Lobatamide C: total synthesis,stereochemical assignment,preparation of simplified analogues,and V-ATPase inhibition studies

The total synthesis and stereochemical assignment of the potent antitumor macrolide lobatamide C, as well as synthesis of simplified lobatamide analogues, is reported. Cu(I)-mediated enamide formation methodology has been developed to prepare the highly unsaturated enamide side chain of the natural product and analogues. A key fragment coupling employs base-mediated esterification of a beta-hydroxy acid and a salicylate cyanomethyl ester. Three additional stereoisomers of lobatamide C have been prepared using related synthetic routes. The stereochemistry at C8, C11, and C15 of lobatamide C was assigned by comparison of stereoisomers and X-ray analysis of a crystalline derivative. Synthetic lobatamide C, stereoisomers, and simplified analogues have been evaluated for inhibition of bovine chromaffin granule membrane V-ATPase. The salicylate phenol, enamide NH, and ortho-substitution of the salicylate ester have been shown to be important for V-ATPase inhibitory activity.     

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.     

Stereoselectivity in reactions of atropisomeric lactams and imides

A range of reactions of cyclic lactam systems is described in which an atropisomeric C-N axis controls the stereochemical outcome of ring substitution or addition. In the case of enantiopure menthol adducts, substitution via N-acyliminium intermediates occurred with essentially complete control. However, the range of nucleophiles that participate in the reaction is very limited and at present the removal of the N-aryl substituent is problematic. A six-membered enamide is of moderate configurational stability and the axis exerts synthetically useful levels of control over enolate alkylations of the system. A novel Lewis acid mediated enamide arylation process was identified.     

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).     

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).     

Acyclic, achiral enamide nucleoside analogues. The importance of the C=C bond in the analogue for its ability to mimic natural nucleosides

The conformations of an acyclic, achiral enamide thymidine analogue 1 have been studied by model building and geometry calculations, as well as by NMR NOE and UV experiments. The results indicate that there are no significant barriers to rotation around any of the sigma bonds, in particular the N1-C1' enamide bond, and that the analogue should be able to accommodate conformations that mimic the conformations of natural nucleosides in A- and B-type helices quite well. For comparison the saturated analogue 2 has been prepared and built into oligonucleotides. It is shown that incorporation of 2 in oligonucleotides results in a much larger depression of the melting temperature (deltaTm -10 to -12.5 degrees C) than does incorporation of 1 (deltaTm -5 to -6.5 degrees C).     

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.     

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.     

Synthesis of enamides related to the salicylate antitumor macrolides using copper-mediated vinylic substitution

[formula: see text] A new approach to the assembly of enamides is described using copper(I) carboxylate-catalyzed substitution of vinyl iodides and amides. Modified reaction conditions have been developed to synthesize the O-methyloxime enamide side chains related to the natural products lobatamides A-F, oximidine I and II, and CJ-12,950.     

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.     

Formal total synthesis of (+/-)-gamma-lycorane and (+/-)-1-deoxylycorine using the [4+2]-cycloaddition/rearrangement cascade of furanyl carbamates

The total syntheses of gamma-lycorane and (+/-)-1-deoxylycorine were accomplished using an intramolecular Diels-Alder cycloaddition of a furanyl carbamate as the key step. The initially formed [4+2]-cycloadduct undergoes nitrogen-assisted ring opening followed by deprotonation/reprotonation of the resulting zwitterion to give a rearranged hexahydroindolinone. The stereochemical outcome of the IMDAF cycloaddition has the side arm of the tethered alkenyl group oriented syn with respect to the oxygen bridge. The key intermediate used in both syntheses corresponds to hexahydroindolinone 20. Removal of the t-Boc group in 20 followed by reaction with 6-iodobenzo[1,3]dioxole-5-carbonyl chloride afforded enamide 22. Treatment of this compound with Pd(OAc)(2) employing the Jeffrey modification of the Heck reaction provided the galanthan tetracycle 24 in good yield. Compound 24 was subsequently converted into (+/-)-gamma-lycorane using a four-step procedure to establish the cis-B,C-ring junction. A radical-based cyclization of the related enamide 33 was used for the synthesis of 1-deoxylycorine. Heating a benzene solution of 33 with AIBN and n-Bu(3)SnH at reflux gave the tetracyclic compound 38 possessing the requisite trans fusion between rings B and C in good yield. After hydrolysis and oxidation of 38 to 40, an oxidative decarboxylation reaction was used to provide the C(2)(-)C(3)(-)C(12) allylic alcohol unit characteristic of the lycorine alkaloids. The resulting enone was eventually transformed into (+/-)-1-deoxylycorine via known synthetic intermediates.     

Transition States of Binap–Rhodium(I)‐Catalyzed Asymmetric Hydrogenation: Theoretical Studies on the Origin of the Enantioselectivity

By using the hybrid IMOMM(B3LYP:MM3) method, we examined the binap–Rh^I‐catalyzed oxidative‐addition and insertion steps of the asymmetric hydrogenation of the enamide 2‐acetylamino‐3‐phenylacrylic acid. We report a path that is energetically more favorable for the major enantiomer than for the minor enantiomer. This path follows the “lock‐and‐key” motif and leads to the major enantiomeric product via an energetically favorable binap–dihydride–Rh^III–enamide complex. Our theoretical results are consistent with the mechanism that takes place via Rh^III dihydride formation, that is, oxidative addition of H₂ followed by enamide insertion.     

