Total synthesis and stereochemical assignment of the salicylate antitumor macrolide lobatamide C(1)

The total synthesis and stereochemical assignment of the potent antitumor macrolide lobatamide C is reported. The synthesis involves Cu(I)-mediated enamide formation and Na(2)CO(3)-mediated esterification of a beta-hydroxy acid and a salicylate cyanomethyl ester. Macrolactonization was accomplished using a Mitsunobu protocol. The stereochemical assignment of lobatamide C was achieved by Mosher ester analysis and comparison with prepared stereoisomers.     

Synthesis and V-ATPase inhibition of simplified lobatamide analogues

[structure: see text] Simplified analogues of the lobatamides have been synthesized and evaluated for inhibition of bovine V-ATPase. The salicylate phenol, enamide NH, and the ortho-substitution of the salicylate ester have been shown to be important for V-ATPase inhibitory activity.     

Synthesis of photoactivatable acyclic analogues of the lobatamides

[structure: see text] The lobatamides and related salicylate enamide natural products are potent mammalian V-ATPase inhibitors. To probe details of binding of the lobatamides to mammalian V-ATPase, three photoactivatable analogues bearing benzophenone photoaffinity labels have been prepared. The analogues were designed on the basis of a simplified acyclic analogue 2. Late-stage installation of the enamide side chain and tandem deallylation/amidation were employed in synthetic routes to these derivatives. Simplified analogue 2 showed strong inhibition against bovine clathrin-coated vesicle V-ATPase (10 nM). Analogues 3-5 were also evaluated for inhibition of bovine V-ATPase in order to select a suitable candidate for future photoaffinity labeling studies.     

Total synthesis and biological evaluation of neodysiherbaine A and analogues

Dysiherbaine (1) and its congener neodysiherbaine A (2) are naturally occurring excitatory amino acids with selective and potent agonistic activity for ionotropic glutamate receptors. We describe herein the total synthesis of 2 and its structural analogues 3-8. Advanced key intermediate 16 was employed as a branching point to assemble a series of these analogues 3-8 with respect to the C8 and C9 functionalities, which would not have been accessible through manipulations of the natural product itself. The synthesis of key intermediate 16 features (i) stereocontrolled C-glycosylation to set the C6 stereocenter, (ii) concise synthesis of the bicyclic ether skeleton through chemo- and stereoselective dihydroxylation of the exo-olefin and stereoselective epoxidation of the endo-olefin, followed by epoxide ring opening/5-exo ring closure, and (iii) catalytic asymmetric hydrogenation of enamide ester to construct the amino acid appendage. A preliminary biological evaluation of analogues for their in vivo toxicity against mice and binding affinity for glutamate receptors showed that both the type and stereochemistry of the C8 and C9 functional groups affected the subtype selectivity of dysiherbaine analogues for members of the kainic acid receptor family.     

Ring-closing metathesis: a powerful tool for the synthesis of simplified salicylihalamide-based V-ATPase inhibitors

Based on our synthetic strategy developed for the total synthesis of the macrocyclic salicylate natural product salicylihalamide, we describe herein the synthesis of a series of simplified salicylihalamide-based analogs. Alterations in the aromatic fragment, the macrolactone scaffold and side-chain were evaluated for in vitro inhibition of V-ATPase activity and human tumor cell growth.     

Total synthesis of dysiherbaine

An efficient total synthesis of dysiherbaine, a potent and subtype-selective agonist for ionotropic glutamate receptors, has been achieved. An advanced key intermediate in the previous synthesis of neodysiherbaine A and its analogues was selected as the starting point, and cis-oriented amino alcohol functionality on the tetrahydropyran ring was installed by using an intramolecular S_N2 cyclization of N-Boc-protected amino alcohol. The amino acid appendage was constructed by catalytic asymmetric hydrogenation of enamide ester.     

Total synthesis of the salicylate enamide macrolide oximidine II

The asymmetric synthesis of the salicylate enamide macrolide oximidine II is reported. The synthesis involves a highly regio- and stereoselective ring-closing metathesis of a bis-diene substrate to construct the macrocyclic triene core. Copper(I)-mediated amidation of a (Z)-vinyl iodide was employed to attach the enamide side chain.     

Total synthesis and structural elucidation of khafrefungin.

Total synthesis and structural elucidation of khafrefungin, a novel antifungal agent isolated from the fermentation culture MF6020, have been achieved. Unlike other inhibitors that inhibit the corresponding enzyme in fungi and mammals to the same extent, khafrefungin does not impair sphingolipid synthesis of mammals. The basic strategy for the structural elucidation is to prepare all stereoisomers of the structurally simplified khafrefungin mimics 1 and 2 that were designed for the elucidation of C10,11,12 and C2',3',4' relative stereochemistry, respectively. The comparison of their spectra with those of natural khafrefungin would result in the identification of eight possible stereoisomers, and the analytical details of these eight stereoisomers have led to the complete stereochemical assignment. On the basis of the structural elucidation, the total synthesis of khafrefungin has been accomplished by using tin(II)-catalyzed asymmetric aldol reactions as key steps.     

