationally derived models have been instrumental in providing atomistic insight into the interactions of the C1 domains with PKC agonists. In this review, we provide an overview of the in silico approaches for seven classes of C1 modulators and outline promising future directions.

Total synthesis of bryostatin 3

Bryostatins are a family of 21 complex marine natural products with a wide range of potent biological activities. Among all the 21 bryostatins, bryostatin 3 is structurally the most complex. Whereas nine total syntheses of bryostatins have been achieved to date, bryostatin 3 has only been targeted once and required the highest number of steps to synthesize (43 steps in the longest linear sequence and 88 total steps). Here, we report a concise total synthesis of bryostatin 3 using 22 steps in the longest linear sequence and 31 total steps through a highly convergent synthetic plan by the use of highly atom-economical and chemoselective transformations in which alkynes played a major role in reducing step count.

CuBr2-catalyzed diastereoselective allylation: total synthesis of decytospolides A and B and their C6-epimers

An efficient CuBr₂-catalyzed diastereoselective allylation of a cyclic hemiacetal with allyltrimethylsilane as a nucleophile has been developed. The protocol offers a cost effective, protecting group tolerant, and operationally simple approach to 2,6-trans-disubstituted tetrahydropyran with excellent diastereoselectivity. Furthermore, the application of this methodology has been demonstrated in the total synthesis of decytospolides A and B and their C6-epimers.

The Phylum Bryozoa: From Biology to Biomedical Potential

Less than one percent of marine natural products characterized since 1963 have been obtained from the phylum Bryozoa which, therefore, still represents a huge reservoir for the discovery of bioactive metabolites with its ~6000 described species. The current review is designed to highlight how bryozoans use sophisticated chemical defenses against their numerous predators and competitors, and which can be harbored for medicinal uses. This review collates all currently available chemoecological data about bryozoans and lists potential applications/benefits for human health. The core of the current review relates to the potential of bryozoan metabolites in human diseases with particular attention to viral, brain, and parasitic diseases. It additionally weighs the pros and cons of total syntheses of some bryozoan metabolites versus the synthesis of non-natural analogues, and explores the hopes put into the development of biotechnological approaches to provide sustainable amounts of bryozoan metabolites without harming the natural environment.

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.

Feedstock Reagents in Metal-Catalyzed Carbonyl Reductive Coupling: Minimizing Preactivation for Efficiency in Target-Oriented Synthesis

Use of abundant feedstock pronucleophiles in catalytic carbonyl reductive coupling enhances efficiency in target-oriented synthesis. For such reactions, equally inexpensive reductants are desired or, ideally, corresponding hydrogen autotransfer processes may be enacted wherein alcohols serve dually as reductant and carbonyl proelectrophile. As described in this Minireview, these concepts allow reactions that traditionally require preformed organometallic reagents to be conducted catalytically in a byproduct-free manner from inexpensive π-unsaturated precursors.

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.

Stereoconfining macrocyclizations in the total synthesis of natural products

This review covers selected examples of point chirality-forming macrocyclizations in natural product total synthesis in the past three decades.

Deletion of the C26 Methyl Substituent from the Bryostatin Analogue Merle 23 Has Negligible Impact on Its Biological Profile and Potency

Important strides are being made in understanding the effects of structural features of bryostatin 1, a candidate therapeutic agent for cancer and dementia, in conferring its potency toward protein kinase C and the unique spectrum of biological responses that it induces. A critical pharmacophoric element in bryostatin 1 is the secondary hydroxy group at the C26 position, with a corresponding primary hydroxy group playing an analogous role in binding of phorbol esters to protein kinase C. Herein, we describe the synthesis of a bryostatin homologue in which the C26 hydroxy group is primary, as it is in the phorbol esters, and show that its biological activity is almost indistinguishable from that of the corresponding compound with a secondary hydroxy group.

Chemoenzymatic Dissection of Polyketide β-Branching in the Bryostatin Pathway

β-Branching is an expansion upon canonical polyketide synthase extension that allows for the installation of diverse chemical moieties in several natural products. Several of these moieties are unique among natural products, including the two vinyl methylesters found in the core structure of bryostatins. This family of molecules is derived from an obligate bacterial symbiont of a sessile marine bryozoan, Bugula neritina. Within this family, bryostatin 1 has been investigated as an anticancer, neuroprotective, and immunomodulatory compound. We have turned to the biosynthetic gene cluster within the bacterial symbiont to investigate the biosynthesis of bryostatins. Recent sequencing efforts resulted in the annotation of two missing genes: bryT and bryU. Using novel chemoenzymatic techniques, we have validated these as the missing enoyl-CoA hydratase and donor acyl carrier protein, essential components of the β-branching cassette of the bryostatin pathway. Together, this cassette installs the vinyl methylester moieties essential to the activity of bryostatins.

