A Study Directed to the Asymmetric Synthesis of the Antineoplastic Macrolide Acutiphycin under Enantioselective Acyclic Stereoselection Based on Chiral Oxazaborolidinone-Promoted Asymmetic Aldol Reactions

Noncovalent Interactions in the Oxazaborolidine-Catalyzed Enantioselective Mukaiyama Aldol

Current models for oxazaborolidine-catalyzed transition-state structures are determined by C–H···O–B and C–H···O=S formyl hydrogen bonding between the electrophile and catalyst. However, selectivity in the oxazaborolidine-catalyzed Mukaiyama aldol cannot be fully rationalized using these models. Combined density functional theory and noncovalent interaction analyses reveal a new reaction model relying on C–H···O, C–H···π, and π–π interactions between the nucleophile, electrophile, and catalyst to induce selectivity.

Total Synthesis of (+)-Neopeltolide by the Macrocyclization/Transannular Pyran Cyclization Strategy

An 11-step synthesis of (+)-neopeltolide was developed. The C1-C7 carboxylic acid and the C8-C16 alcohol were prepared, each in six steps, from (R)- and (S)-epichlorohydrin, respectively. After esterification, our tandem macrocyclization/transannular pyran cyclization strategy was applied to a stereocontrolled construction of the neopeltolide macrolactone. The side chain was synthesized in six steps from ethyl 4-oxazolecarboxylate through palladium-catalyzed cross-couplings. A Mitsunobu reaction of the neopeltolide macrolactone and the side chain completed the synthesis.

Synthetic efforts on the road to marine natural products bearing 4-O-2,3,4,6-tetrasubstituted THPs: an update

Scientific literature is inundated with secondary metabolites from marine sources. In this ocean of natural products, the presence of recurring patterns has traditionally led scientists to unravel the biosynthetic mechanisms that naturally yield these products, as well as to imitate Nature to prepare them in the laboratory, especially when promising bioactivities and stimulating molecular architectures are involucrate. For instance, natural products containing multisubstituted oxygenated rings and macrocyclic lactones are recurrently selected as targets for developing total syntheses. Thus, in the last decades a noteworthy number of synthetic works regarding miyakolide, madeirolide A and representative compounds of polycavernosides, lasonolides and clavosolides have come to fruition. Up to now, these families of macrolides are the only marine natural products bearing a tetrasubstituted tetrahydropyran ring with carbon substituents at positions 2, 3 and 6, as well as an oxygen at position 4. Their splendid structures have received the attention of the synthetic community, up to the point of starring in dozens of articles, and even some reviews. This work covers all the synthetic studies towards miyakolide and madeirolide A, as well as the synthetic efforts performed after the previous specialised reviews about lasonolide A, polycavernoside A and clavosolides, published in 2006, 2007 and 2016, respectively. In total, this review summarises 22 articles in which these marine natural products with 4-O-2,3,4,6-tetrasubstituted tetrahydropyrans have the leading role.

Structure determination, correction, and disproof of marine macrolide natural products by chemical synthesis

Integration of chemical synthesis, NMR spectroscopy, and various analytical means is key to success in the structure elucidation of stereochemically complex marine macrolide natural products.

A counterintuitive stereochemical outcome from a chelation-controlled vinylmetal aldehyde addition leads to the configurational reassignment of phormidolide A

Detailed NMR spectroscopic comparisons of a series of synthetic C10–C23 fragments with a known derivative of phormidolide A resulted in the configurational reassignment of seven of its 11 stereocentres.

Total Synthesis of Polycavernosides A and B, Two Lethal Toxins from Red Alga

Polycavernosides A and B are glycosidic macrolide natural products isolated as the toxins causing fatal human poisoning by the edible red alga Gracilaria edulis (Polycavernosa tsudai). Total synthesis of polycavernosides A and B has been achieved via a convergent approach. The synthesis of the macrolactone core structure is highlighted by the catalytic asymmetric syntheses of the two key fragments using hetero-Diels-Alder reaction and Kiyooka aldol reaction as the key steps, their union through Suzuki-Miyaura coupling, and Keck macrolactonization. Finally, glycosylation with the l-fucosyl-d-xylose unit and construction of the polyene side chain through Stille coupling completed the total synthesis of polycavernosides A and B.