Approaches to oximidines, lobatamides and related natural products: the coupling reactions of 3-iodoacrolein O-methyl oximes

Both 2E- and 2Z-3-iodoacrolein O-methyl oximes are prepared in two steps from ethyl propiolate. Lithium–iodine exchange is effected and the resulting organolithium reagents added to several electrophiles, including styryl isocyanate which gives a conjugated O-methyl oxime enamide of the type found in the side chains of the oximidine, lobatamide and CJ-12950 natural products. The Pd(0) catalysed cross-coupling of these iodoalkenes is also explored.     

Rapid and efficient synthesis of dysiherbaine and analogues to explore structure-activity relationships

A rapid and efficient total synthesis of dysiherbaine (1), a potent and subtype-selective agonist for ionotropic glutamate receptors, has been accomplished. A key intermediate 15 was synthesized by two approaches. The first synthetic route utilized compound 9, an advanced intermediate in our previous total synthesis of neodysiherbaine A, as the starting point, and the cis-oriented amino alcohol functionality on the tetrahydropyran ring was installed by using an intramolecular S(N)2 cyclization of N-Boc-protected amino alcohol 20. An alternative and even more efficient synthetic approach to 15 featured stereoselective introduction of an amino group at C8 by iodoaminocyclization prior to constructing the bicyclic ether skeleton. The amino acid appendage was efficiently constructed by a catalytic asymmetric hydrogenation of enamide ester 36. The synthetic route developed here provided access to several dysiherbaine analogues, including 9-epi-dysiherbaine (38), 9-deoxydysiherbaine (39), 9-methoxydysiherbaine (40), and N-ethyldysiherbaine (41). The preliminary structure-activity relationship studies revealed that the presence and stereochemistry of the C9 hydroxy group in dysiherbaine is important for high-affinity and selective binding to glutamate subtype receptors.     

Expedient asymmetric synthesis of all four isomers of N,N'-protected 2,3-diaminobutanoic acid

2,3-Diaminobutanoic acid (DAB) is found in several peptide antibiotics, toxins, and biologically active molecules. This paper describes the practical and highly enantioselective synthesis of all four N,N'-protected DAB stereoisomers using an asymmetric Rh(I)-phosphine-catalyzed hydrogenation of isomeric enamides as the key step. Thermal and photochemical isomerization of the enamide hydrogenation substrates coupled with catalyst-geometric isomer pairing allows targeted synthesis of single DAB isomers in maximum yield.     

Synthesis and cell cycle inhibition of the peptide enamide natural products terpeptin and the aspergillamides

Total syntheses of the peptide enamide natural products terpeptin and aspergillamides A and B are reported. An oxidative decarboxylation-elimination protocol is employed to construct the indolic enamide moiety. Unambiguous stereochemical assignment of (−)-terpeptin is accomplished by synthesis of all possible stereochemical analogues. Select compounds have been evaluated in cell cycle inhibitor assays which show that the natural amino acid configuration of terpeptin has the most potent inhibitory activity.     

Solid‐Phase Total Synthesis of Yaku'amide B Enabled by Traceless Staudinger Ligation

We report a solid‐phase strategy for total synthesis of the peptidic natural product yaku'amide B ( 1 ), which exhibits antiproliferative activity against various cancer cells. Its linear tridecapeptide sequence bears four β,β‐dialkylated α,β‐dehydroamino acid residues and is capped with an N‐terminal acyl group (NTA) and a C‐terminal amine (CTA). To realize the Fmoc‐based solid‐phase synthesis of this complex structure, we developed new methods for enamide formation, enamide deprotection, and C‐terminal modification. First, traceless Staudinger ligation enabled enamide formation between sterically encumbered alkenyl azides and newly designed phosphinophenol esters. Second, application of Eu(OTf)₃ led to chemoselective removal of the enamide Boc groups without detaching the resin linker. Finally, resin‐cleavage and C‐terminus modification were simultaneously achieved with an ester–amide exchange reaction using CTA and AlMe₃ to deliver 1 in 9.1 % overall yield (24 steps from the resin).     

Synthesis of monoacyl A-ring precursors of 1alpha,25-dihydroxyvitamin D3 through selective enzymatic hydrolysis

An efficient synthesis of monoacylated 1alpha,25-dihydroxyvitamin D3 A-ring precursors 15, 16, 18, and 19 has been described through an enzymatic hydrolysis process. Candida antarctica A lipase (CAL-A) hydrolyzes the C-5 acetate ester in trans stereoisomers 9 and 13, with complete and high selectivity, respectively. In the case of cis isomers 11 and 14, Chromobacterium viscosum lipase (CVL) is the enzyme of choice, exhibiting opposite selectivity for these two enantiomers. This lipase selectively catalyzes the hydrolysis at the C-3 acetate in diester 11 and at C-5 position in diester 14. It is noteworthy that through a hydrolysis reaction CAL-A and CVL allow the synthesis of the four A-ring monoacetylated precursors of 1alpha,25-dihydroxyvitamin D3, precursors which are complementary to those obtained by the enzymatic acylation process. In addition, with excellent yield CVL selectively hydrolyzes the C-3 chloroacetate ester instead of the C-5 acetate in diester 22, a key intermediate in the synthesis of new A-ring modified 1alpha,25-dihydroxyvitamin D3 analogues.     

