BIOMIMETIC AND SYNTHETIC APPROACHES TO MARINE SPONGE ALKALOIDS DERIVED FROMBIS-PYRIDINE MACROCYCLES. A REVIEW

Regiocontrolled Dimerization of Densely Functionalized 1,6-Dihydropyridines for the Biomimetic Synthesis of a Halicyclamine-type Scaffold by Preventing Disproportionation

The biomimetic dimerization of 1,6-dihydropyridines (DHPs) remains a daunting challenge due to competitive disproportionation pathways. Herein we report the regioselective dimerization of densely functionalized 1,6-DHPs that allow direct access to the bis-nitrogen bicyclic scaffold of halicyclamines. Disproportionation triggered by the hydride shift of 1,6-DHP was suppressed by the use of geminal disubstituted substrates. Installation of an electron-withdrawing substituent at the C3 position was demonstrated to be crucial for facilitating biomimetic dimerization under metal-free conditions, with exquisite control of regioselectivity at ambient temperature. Our approach, featuring an appropriately functionalized and substantially stabilized substrate rather than merely adopting the highly reactive and labile hypothetical biosynthetic intermediate, allowed gram-scale and atom-economical synthesis of the bis-nitrogen bicyclic scaffold. Furthermore, conversion of a series of 1,6-DHPs provided mechanistic insights by circumventing the competitive disproportionation reaction. This revealed not only the innate reactivity of the conjugate diene system for [4 + 2] cycloaddition but also the reversibility of the dimerization reaction with multiple cationic intermediates in equilibrium.

The alkaloids of the madangamine group

This chapter is focused on madangamines, a small group of complex diamine alkaloids isolated from marine sponges of the order Haplosclerida, and covers their isolation, characterization, biogenesis, biological activity, and synthesis. Structurally, madangamines are pentacyclic alkaloids with an unprecedented skeletal type, characterized by a common diazatricyclic core and two peripheral macrocyclic rings. The isolation of these alkaloids from Xestospongia ingens (madangamines A-E) and Pachychalina alcaloidifera (madangamine F) is described in detail. Physical and complete spectroscopic 1H and 13C NMR data are included. The proposed biogenesis of madangamines from ammonia, a functionalized three-carbon unit, and saturated or unsaturated linear long-chain dialdehydes, via partially reduced bis-alkylpyridine macrocycles, is discussed. The synthesis of alkaloids of the madangamine group has been little explored, with only one total synthesis reported so far, that of (+)-madangamine D. This review also describes several model synthetic approaches to the diazatricyclic ABC core of these alkaloids, as well as model studies on the construction of the (Z,Z)-unsaturated 11-membered E macrocycle common to madangamines A-E, the 13- and 14-membered D rings of madangamines C-E, and the all-cis-triunsaturated 15-membered D ring of madangamine A. Some members of this group have shown significant in vitro cytotoxicity against a number of cancer cell lines.

Natural products with anti-Bredt and bridgehead double bonds

Well over a hundred years ago, Professor Julius Bredt embarked on a career pursuing and critiquing bridged bicyclic systems that contained ring strain induced by the presence of a bridgehead olefin. These endeavors founded what we now know as Bredt's rule (Bredtsche Regel). Physical, theoretical, and synthetic organic chemists have intensely studied this premise, pushing the boundaries of such systems to arrive at a better understood physical phenomenon. Mother nature has also seen fit to construct molecules containing bridgehead double bonds that encompass Bredt's rule. For the first time, this topic is reviewed in a natural product context.

Neurotrophic natural products: chemistry and biology

Neurodegenerative diseases and spinal cord injury affect approximately 50 million people worldwide, bringing the total healthcare cost to over 600 billion dollars per year. Nervous system growth factors, that is, neurotrophins, are a potential solution to these disorders, since they could promote nerve regeneration. An average of 500 publications per year attests to the significance of neurotrophins in biomedical sciences and underlines their potential for therapeutic applications. Nonetheless, the poor pharmacokinetic profile of neurotrophins severely restricts their clinical use. On the other hand, small molecules that modulate neurotrophic activity offer a promising therapeutic approach against neurological disorders. Nature has provided an impressive array of natural products that have potent neurotrophic activities. This Review highlights the current synthetic strategies toward these compounds and summarizes their ability to induce neuronal growth and rehabilitation. It is anticipated that neurotrophic natural products could be used not only as starting points in drug design but also as tools to study the next frontier in biomedical sciences: the brain activity map project.

