Studies in polypropionate synthesis: High π-face selectivity in syn and anti aldol reactions of chiral boron enolates of lactate-derived ketones

(2R,4S,5S)-5-Methoxy-4-methyl-3-oxohept-6-en-2-yl benzoate

The title compound, C16H20O4, was synthesized in the course of the total synthesis of fusaequisin A in order to verify and confirm the configurations of the stereogenic centers and to exclude the possibility of epimerization during the methylation process. The crystal structure of the title compound at 100 K has orthorhombic (P212121) symmetry. The absolute configuration was determined by anomalous dispersion and agrees with the configuration of the allylic alcohol used in the synthesis.

Conquering peaks and illuminating depths: developing stereocontrolled organic reactions to unlock nature's macrolide treasure trove

The structural complexity and biological importance of macrolide natural products has inspired the development of innovative strategies for their chemical synthesis. With their dense stereochemical content, high level of oxygenation and macrocyclic cores, we viewed the efficient total synthesis of these valuable compounds as an aspirational driver towards developing robust methods and strategies for their construction. Starting out from the initial development of our versatile asymmetric aldol methodology, this personal perspective reflects on an adventurous journey, with all its trials, tribulations and serendipitous discoveries, across the total synthesis, in our group, of a representative selection of six macrolide natural products of marine and terrestrial origin - swinholide A, spongistatin 1, spirastrellolide A, leiodermatolide, chivosazole F and actinoallolide A.

On the Mechanism of the Asymmetric Aldol Addition of Chiral N-Amino Cyclic Carbamate Hydrazones: Evidence of Non-Curtin-Hammett Behavior

he mechanistic details of the aldol addition of N-amino cyclic carbamate (ACC) hydrazones is provided herein from both an experimental and computational perspective. When the transformation is carried out at room temperature the anti-aldol product is formed exclusively. Under these conditions the anti- and syn-aldolate intermediates are in equilibrium and the transformation is under thermodynamic control. The anti-aldolate that leads to the anti-aldol product was calculated to be 3.7 kcal mol^-1 lower in energy at room temperature than that leading to the syn-aldol product, which sufficiently accounts for the exclusive formation of the anti-aldol product. When the reaction is conducted at -78 °C it is under kinetic control and favors formation of the syn-aldol addition product. In this case, it was found that a solvent separated aza-enolate anion and aldehyde form a σ-intermediate in which the lithium cation is coordinated to the aldehyde. The σ-intermediate collapses with a very small activation barrier to form the β-alkoxy hydrazone intermediate. The chiral nonracemic lithium aza-enolate discriminates between the two diastereotopic faces of the pro-chiral aldehyde, and there is no rapid direct pathway that interconverts the two diastereomeric intermediates. Consequently, the reaction does not follow the Curtin-Hammett principle and the stereochemical outcome at low temperature instead depends on the relative energies of the two σ-intermediates.

Synthesis of Cytospolide Analogues and Late-State Diversification Thereof

The cytospolides are a novel group of fungal secondary metabolites first described in 2011. Although all 17 natural derivatives share the same C-14 polyketide backbone, they exhibit a fairly broad structural diversity regarding their oxygenation and acetylation pattern as well as macrolide structure, e.g., monocyclic nonanolide core or bicyclic ring systems with a bridging THF ring. In the present work, the prospects for an extension of the structural diversity of cytospolides have been investigated. On the basis of a previously established synthesis of cytospolide D, a modified route to a truncated analogue carrying an alkyne instead of the natural n-pentyl side chain has been developed. In a bioinspired approach the so-derived cytospolide D alkyne analogue has been further converted to bicyclic and THF ring containing derivatives with a different backbone architecture. Finally, Sonogashira couplings or Huisgen-Sharpless click reactions have been used for late-stage diversifications. Thus, a set of 15 novel and structurally diverse natural product derivatives has been synthesized in a highly efficient manner.

Studies toward the Synthesis of Iriomoteolide-2a: Construction of the C(6)-C(28) Fragment

The synthesis of an appropriately functionalized advanced C(6-28) fragment (3) of the marine macrolide iriomoteolide-2a (1) has been achieved in a highly efficient manner. The C(6)-C(18) fragment of 1 is prepared via a radical cyclization of a vinyl ether intermediate and palladium-promoted hydrostannylation/iodination. Paterson aldol reaction and Peterson olefination are used to construct the C(19)-C(28) fragment. The union of the C(6)-C(18) and C(19)-C(28) fragments is accomplished via a Suzuki-Miyaura coupling reaction.

