Finding activity through rigidity: syntheses of natural products containing tricyclic bridgehead carbon centers

Covering: up to 2022Tricyclic bridgehead carbon centers (TBCCs) are a synthetically challenging substructure found in many complex natural products. Here we review the syntheses of ten representative families of TBCC-containing isolates, with the goal of outlining the strategies and tactics used to install these centers, including a discussion of the evolution of the successful synthetic design. We provide a summary of common strategies to inform future synthetic endeavors.

Recent progress in the total synthesis of marine brominated sesquiterpene aplydactone

Aplydactone is a brominated sesquiterpene isolated from the sea hare Aplysia dactylomela. Structurally, it features a complex cage-like skeleton containing a highly strained tricyclic-[4.2.0.0^3,8]-4-decanone system. Its unique structural features have fascinated many synthetic chemists. In this review, the synthetic efforts towards aplydactone in the last five years are summarized in two categories including nonbiomimetic synthesis and biomimetic synthesis based on the core synthetic strategy. These syntheses set a classical and instructive example for the syntheses of other marine natural products.

Iterative Polyketide Synthesis via a Consecutive Carbonyl-Protecting Strategy

To address the difficulty in protecting a β-polycarbonyl compound, a method for the sequential protection of elongating carbonyl groups was demonstrated. The iterative chain elongation of a carboxylic acid with malonic acid half thioester followed by the protection of the resulting β-ketothioester was performed via the stepwise formation of an isoxazole ring using an O-protected oxime functionality. Yangonin and isosakuranetin were synthesized according to this procedure.

Biomimetic iterative method for polyketide synthesis

An iterative method for synthesizing polyketides was demonstrated, in which the chain elongation of a carboxylic acid was performed by decarboxylative dehydration condensation with a malonic acid half thioester. After transforming the resulting β-ketothioester into an appropriate form, the carboxylic acid functionality was regenerated for the next elongation step.

Application of the aza-Diels–Alder reaction in the synthesis of natural products

The Diels–Alder reaction that involves a nitrogen atom in the diene or dienophile is termed the aza-Diels–Alder reaction.

Marine natural products

This review covers the literature published in 2013 for marine natural products (MNPs), with 982 citations (644 for the period January to December 2013) referring to compounds isolated from marine microorganisms and phytoplankton, green, brown and red algae, sponges, cnidarians, bryozoans, molluscs, tunicates, echinoderms, mangroves and other intertidal plants and microorganisms. The emphasis is on new compounds (1163 for 2013), together with the relevant biological activities, source organisms and country of origin. Reviews, biosynthetic studies, first syntheses, and syntheses that lead to the revision of structures or stereochemistries, have been included.

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.

Antioxidant and acetylcholinesterase inhibition properties of novel bromophenol derivatives

In this study, series of novel bromophenol derivatives were synthesized and investigated for their antioxidant and AChE inhibition properties. Novel brominated diarylmethanones were obtained from the acylation reactions of benzoic acids with substituted benzenes. One of the bromodiarylmethanone was synthesized from the bromination of diarylmethanone with molecular bromine. All diarylmethanones were converted into their bromophenol derivatives with BBr3. The antioxidant activities of all synthesized compounds were elucidated by using various bioanalytical assays. Radical scavenging activities of compounds 10-24 were evaluated by means of DPPH and ABTS(+) scavenging activities. In addition, reducing ability of 10-24 were determined by Fe(3+), Cu(2+), and [Fe(3+)-(TPTZ)2](3) reducing activities. α-Tocopherol, trolox, BHA, and BHT were used as positive antioxidant and radical scavenger molecules. On the other hand, IC50 values were calculated for DPPH, ABTS(+) scavenging, and AChE inhibition effects of novel compounds. The results obtained from the current studies clearly show that novel bromophenol derivatives 20-24 have considerable antioxidant, antiradical, and AChE inhibition effects.

Total synthesis of 7'-desmethylkealiiquinone, 4'-desmethoxykealiiquinone, and 2-deoxykealiiquinone

Synthetic approaches to the imidazonaphthoquinone core of kealiiquinone and related Leucetta-derived alkaloids are described. The polysubstituted benzimidazole framework can be constructed through intramolecular Diels-Alder reactions of propiolate-derived enynes followed by oxidation. Adjustment of the oxidation state of the thus formed lactone allows introduction of the 2,3-dihydroxybenzoquinone moiety through a presumed benzoin-like condensation between a phthaldehyde derivative and a masked glyoxal equivalent catalyzed by a cyanide ion. Oxidation of the C2-position can be accomplished through application of an operationally simple treatment of an imidazolium salt with bleach, thus producing the corresponding 2-imidazolone. Debenzylation of a late stage intermediate en route to kealiiquinone was compromised by concomitant O-demethylation upon treatment with triflic acid resulting in the formation of non-natural 7'-desmethylkealiiquinone. Other endgame strategies were evaluated; however, these efforts did not lead to completion of a synthesis of kealiiquinone but did provide access to other closely related analogues.

Marine natural products

This review covers the literature published in 2012 for marine natural products, with 1035 citations (673 for the period January to December 2012) referring to compounds isolated from marine microorganisms and phytoplankton, green, brown and red algae, sponges, cnidarians, bryozoans, molluscs, tunicates, echinoderms, mangroves and other intertidal plants and microorganisms. The emphasis is on new compounds (1241 for 2012), together with the relevant biological activities, source organisms and country of origin. Biosynthetic studies, first syntheses, and syntheses that lead to the revision of structures or stereochemistries, have been included.