The Saraine Alkaloids

Saraines have been isolated by the group of Cimino from the marine sponge Reniera sarai collected in the Bay of Naples. These alkaloids can be classified into two categories, depending on the type of their structure. Saraines 1-3 and isosaraines 1-3 are characterized by a trans-2-oxoquinolizidine moiety bound directly to a tetrahydropyridine ring, both nuclei being also linked by two alkyl chains of different lengths. On the other hand, saraines A-C and misenine present a diazatricyclic central core, showing an unusual interaction between an aldehyde and a tertiary amine and giving them a zwitterionic character, and flanked with two chains forming macrocycles. In a first part of the chapter, the isolation, spectral data, and structure elucidation of saraines are presented. Biological properties and biogenetic proposals concerning these compounds are then described. The most important part is devoted to synthetic approaches and total synthesis of saraine A.

Cyclic Bis-1,3-dialkylpyridiniums from the sponge Haliclona sp

Eight novel cyclic bis-1,3-dialkylpyridiniums, as well as two known compounds from the cyclostellettamine class, were isolated from the sponge Haliclona sp. from Korea. Structures of these novel compounds were determined using combined NMR and FAB-MS/MS analyses. Several of these compounds exhibited moderate cytotoxic and antibacterial activities against A549 cell-line and Gram-positive strains, respectively. The structure-activity relationships of cyclostellettamines are discussed based on their bioactivities.

An Approach to the Tricyclic Core of Madangamines Based upon a Biogenetic Scheme

[reaction: see text] A short synthesis of intermediates possessing the tricyclic core of natural madangamines, bioactive alkaloids found in marine sponges, is described. The key reaction entails the condensation of the sodium salt of diethylacetonedicarboxylate with a dihydropyridinium salt derivative. This new approach is modeled on a biogenetic proposal linking madangamines to ircinals, related alkaloids occurring in sponges of the same order.

Synthesis and biological studies of 1-amino beta-carbolines

A selection of 1-amino-substituted beta-carbolines have been prepared by amination of 1-chloro-beta-carboline as simple mimics of manzamine A and chloroquine and their intercalating ability, anticancer and antimalarial activity were studied.

Tetradehydrohalicyclamine A and 22-hydroxyhalicyclamine A, new cytotoxic bis-piperidine alkaloids from a marine sponge Amphimedon sp

Two new 3-alkylpiperidine alkaloids, tetradehydrohalicyclamine A (2) and 22-hydroxyhalicyclamine A (3), have been isolated from a marine sponge Amphimedon sp. as cytotoxic constituents. Their structures were elucidated by spectroscopic analysis. Compounds 2 and 3 inhibited growth of P388 cells with IC50 values of 2.2 and 0.45 microg/mL, respectively.

An Access to the Bicyclic Nucleus of the Sponge Alkaloid Halicyclamine A by Successive Condensation of Malondialdehyde Units, Aldehyde Derivatives, and Primary Amines

First results of an evaluation of the synthetic sequence depicted in Scheme 4 are reported. This sequence is based upon biosynthetic considerations concerning the manzamine family of sponge alkaloids. Stable equivalents 21 and 31 of the tetraaldehyde 7 have thus been obtained by using the chemistry of malondialdehyde previously reported by Tietze. These compounds afforded pyridinium salts 23 and 33 when treated with a primary amine in acidic medium. Further reductive amination and cyclization yielded bicyclic derivatives 25 and 35, thus demonstrating the feasibility of this synthetic approach for the preparation of halicyclamine derivatives 13. Products resulting from cycloaddition reactions leading to 9 were not observed.

Synthetic studies toward sarain A. Formation of the western macrocyclic ring

Starting with the tricyclic core 2b, annulation to form the 13-membered western ring of sarain A has been achieved to afford the macrocycle 30a by initial construction of the sterically congested quaternary center at C-3, followed by elaboration of the C-3 side-chain and ring-closing olefin metathesis. Also included is a parallel conversion of tricycle 2c to macrocycle 30b containing a functionalized side-chain at N-1 suitable for attachment of the eastern macrocyclic ring.