Lipid structure influences the ability of glucose monocorynomycolate to signal through Mincle

Mincle (macrophage-inducible C-type lectin) is a C-type lectin receptor that provides the capacity for immune sensing of a range of pathogen- and commensal-derived glycolipids. Mincle can recognize mycolic and/or corynomycolic acid esters of trehalose, glycerol and glucose from mycobacteria and corynebacteria. While simple straight-chain long fatty acids (e.g. behenic acid) can substitute for mycolic acid on trehalose and glycerol and maintain robust signalling through Mincle, glucose monobehenate has been reported to be much less active than glucose monocorynomycolate (GMCM). We report the preparation of a range of analogues of GMCM to explore structural requirements in the lipid chain for signalling through Mincle. GMCM analogues bearing simple straight chain or branched fatty acid esters provided only weak signalling through human and mouse Mincle. A GMCM variant with a truncated (pentyl) α-chain provided attenuated signalling, whereas an analogue with an extended (tricosyl; C23) α-chain signalled as potently as GMCM. This work suggests that Mincle has the ability to survey mycolate-derived glycolipids from actinomycetes, distinguishing non-pathogenic (e.g. Rhodococcus spp.) and pathogenic (e.g. Mycobacterium tuberculosis) species on the basis of α-chain length. Finally, an α-phenyldodecyl analogue of GMCM possessed similar potency to GMCM and was only slightly less potent than trehalose dimycolate (cord factor), showing that large functional groups may be tolerated in the α-chain.

(2R,4S,5S)-5-Hydroxy-4-methyl-3-oxohept-6-en-2-yl benzoate

The title compound, C15H18O4, which crystallizes with two molecules in the asymmetric unit, was obtained in the course of the total synthesis of curvicollides A–C and fusaequisin A. It features the relative configuration of the Western aldol part of the natural products. In the crystal, molecules are linked by C—H...O hydrogen bonds.

Total Synthesis and Structural Assignment of Curvicollide C

The first total synthesis of (+)-curvicollide C has been accomplished. Cross-metathesis and Julia-Kocienski olefination were instrumental in the synthesis of 1,3-diene segments and allowed for a ternary-convergent synthetic design. A full structural assignment is proposed for (-)-curvicollide C, a uniquely structured polyketide of fungal origin.

Total Synthesis of Cytospolide D and Its Biomimetic Conversion to Cytospolides M, O, and Q

A total synthesis of cytospolide D, starting from l-glutamic acid, is described. The critical macrolactonization to the 10-membered lactone containing an (E)-configured double bond was successfully achieved by Shiina esterification. Conversion of cytospolide D to its bicyclic derivatives M, O, and Q was accomplished under mild conditions, lending support to the proposed biosynthetic hypothesis.

HM, Nair VA. Anti selective glycolate aldol reactions of (S)-4-isopropyl-1-[(R)-1-phenylethyl]imidazolidin-2-one: application towards the asymmetric synthesis of 8-4′-oxyneolignans

The anti selective glycolate aldol reactions of (S)-4-isopropyl-1-[(R)-1-phenylethyl]imidazolidin-2-one auxiliary have been standardized with high yields and excellent diastereoselectivities on various substituted aryl, allyl and alkyl aldehydes.

A Highly Convergent and Biomimetic Total Synthesis of Portentol

An efficient total synthesis of the unusual polyketide portentol is reported. Three boron aldol reactions were used to assemble the linear carbon chain of the natural product, which contains two challenging anti-anti stereotriads. A biomimetic double cyclization cascade, triggered by an oxidation, then afforded portentol and its known dehydration product, anhydroportentol. The biosynthesis of portentol and the biosynthetic relevance of our key step are discussed.

Corynomycolic acid-containing glycolipids signal through the pattern recognition receptor Mincle

Glucose monocorynomycolate is revealed to signal through both mouse and human Mincle. Glycerol monocorynomycolate is shown to selectively signal through human Mincle, with the activity residing predominantly in the 2′S-isomer.

Synthesis of (+)-discodermolide by catalytic stereoselective borylation reactions

The marine natural product (+)-discodermolide was first isolated in 1990 and, to this day, remains a compelling synthesis target. Not only does the compound possess fascinating biological activity, but it also presents an opportunity to test current methods for chemical synthesis and provides an inspiration for new reaction development. A new synthesis of discodermolide employs a previously undisclosed stereoselective catalytic diene hydroboration and also establishes a strategy for the alkylation of chiral enolates. Furthermore, this synthesis of discodermolide provides the first examples of the asymmetric 1,4-diboration of dienes and borylative diene-aldehyde couplings in complex-molecule synthesis.

