Gordon J. Florence

Highly chemoselective oxidation of 1,5-diols to δ-lactones with TEMPO/BAIB

Abstract The selective oxidative conversion of a variety of highly functionalized 1°,2°-1,5-diols into the corresponding δ-lactones has been effected simply and efficiently using a reagent system comprised of catalytic 2,2,6,6-tetrmethylpiperidinooxy (TEMPO) and excess bis-acetoxyiodobenzene (BAIB).

Development of practical syntheses of the marine anticancer agents discodermolide and dictyostatin.

Initially isolated in trace quantities from deep-sea sponges, the structurally related polyketides discodermolide and dictyostatin share the same microtubule-stabilizing antimitotic mechanism as Taxol. Discodermolide has been the focus of intense research activity in order to develop a practical supply route, and these efforts ultimately allowed its large-scale synthesis and the initiation of clinical trials as a novel anticancer drug. Similarly, the re-isolation and synthesis of dictyostatin continues to stimulate the biological and chemical communities in their quest for the development of new chemotherapeutic agents. This comprehensive review chronicles the synthetic endeavours undertaken over the last 15 years towards the development and realization of practical chemical syntheses of discodermolide and, more recently, dictyostatin, focusing on the methods and strategies employed for achieving overall stereocontrol and key fragment unions, as well as the design and synthesis of novel hybrid structures.

Synthesis of (+)-discodermolide and analogues by control of asymmetric induction in aldol reactions of γ-chiral (Z)-enals

Abstract The boron-mediated aldol reactions of the ( Z )-enals 3 and 7 proceed with high levels of 1,4-stereoinduction arising from the γ-substituent. Reagent control from (+)-Ipc 2 BCl can be used effectively to overturn this substrate bias, thus enabling the stereocontrolled formation of (+)-discodermolide ( 1 ) and related analogues 15 – 18 .

Discodermolide interferes with the binding of tau protein to microtubules

We investigated whether discodermolide, a novel antimitotic agent, affects the binding to microtubules of tau protein repeat motifs. Like taxol, the new drug reduces the proportion of tau that pellets with microtubules. Despite their differing structures, discodermolide, taxol and tau repeats all bind to a site on β‐tubulin that lies within the microtubule lumen and is crucial in controlling microtubule assembly. Low concentrations of tau still bind strongly to the outer surfaces of preformed microtubules when the acidic C‐terminal regions of at least six tubulin dimers are available for interaction with each tau molecule; otherwise binding is very weak.

Total Synthesis of ( )-Reidispongiolide A, an Actin-Targeting Macrolide Isolated from the Marine Sponge Reidispongia coerulea

A stereocontrolled total synthesis of the microfilament-destabilizing cytotoxic macrolide (-)-reidispongiolide A, isolated from the New Caledonian marine sponge Reidispongia coerulea, is described. This synthesis utilizes a convergent aldol-based strategy to construct the 26-membered macrolactone, followed by the late-stage coupling of a derived aldehyde with an N-vinylformamide-containing ketone subunit to install the full side chain. Two alternative routes were examined for the introduction of the 2E,4E-dienoate region, and a complex Mukaiyama aldol coupling was used to connect the northern and southern hemispheres to install the C13 stereocenter. This constitutes the first chemical synthesis of any member of the reidispongiolide/sphinxolide family of marine macrolides and unequivocally establishes the relative and absolute configuration.

An internal thioester in a pathogen surface protein mediates covalent host binding

To cause disease and persist in a host, pathogenic and commensal microbes must adhere to tissues. Colonization and infection depend on specific molecular interactions at the host-microbe interface that involve microbial surface proteins, or adhesins. To date, adhesins are only known to bind to host receptors non-covalently. Here we show that the streptococcal surface protein SfbI mediates covalent interaction with the host protein fibrinogen using an unusual internal thioester bond as a ‘chemical harpoon’. This cross-linking reaction allows bacterial attachment to fibrin and SfbI binding to human cells in a model of inflammation. Thioester-containing domains are unexpectedly prevalent in Gram-positive bacteria, including many clinically relevant pathogens. Our findings support bacterial-encoded covalent binding as a new molecular principle in host-microbe interactions. This represents an as yet unexploited target to treat bacterial infection and may also offer novel opportunities for engineering beneficial interactions. DOI: http://dx.doi.org/10.7554/eLife.06638.001