Synthesis and Biological Evaluation of Fluorescent Bryostatin Analogues

To investigate the cellular distribution of tumor-promoting vs. non-tumor-promoting bryostatin analogues, we synthesized fluorescently labeled variants of two bryostatin derivatives that have previously shown either phorbol ester-like or bryostatin-like biological activity in U937 leukemia cells. These new fluorescent analogues both displayed high affinity for protein kinase C (PKC) binding and retained the basic properties of the parent unlabeled compounds in U937 assays. The fluorescent compounds showed similar patterns of intracellular distribution in cells, however; this argues against an existing hypothesis that various patterns of intracellular distribution are responsible for differences in biological activity. Upon further characterization, the fluorescent compounds revealed a slow rate of cellular uptake; correspondingly, they showed reduced activity for cellular responses that were only transient upon treatment with phorbol ester or bryostatin 1.

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.

Scalable synthesis of bryostatin 1 and analogs, adjuvant leads against latent HIV

Bryostatin 1 is an exceedingly scarce marine-derived natural product that is in clinical development directed at HIV/AIDS eradication, cancer immunotherapy, and the treatment of Alzheimer's disease. Despite this unique portfolio of indications, its availability has been limited and variable, thus impeding research and clinical studies. Here, we report a total synthesis of bryostatin 1 that proceeds in 29 total steps (19 in the longest linear sequence, >80% average yield per step), collectively produces grams of material, and can be scaled to meet clinical needs (~20 grams per year). This practical solution to the bryostatin supply problem also opens broad, facile, and efficient access to derivatives and potentially superior analogs.

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.

Structural Characterization and Unique Catalytic Performance of Silyl-Group-Substituted Geminal Dichromiomethane Complexes Stabilized with a Diamine Ligand

Stabilization by a silyl group on the methylene carbon and a diamine ligand led to the isolation of gem-dichromiomethane species. X-ray crystallography confirmed the identity of the structure of this rare example of reactive gem-dimetalloalkane species. The isolated gem-dichromiomethane complex acted as a storable silylmethylidene carbene equivalent, with reactivity that could be changed dramatically upon addition of a Lewis acid (ZnCl₂) and a base (TMEDA) to promote both silylalkylidenation of polar aldehydes and silylcyclopropanation of nonpolar alkenes. Identification of a key reactive species also identified the catalytic version of these transformations and provided insights into the reaction mechanism. In contrast to Simmons-Smith cyclopropanation, the real reactive species for the current cyclopropanation was a chromiocarbene species, not a chromium carbenoid species.

Advances in the synthesis of glycosidic macrolides: clavosolides A-D and cyanolide A

Covering: 2005 to 2016Clavosolides A-D and cyanolide A are glycosidic macrolides and represent a new family of marine natural products. They possess a number of unusual structural features and have attracted considerable interest from the synthetic community. This review presents a comprehensive survey of all aspects of the clavosolides A-D and cyanolide A. Specific topics include isolation, structure determination, biological activity, and synthetic approaches.

Synthetic strategies towards mycolactone A/B, an exotoxin secreted by Mycobacterium ulcerans

Pitfalls and dead-ends pave the way to mycolactone A/B. This full account reports synthetic efforts towards this natural product that eventually culminated in a de novo total synthesis.

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.

Replacement of the Bryostatin A- and B-Pyran Rings With Phenyl Rings Leads to Loss of High Affinity Binding With PKC

We describe a convergent synthesis of a bryostatin analogue in which the natural A- and B-ring pyrans have been replaced by phenyl rings. The new analogue exhibited PMA like behavior in cell assays, but failed to maintain high affinity binding for PKC, despite retaining an unaltered C-ring 'binding domain'.

TMSBr/InBr3-promoted Prins cyclization/homobromination of dienyl alcohol with aldehyde to construct cis-THP containing an exocyclic E-alkene

A TMSBr/InBr3-promoted Prins cyclization/homobromination reaction of dienyl alcohol with aldehyde has been developed to construct a unique cis-E THP shown as the A ring in (-)-exiguolide and the B ring in bryostatins.