Total synthesis of the antimitotic marine macrolide (-)-leiodermatolide

Leiodermatolide is an antimitotic macrolide isolated from the marine sponge Leiodermatium sp. whose potentially novel tubulin-targeting mechanism of action makes it an exciting lead for anticancer drug discovery. In pursuit of a sustainable supply, we report a highly stereocontrolled total synthesis (3.2% yield) based on a convergent sequence of palladium-mediated fragment assembly and macrolactonization. Boron-mediated aldol reactions were used to configure the three key fragments 2, 5, and 6 by employing the appropriate enantiomer of the lactate-derived ketone 7.

Asymmetric anti-aldol addition of achiral ketones via chiral N-amino cyclic carbamate hydrazones

The asymmetric anti-aldol addition of ketone-derived donors and aldehyde acceptors is described. Asymmetric induction is achieved through the use of chiral N-amino cyclic carbamate (ACC) auxiliaries. The transformation exhibits essentially perfect anti-diastereoselectivity and enantioselectivity, and has the unusual feature of proceeding via thermodynamic, rather than kinetic control.

The impact of the Mukaiyama aldol reaction in total synthesis

Four decades since Mukaiyama's first reports on the successful application of silicon and boron enolates in directed aldol reactions, the ability of this highly controlled carbon-carbon bond-forming method to simultaneously define stereochemistry, introduce complexity, and construct the carbon skeleton with a characteristic 1,3-oxygenation pattern has made it a powerful tool for natural product synthesis. This Minireview highlights a number of representative total syntheses that demonstrate the impact of the Mukaiyama aldol reaction and discusses the underlying mechanistic rationale that determines the stereochemical outcomes.

Total synthesis of (+)-spirastrellolide A methyl ester: Challenges and discoveries

This review focuses on recent synthetic efforts by our group towards spirastrellolide A methyl ester, a complex marine macrolide containing two spiroacetal ring systems that shows promising anticancer properties. The evolution of a flexible, modular strategy leading to the first total synthesis of (+)-spirastrellolide A methyl ester, and the associated challenges overcome, are highlighted, particularly in dealing with the initial structural ambiguities. This work enabled the development of an improved second-generation synthesis, which revealed a critical dependence of the key macrolactonization step on the nature of the protecting groups in the linker region between the spiroacetal motifs.

The stereocontrolled total synthesis of spirastrellolide A methyl ester. Expedient construction of the key fragments

Due to a combination of their promising anticancer properties, limited supply from the marine sponge source and their unprecedented molecular architecture, spirastrellolides represent attractive and challenging synthetic targets. A modular strategy for the synthesis of spirastrellolide A methyl ester, which allowed for the initial stereochemical uncertainties in the assigned structure was adopted, based on the envisaged sequential coupling of a series of suitably functionalised fragments; in this first paper, full details of the synthesis of these fragments are described. The pivotal C26-C40 DEF bis-spiroacetal was assembled by a double Sharpless asymmetric dihydroxylation/acetalisation cascade process on a linear diene intermediate, configuring the C31 and C35 acetal centres under suitably mild acidic conditions. A C1-C16 alkyne fragment was constructed by application of an oxy-Michael reaction to introduce the A-ring tetrahydropyran, a Sakurai allylation to install the C9 hydroxyl, and a 1,4-syn boron aldol/directed reduction sequence to establish the C11 and C13 stereocentres. Two different coupling strategies were investigated to elaborate the C26-C40 DEF fragment, involving either a C17-C25 sulfone or a C17-C24 vinyl iodide, each of which was prepared using an Evans glycolate aldol reaction. The remaining C43-C47 vinyl stannane fragment required for introduction of the unsaturated side chain was prepared from (R)-malic acid.

Total synthesis of auripyrone B using a non-aldol aldol-cuprate opening process

A non-aldol aldol-cuprate opening generates the polypropionate 11 from the epoxy ether 14 in eight steps as a single diastereomer. A highly stereoselective aldol reaction of 8 with 9 gives the aldol product 7 in high yield and excellent diastereoselectivity, due to double stereodifferentiation. This compound was used for an efficient synthesis of the natural product auripyrone B 2 in only 20 steps and 8% overall yield from 14 using a late-stage spiroketalization onto a stable hemiketal as the final key step.