Diastereoselective synthesis of (2S,5R)-5-hydroxypipecolic acid and 6-substituted derivatives

[reaction: see text] Herein, we report a diastereoselective synthesis of the natural product (2S,5R)-5-hydroxypipecolic acid and 6-substituted derivatives thereof. The key step in the synthetic sequence is a novel highly diastereoselective epoxidation reaction of an enantiomerically pure cyclic enamide intermediate.     

The Thioesterase of the Erythromycin-Producing Polyketide Synthase: Influence of Acyl Chain Structure on the Mode of Release of Substrate Analogues from the Acyl Enzyme Intermediates

The production of genetically engineered polyketides depends critically on thioesterase activity for product release. In vitro studies with the thioesterase from the erythromycin polyketide synthase (PKS) have demonstrated that the ability of this enzyme to act as a universal decoupler is limited, but stereochemical variation is readily tolerated. Synthetic analogues with all four stereochemical configurations of the natural substrate's 2-methyl-3-hydroxy substitution pattern ( 1 – 4 ; X=p-nitrophenoxy) were substrates for the enzyme.     

Stereospecific total synthesis of somocystinamide A

The first total synthesis of somocystinamide A, a disulfide dimer with extremely labile enamide functional groups, was accomplished in a concise and stereospecific manner. Somocystinamide A is reported to possess exceptionally potent antiangiogenic and tumoricidal activities. The current work should enable further pharmacological investigation of this important natural product.     

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.     

Synthesis,Structural Reassignment,and Antibacterial Evaluation of 2,18‐Seco‐Lankacidinol B

Lankacidins are a group of polyketide natural products with activity against several strains of Gram‐positive bacteria. We developed a route to stereochemically diverse variants of 2,18‐seco‐lankacidinol B and found that the stereochemical assignment at C4 requires revision. This has interesting implications for the biosynthesis of natural products of the lankacidin class, all of which possessed uniform stereochemistry prior to this finding. We have evaluated 2,18‐seco‐lankacidinol B and three stereochemical derivatives against a panel of pathogenic Gram‐positive and Gram‐negative bacteria.     

Synthesis,Structural Reassignment,and Antibacterial Evaluation of 2,18‐Seco‐Lankacidinol B

Lankacidins are a group of polyketide natural products with activity against several strains of Gram‐positive bacteria. We developed a route to stereochemically diverse variants of 2,18‐seco‐lankacidinol B and found that the stereochemical assignment at C4 requires revision. This has interesting implications for the biosynthesis of natural products of the lankacidin class, all of which possessed uniform stereochemistry prior to this finding. We have evaluated 2,18‐seco‐lankacidinol B and three stereochemical derivatives against a panel of pathogenic Gram‐positive and Gram‐negative bacteria.     

Intermediates in asymmetric hydrogenation : The structure and 31P NMR spectra of rhodium enamide complexes containing 1R, 2R-trans-1,2-bis (diphenylphosphinomethyl) cyclo buta

Hydrogenation of bicyclo[2.2.1]heptadiene-1R,2R-(trans-1,2-bis(diphenylphosphinomethyl)cyclbutane rhodium (I) tetrafluoroborate in methanol gives a solvent adduct which reacts with N-acyldehydroamino acids and their esters to give air-unstable scarlet to yellow complexes. Effects of structure variation in the enamide on the ³¹P NMR spectra of these species are reported and discussed with reference to (a) the equilibrium between methanol and enamide complexes; (b) the ratio of diastereomeric enamide complexes formed; (c) the temperature dependence of this ratio; (d) the rate of complexation equilibria and (e) the structure of enamide complexes, which are normally bidentate with binding via the olefin and amide groups. In certain cases the complex may be terdentate and E-enamides bind through the olefin and carboxyl groups. Each mode of binding gives rise to characteristic ³¹P NMR spectra with regard to P-P and P-Rh coupling constants.     

Programming the supramolecular helical polymerization of dendritic dipeptides via the stereochemical information of the dipeptide

Many natural biomacromolecules are homochiral and are built from constituents possessing identical handedness. The construction of synthetic molecules, macromolecules, and supramolecular structures with tailored stereochemical sequences can detail the relationship between chirality and function and provide insight into the process that leads to the selection of handedness and amplification of chirality. Dendritic dipeptides, previously reported from our laboratory, self-assemble into helical porous columns and serve as fundamental mimics of natural porous helix-forming proteins and supramolecular polymers. Herein, the synthesis of all stereochemical permutations of a self-assembling dendritic dipeptide including homochiral, heterochiral, and differentially racemized variants is reported. A combination of CD/UV-vis spectroscopy in solution and in film, X-ray diffraction, and differential scanning calorimetry studies in solid state established the role of the stereochemistry of the dipeptide on the thermodynamics and mechanism of self-assembly. It was found that the highest degree of stereochemical purity, enantiopure homochiral dendritic dipeptides, exhibits the most thermodynamically favorable self-assembly process in solution corresponding to the greatest degree of helical order and intracolumnar crystallization in solid state. Reducing the stereochemical purity of the dendritic dipeptide through heterochirality or by partially or fully racemizing the dendritic dipeptide destructively interferes with the self-assembly process. All dendritic dipeptides were shown to coassemble into single columns regardless of their stereochemistry. Because these columns exhibit no deracemization, the thermodynamic advantage of enantiopurity and homochirality suggests a mechanism for stereochemical selection and chiral amplification.