Enantioselective Alcohol C-H Functionalization for Polyketide Construction: Unlocking Redox-Economy and Site-Selectivity for Ideal Chemical Synthesis

The development and application of stereoselective and site-selective catalytic methods that directly convert lower alcohols to higher alcohols are described. These processes merge the characteristics of transfer hydrogenation and carbonyl addition, exploiting alcohols and π-unsaturated reactants as redox pairs, which upon hydrogen transfer generate transient carbonyl-organometal pairs en route to products of C-C coupling. Unlike classical carbonyl additions, stoichiometric organometallic reagents and discrete alcohol-to-carbonyl redox reactions are not required. Additionally, due to a kinetic preference for primary alcohol dehydrogenation, the site-selective modification of glycols and higher polyols is possible, streamlining or eliminating use of protecting groups. The total syntheses of several iconic type I polyketide natural products were undertaken using these methods. In each case, the target compounds were prepared in significantly fewer steps than previously achieved.

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.

Isolation, analytical measurements, and cell line studies of the iron-bryostatin-1 complex

Bryostatin-1 is a marine natural product that has demonstrated medicinal activity in pre-clinical and clinical trials for the treatment of cancer, Alzheimer's disease, effects of stroke, and HIV. In this study, iron-bryostatin-1 was obtained using a pharmaceutical aquaculture technique developed by our lab that cultivates marine bacteria for marine natural product extraction. Analytical measurements (1)H and (13)C NMR, mass spectrometry, and flame atomic absorption were utilized to confirm the presence of an iron-bryostatin-1 complex. The iron-bryostatin-1 complex produced was then tested against the National Cancer Institute's 60 cell line panel. Adding iron to bryostatin-1 lowered the anti-cancer efficacy of the compound.

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.

Total Synthesis of (-)-Exiguolide

A concise total synthesis of (-)-exiguolide has been completed in an overall 2.8% yield over 20 steps in the longest linear path. The key strategies involve (1) Prins cyclization/homobromination of dienyl alcohol with the B ring-substituted aldehyde, prepared by Prins cyclization/bromination, to construct the A ring with excellent cis-Z stereochemical control and (2) an unusual side chain installation/macrocyclization strategy featuring Sonogashira cross-coupling followed by a ring-closing metathesis reaction to deliver the target.

Phosphonoselenoic acid esters from the reaction between phosphoroselenoyl chlorides and Grignard reagents: synthetic and stereochemical aspects

Substitution reactions between a phosphoroselenoyl chloride and Grignard reagents proceeded smoothly at the phosphorus atom to furnish phosphonoselenoic acid esters in moderate to high yields.

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

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

Dynamics and Membrane Interactions of Protein Kinase C

Protein kinase C (PKC) is a family of Ser/Thr kinases that regulate a multitude of cellular processes through participation in the phosphoinositide signaling pathway. Significant research efforts have been directed at understanding the structure, function, and regulatory modes of the enzyme since its discovery and identification as the first receptor for tumor-promoting phorbol esters. The activation of PKC involves a transition from the cytosolic autoinhibited latent form to the membrane-associated active form. The membrane recruitment step is accompanied by the conformational rearrangement of the enzyme, which relieves autoinhibitory interactions and thereby allows PKC to phosphorylate its targets. The multidomain structure and intrinsic flexibility of PKC present remarkable challenges and opportunities for the biophysical and structural biology studies of this class of enzymes and their interactions with membranes, the major focus of this Current Topic. I will highlight the recent advances in the field, outline the current challenges, and identify areas where biophysics and structural biology approaches can provide insight into the isoenzyme-specific regulation of PKC activity.

Considerations of the chemical biology of microbial natural products provide an effective drug discovery strategy

Conventional approaches to natural product drug discovery rely mainly on random searches for bioactive compounds using bioassays. These traditional approaches do not incorporate a chemical biology perspective. Searching for bioactive molecules using a chemical and biological rationale constitutes a powerful search paradigm. Here, the authors review recent examples of the discovery of bioactive natural products based on chemical and biological interactions between hosts and symbionts, and propose this method provides a more effective means of exploring natural chemical diversity and eventually of discovering new drugs.

An iodine catalyzed metal free domino process for the stereoselective synthesis of oxygen bridged bicyclic ethers

A domino reaction has been developed for the synthesis of oxygen bridged bicyclic ethers through the coupling of 4-(2-hydroxyethyl)cyclohex-3-enols with aldehydes in the presence of 10 mol% of molecular iodine in dichloromethane at 25 °C. This method is highly diastereoselective affording the corresponding bicyclic ethers, i.e. octahydro-4a,7-epoxyisochromenes in good yields with high selectivity. It is the first report on the synthesis of oxygen bridged bicyclic ethers using a domino Prins strategy.