Synthesis and stereochemical assignment of (+)-chamuvarinin

A stereocontrolled total synthesis of (+)-chamuvarinin, isolated from the root extract of Uvaria Chamae, utilizes a convergent modular strategy to construct the adjacently linked C15−C28 ether array, followed by a late-stage Julia−Kocienski olefination to append the butenolide motif. This constitutes the first total synthesis of (+)-chamuvarinin, defining the relative and absolute configuration of this unique annonaceous acetogenin.

A quantitative evaluation of the effects of inhibitors of tubulin assembly on polymerization induced by discodermolide, epothilone B, and paclitaxel

PurposeTo determine whether inhibitors of microtubule assembly inhibit polymerization induced by discodermolide and epothilone B, as well as paclitaxel, and to quantitatively measure such effects.MethodsInhibition was quantitated by measuring polymer formation either by turbidimetry or by centrifugation, and the amount of inhibitor required to inhibit 50% relative to an appropriate control reaction was determined.ResultsThe inhibitory drugs evaluated were four colchicine site agents (combretastatin A-4, podophyllotoxin, nocodazole, and N-acetylcolchinol-O-methyl ether), maytansine, which competitively inhibits the binding of Catharanthus alkaloids to tubulin, halichondrin B and phomopsin A, which noncompetitively inhibit the binding of Catharanthus alkaloids to tubulin, and the depsipeptide dolastatin 15. While relative inhibitory effects were highly variable, a few broad generalizations can be made. First, assembly reactions that were either enhanced or dependent upon all three stimulatory drugs were subject to inhibition by all inhibitors. Second, the more readily the tubulin assembled, the greater the concentration of inhibitor required to inhibit polymerization. Drug IC50 values were generally lowest with no stimulatory drug and highest when discodermolide was present; IC50 values were higher as reaction temperature increased; and IC50 values were higher as the tubulin concentration increased. Third, inhibition of assembly by inhibitors of Catharanthus alkaloid binding to tubulin changed much less as a function of changes in reaction conditions than inhibition by inhibitors of colchicine binding.ConclusionsSince there was no apparent quantitative predictability of combined drug interactions with tubulin, any combination of interest must be studied in detail.

Synthesis of the Originally Proposed Structure of Palmerolide C

A stereoselective synthesis of the proposed structure of palmerolide C (1), a cytotoxic marine macrolide isolated from the Antarctic tunicate Synoicum adareanum, utilizes a convergent carbonyl-based coupling strategy to construct the C1-C24 carbon skeleton (see scheme). Compound 1 was shown to be a diastereomer of palmerolide C, and the structural revision of the natural product is proposed.

The Chemical Synthesis of Discodermolide

The marine sponge-derived polyketide discodermolide is a potent antimitotic agent that represents a promising natural product lead structure in the treatment of cancer. Discodermolide shares the same microtubule-stabilising mechanism of action as Taxol(®), inhibits the growth of solid tumours in animal models and shows synergy with Taxol. The pronounced cytotoxicity of discodermolide, which is maintained against cancer cell lines that display resistance to Taxol and other drugs, combined with its scarce availability from its natural source, has fuelled significant academic and industrial interest in devising a practical total synthesis as a means of ensuring a sustainable supply for drug development. This chapter surveys the various total syntheses of discodermolide that have been completed over the period 1993-2007, focusing on the strategies employed for introduction of the multiple stereocentres and achieving control over the alkene geometry, along with the various methods used for realising the pivotal fragment couplings to assemble progressively the full carbon skeleton. This dedicated synthetic effort has triumphed in removing the supply problem for discodermolide, providing sufficient material for extensive biological studies and enabling its early stage clinical development, as well as facilitating SAR studies for lead optimisation.