Synthesis and antiproliferative activity of new tonantzitlolone-derived diterpene derivatives

The synthesis of the diterpene (+)-tonantzitlolone A and a series of derivatives is reported. The study includes the determination of their antiproliferative activities against selected cancer cell lines.

Enzymatic reductions for the regio- and stereoselective synthesis of hydroxy-keto esters and dihydroxy esters

Ketoreductases were utilized for the stereoselective synthesis of δ-hydroxy-β-keto esters, β-hydroxy-δ-keto esters, and β,δ-dihydroxy esters. Seven out of eight possible stereoisomers were obtained from the enzymatic reduction of the corresponding β,δ-diketo ester in high enantio- and diastereomeric excess.

Synthesis of a new [6]-gingerol analogue and its protective effect with respect to the development of metabolic syndrome in mice fed a high-fat diet

To determine the effects of a [6]-gingerol analogue (6G), a major chemical component of the ginger rhizome, and its stable analogue after digestion in simulated gastric fluid, aza-[6]-gingerol (A6G), on diet-induced body fat accumulation, we synthesized 6G and A6G. Mice were fed either a control regular rodent chow, a high-fat diet (HFD), or a HFD supplemented with 6G and A6G. Magnetic resonance imaging adiposity parameters of the 6G- and A6G-treated mice were compared with those of control mice. Supplementation with 6G and A6G significantly reduced body weight gain, fat accumulation, and circulating levels of insulin and leptin. The mRNA levels of sterol regulatory element-binding protein 1c (SREBP-1c) and acetyl-CoA carboxylase 1 in the liver were significantly lower in mice fed A6G than in HFD control mice. Our findings indicate that A6G, rather than 6G, enhances energy metabolism and reduces the extent of lipogenesis by downregulating SREBP-1c and its related molecules, which leads to the suppression of body fat accumulation.

Introduction of the (-)-berkelic acid side chain and assignment of the C-22 stereochemistry

A Kiyooka aldol condensation of an aldehyde with a trimethylsilyl ketene acetal and the oxazaborolidinone prepared from N-Ts-(S)-valine gives two of the four possible aldol adducts, which were oxidized and deprotected to complete the synthesis of (-)-berkelic acid and (-)-22-epi-berkelic acid. This synthesis establishes the absolute stereochemistry and assigns the stereochemistry at C-22. A biosynthetic pathway is proposed that is consistent with the known absolute stereochemistry at the quaternary carbon of spiciferone A, spicifernin, and berkelic acid and provides a simple explanation for the differing stereochemistry at C-18 and C-19 of spicifernin and berkelic acid.

Synthesis of (-)-berkelic acid

An extremophilic challenge: Stereospecific condensation of a fully functionalized ketal aldehyde and a 2,6-dihydroxybenzoic acid is the key step in the synthesis of (-)-berkelic acid confirming Fürstner's reassignment of the stereochemistry at C18 and C19, establishing the absolute stereochemistry, and tentatively assigning the stereochemistry at C22.

Total synthesis of (+)-acutiphycin

Synthetic studies toward the total synthesis of (+)-acutiphycin (1) resulted in the discovery of additive-free, highly regioselective nickel-catalyzed reductive coupling reactions of aldehydes and 1,6-enynes and the construction of an advanced intermediate in studies directed toward the synthesis of 1. Ultimately, although not employing the nickel-catalyzed reaction, a highly convergent total synthesis of (+)-acutiphycin featuring an intermolecular SmI2-mediated Reformatsky coupling reaction and macrolactonization initiated by a retro-ene reaction of an alkoxyalkyne was achieved. The resulting synthesis was 18 steps in the longest linear sequence from either methyl acetoacetate or isobutyraldehyde.

A new construction of 2-alkoxypyrans by an acylation-reductive cyclization sequence

A new convergent synthetic approach to a pyran motif common to many naturally occurring structures is described. In this approach, two fragments are joined by esterification, and a subsequent intramolecular reductive cyclization affords the 2-hydroxypyran.

Marine cytotoxic macrolides haterumalides and biselides, and related natural products

Marine animals and plants are rich sources of bioactive natural products. Haterumalides and biselides, isolated from Okinawan marine animals, are 14-membered macrolides with strong cytotoxicity against human cancer cell lines. This review highlights the isolation, structures, bioactivities, and total synthesis of haterumalides, biselides, and related natural products.