Synthesis of the ABC ring system of manzamine A

A synthesis of the core ABC ring system of the manzamine alkaloids is described, starting from arecoline. The key steps involve a Claisen rearrangement to set up a 4-substituted-3-methylenepiperidine and a stereoselective azomethine ylide dipolar cycloaddition reaction. Condensation of the aldehyde 6 and sarcosine ethyl ester hydrochloride salt gives an intermediate azomethine ylide, which undergoes an intramolecular cycloaddition reaction to set up two new rings and three new chiral centers stereoselectively. The aldehyde 6 was not a suitable substrate for related azomethine ylide cycloaddition reactions with other amines. However, the related dimethyl acetal 26 could be condensed with a variety of amines to give the desired tricyclic products. The cycloaddition reaction with N-methyl or N-allyl glycine ethyl ester gave almost exclusively the exo adduct, whereas cycloaddition with glycine ethyl ester gave the endo adduct.

From a biogenetic scenario to a synthesis of the ABC ring of manzamine A

On the basis of a biogenetic proposal for explaining the biogenesis of manzamine A, the cycloaddition of dihydropyridinium salt 26 with diene derivative 5 leads to adducts 27. These adducts, as well as their related and previously described analogues 9, are now shown to be precursors of diene derivatives such as 10, 13, and 28. Treatment of diene 32 with sodium azide resulted in a one-step formation of the tricyclic imino derivative 34. This key intermediate was further transformed into tricyclic derivative 40, which possesses the essential features of the ABC ring of manzamine A.

Synthesis of aza polycyclic compounds derived from pyrrolidine, indolizidine, and indole via intramolecular Diels-Alder cycloadditions of neutral 2-azadienes

A method for the preparation of novel oxaza and diaza polycyclic 9-oxa-4-azaphenanthrene, 5H-pyrido[2,3-a]pyrrolizine, 5H,6H-pyrido[3,2-g]indolizine, and 5H,6H-indeno[2,1-a]indole is described, based on tandem reactions: aza-Wittig reaction of N-vinylic phosphazenes with functionalized aldehydes and an intramolecular aza-Diels-Alder reaction.

Indenylidene complexes of ruthenium: optimized synthesis, structure elucidation, and performance as catalysts for olefin metathesis--application to the synthesis of the ADE-ring system of nakadomarin A

An optimized and large scale adaptable synthesis of the ruthenium phenylindenylidene complex 3 is described which employs commercially available diphenyl propargyl alcohol 5 as a stable and convenient carbene source. Previous ambiguities as to the actual structure of the complex have been ruled out by a full analysis of its NMR spectra. A series of applications to ring closing metathesis (RCM) reactions shows that complex 3 is as good as or even superior to the classical Grubbs carbene 1 in terms of yield, reaction rate, and tolerance towards polar functional groups. Complex 3 turns out to be the catalyst of choice for the synthesis of the enantiopure core segment 77 of the marine alkaloid nakadomarin A 60 comprising the ADE rings of this target. Together with a series of other examples, this particular application illustrates that catalyst 3 is particularly well suited for the cyclization of medium-sized rings by RCM. Other key steps en route to nakadomarin A are a highly selective intramolecular Michael addition setting the quaternary center at the juncture of the A and D rings and a Takai-Nozaki olefination of aldehyde 73 with CH2I2, Ti(OiPr)4 and activated zinc dust.

Selective oxidation of a keramaphidin B model

A crucial step in the Baldwin and Whitehead proposal for explaining the biogenesis of the marine alkaloid manzamine A is the selective oxidation of natural keramaphidin B to an iminium salt 3, which is then hydrolyzed to give the aldehyde 4. Conditions are now presented in which this selective oxidation can be performed on model compound 8, leading to the iminium salt 16. Although this salt can be considered as a model equivalent of the proposed aldehyde intermediate 4, it was found to be very resistant to hydrolysis as was the corresponding amide 20. From a synthetic point of view, the reported results illustrate the usefulness of the temporary protection of tertiary amines as aminoborane derivatives and constitute a good method for the oxidation of a sterically hindered tertiary nitrogen atom in the presence of a second nitrogen.

Facile synthesis of the tricyclic core of sarain A. 3-Oxidopyridinium betaine cycloaddition approach

[formula: see text] A new approach to a suitably functionalized tricyclic core of sarains has been developed by means of Katritzky's cycloaddition using 3-oxidopyridinium betaines. A key step was the regioselective differentiation of the two nearly identical hydroxy groups derived from oxidative cleavage of the double bond in 8 to afford 14. A stereocontrolled construction of the tricyclic core 20 of sarains containing the requisite side chain at C-3' was achieved by an intramolecular conjugate addition.