Solution- and solid-phase synthesis of natural product-like tetrahydroquinoline-based polycyclics having a medium size ring

A solid-phase synthesis of tetrahydroquinoline-derived polycyclic 4, having a medium size ring with an enamide functionality, was achieved from tetrahydroquinoline derivative 3 in five steps with overall 40-45% yield. An enantiopure, tetrahydroquinoline-derived beta-amino ester, 1, was converted into compound 2 that has a free phenolic hydroxyl group as an anchoring site for solid-phase synthesis. The solid-phase worked well for this sequence, in which the synthesis of the unsaturated eight-membered enamide lactam was obtained by a ring-closing metathesis approach. Compound 4 is a novel, natural product-like polycyclic derivative that could further be utilized in library generation for developing small molecule chemical probes.     

N-Acetylneuraminic Acid: Neoglycoproteins and Pseudopolysaccharides

Abstract

A simplified procedure for the synthesis of acetochloroneuraminic acid is described starting from the methyl ester. N-acetyl neuraminic acid α-allyl glycoside was prepared in high yield using silver salicylate as catalyst. Reductive ozonolysis of the allyl group, followed by direct reductive ami nation to protein carriers, gave immunogenic neoglycoproteins. Copolymerization of the α-allyl glycoside with acrylamide produced a water-soluble pseudopolysaccharide useful for the EL ISA technique.     

Solid-Phase Total Synthesis of Yaku'amide B Enabled by Traceless Staudinger Ligation

We report a solid-phase strategy for total synthesis of the peptidic natural product yaku'amide B ( 1 ), which exhibits antiproliferative activity against various cancer cells. Its linear tridecapeptide sequence bears four β,β-dialkylated α,β-dehydroamino acid residues and is capped with an N-terminal acyl group (NTA) and a C-terminal amine (CTA). To realize the Fmoc-based solid-phase synthesis of this complex structure, we developed new methods for enamide formation, enamide deprotection, and C-terminal modification. First, traceless Staudinger ligation enabled enamide formation between sterically encumbered alkenyl azides and newly designed phosphinophenol esters. Second, application of Eu(OTf)₃ led to chemoselective removal of the enamide Boc groups without detaching the resin linker. Finally, resin-cleavage and C-terminus modification were simultaneously achieved with an ester–amide exchange reaction using CTA and AlMe₃ to deliver 1 in 9.1 % overall yield (24 steps from the resin).     

Three-component Biginelli cyclocondensation reaction using C-glycosylated substrates. Preparation of a collection of dihydropyrimidinone glycoconjugates and the synthesis of C-glycosylated monastrol analogues

The aldehyde-ketoester-urea cyclocondensation reaction has been revisited using C-glycosylated reagents with the aim of exploring a potential entry to a library of dihydropyrimidinone glycoconjugates. A collection of 13 mono- and bis-C-glycosylated dihydropyrimidinones has been prepared by a parallel synthesis approach using the three-component promoter CuCl/AcOH/BF(3) x Et(2)O. The sugar residues have been installed at either N1, C4, or C6 in the monoglycosylated derivatives and at both the C4 and C6 in the bisglycosylated products. The mono- and bisglycosylated products at C4 and C6 were obtained as mixtures of diastereoisomers with good to excellent selectivities due to the asymmetric induction by the sugar residue in the formation of the C4 stereocenter of the dihydropyrimidinone ring. Individual stereoisomers were isolated as pure compounds and their structures assigned with the aid of X-ray crystallography and chiroptical properties. As a demonstration of this new concept in the Biginelli reaction, the synthesis of two C4 epimer monastrol analogues bearing the ribofuranosyl moiety at C6 has been described.     

Asymmetric synthesis of (S)-(−)-N-acetylcolchinol via Ullmann biaryl coupling

A modified Ziegler Ullmann coupling process has been developed as the key step in an effective synthesis of (S)-(−)-N-acetylcolchinol, analogues of which are selective vascular targeting agents with potential importance in cancer chemotherapy. Asymmetric induction is achieved by enamide hydrogenation using FerroTANE catalysts.     

Toward breast cancer resistance protein (BCRP) inhibitors: design, synthesis of a series of new simplified fumitremorgin C analogues

In this study, we report the design and synthesis of a series of new simplified fumitremorgin C analogues. The preliminary biological study indicated some of these simplified fumitremorgin C might be developed into breast cancer resistance inhibitors.