An organocatalytic approach to the core of eunicellin

A stereocontrolled synthesis of the core of eunicellin is described featuring a Claisen rearrangement and a diastereoselective organocatalytic Diels-Alder reaction as the key steps.

Highly convergent total synthesis of (+)-acutiphycin

An enantioselective, convergent, total synthesis of (+)-acutiphycin (18 steps, longest linear sequence from commercial materials) features the first application of an alkynyl ether as a macrolactone precursor in total synthesis, as well as the first example of an intermolecular, SmI2-mediated, Reformatsky fragment coupling reaction. The high convergence, efficiency, and modular nature of this synthesis make it amenable to the synthesis of structurally related compounds.

Total synthesis and elucidation of the absolute configuration of the diterpene tonantzitlolone

[structure: see text] The first enantioselective total synthesis of tonantzitlolone, a novel 15-membered macrocyclic diterpene, utilized a Julia olefination, a highly selective, potassium enolate-based anti-Felkin aldol reaction, and an E-selective ring-closing metathesis as key C-C bond-forming steps. The absolute configuration of tonantzitlolone is established.

Enantioselective Synthesis of Apoptolidinone: Exploiting the Versatility of Thiazolidinethione Chiral Auxiliaries

An efficient, enantioselective synthesis of apoptolidinone has been completed, demonstrating the versatility of thiazolidinethione auxiliaries. Three propionate aldol additions and two asymmetric glycolate alkylations function to establish 8 of the 12 stereogenic carbon centers. A cross-metathesis reaction is utilized to assemble the C1-C10 trieneoate fragment and the C11-C28 polypropionate region of the molecule.

Roles of C-H...O=S and pi-stacking interactions in the 2-bromoacrolein complex with N-tosyl-(S)-tryptophan-derived oxazaborolidinone catalyst

[structures: see text] Ab initio and density functional calculations were employed to examine the structures and binding energies of various complexes between 2-bromolacrolein and N-tosyl-(S)-tryptophan-derived B-butyl-1,3,2-oxazaborolidin-5-one (NTOB), a catalyst commonly used for Diels-Alder reactions. Our calculations show that the chiral oxazaborolidinone catalyst serves as a tridentate complexation agent via B...O donor-acceptor, C-H...O hydrogen-bonded, and pi-stacking interactions. The most stable complex (1TS) is predicted to have a binding energy of -93 kJ mol(-1) (deltaG(298) = -29 kJ mol(-1)). The formyl C-H...O hydrogen bond and pi-stacking interaction are the key factors governing the relative stabilities of the four acrolein-NTOB complexes examined. The calculated structure and binding properties of 1TS are consistent with the experimental results on the absorption spectrum of the acrolein-NTOB complex and the effects of substituents on the reactivity of Diels-Alder reactions. 1TS differs from Corey's proposed model of transition-state assembly in two aspects: (1) it involves the s-trans-acrolein and (2) it favors a C-H...O interaction via the sulfonyl oxygen (C-H...O=S), rather than the ring oxygen (C-H...O-B). This calculated structure of the acrolein-catalyst complex provides an alternate explanation of the origin of stereoselectivity in the NTOB-catalyzed Diels-Alder reactions.

Catalytic, Enantioselective, Vinylogous Aldol Reactions

In 1935, R. C. Fuson formulated the principle of vinylogy to explain how the influence of a functional group may be felt at a distant point in the molecule when this position is connected by conjugated double-bond linkages to the group. In polar reactions, this concept allows the extension of the electrophilic or nucleophilic character of a functional group through the pi system of a carbon-carbon double bond. This vinylogous extension has been applied to the aldol reaction by employing "extended" dienol ethers derived from gamma-enolizable alpha,beta-unsaturated carbonyl compounds. Since 1994, several methods for the catalytic, enantioselective, vinylogous aldol reaction have appeared, with which varying degrees of regio- (site), enantio-, and diastereoselectivity can be attained. In this Review, the current scope and limitations of this transformation, as well as its application in natural product synthesis, are discussed.

Synthesis of ent-haterumalide NA (ent-oocydin A) methyl ester

[reaction: see text] Stille coupling of allylic chloride 26 with vinylstannane 4a afforded 65% of 27 with the requisite skipped diene, vinyl chloride, and allylic oxygen functionality. Yamaguchi macrolactonization of 28 provided 65% of 29, which was elaborated to haterumalide NA methyl ester (32) by a Nozaki-Hiyama-Kishi coupling with 31.