Asymmetric crotylation reactions in synthesis of polypropionate-derived macrolides: application to total synthesis of oleandolide

Complete details of a convergent asymmetric synthesis of oleandolide (1), the aglycon of the macrolide antibiotic oleandomycin, is described. The synthesis has been achieved through the assembly and coupling of the left- and right-hand subunits 12 and 38, respectively. These subunits were prepared from chiral silane-based asymmetric crotylation reactions to control the stereochemical relationships. The left- and right-hand subunits (C1-C7 and C8-C14) were brought together through a Pd(0)-catalyzed sp3-sp2 cross-coupling reaction between the zinc intermediate 40 and vinyl triflate 38 to give 27. This product was converted to seco acid 42a and cyclized to lactone 35 under Yamaguchi conditions. This material was then epoxidized with m-chloroperbenzoic acid (m-CPBA) to install the correct C8 epoxide as a single diastereomer, which after a short deprotection sequence completed the synthesis of oleandolide.     

Total synthesis of (-)-callystatin A

[reaction: see text] A convergent enantioselective synthesis of the natural product (-)-callystatin A (1) is described. Key features of the synthesis include a lipase-mediated kinetic resolution to install the C5 lactone stereochemistry, a hydrozirconation-based approach to the C8-C9 trisubstituted (Z)-olefin, and a stereoselective cross-coupling of a vinyl dibromide to install the C14-C15 trisubstituted (E)-olefin.     

The first total synthesis of concanamycin f (concanolide a)

A highly stereoselective total synthesis of the macrolide antibiotic concanamycin F (1), a specific and potent inhibitor of vacuolar H(+)-ATPase, has been achieved by a convergent route involving the synthesis and coupling of its 18-membered tetraenic lactone and beta-hydroxyl hemiacetal side chain subunits. The C1-C19 18-membered lactone aldehyde 4 was synthesized through the intermolecular Stille coupling of the C5-C13 vinyl iodide 24 and the C14-C19 vinyl stannane 25, followed by construction of the C1-C4 diene and macrolactonization. Synthesis of 4 via a second convergent route including the esterification of the C1-C13 vinyl iodide 45 and the C14-C19 vinyl stannane 47 followed by the intramolecular Stille coupling was also realized. The highly stereoselective aldol coupling of 4 and the C20-C28 ethyl ketone 5 followed by desilylation provided 1 which was identical with natural concanamycin F.     

Enantioselective synthesis of aurisides A and B, cytotoxic macrolide glycosides of marine origin

The enantioselective synthesis of aurisides A and B, macrolide glycosides of marine origin, was achieved by a convergent approach. The C1-C9 segment 4 was prepared from (R)-pantolactone, and the C10-C17 segment 14 was synthesized from (R)-glycidyl trityl ether. The Nozaki-Hiyama-Kishi reaction between 4 and 14 and subsequent reactions gave seco acid 10, which was converted into the aglycon (3) of aurisides by construction of the 14-membered lactone and bromine-substituted conjugated diene. The glycosylation reaction of the aglycon provided aurisides A and B.     

New fatty monoesters of erythromycin A

New fatty polyenic (linoleic, linolenic, arachidonic, linoelaidic) mono esters of erythromycin A have been synthesized by using various reagents such as acyl chloride, carboxylic acid anhydride, and mixed carbonic anhydride. These different ways of activating the fatty acid allowed a regioselectivity of esterification at position 2' of the desosamine ring or position 4" of the cladinose ring of erythromycin A. The in vitro antibacterial properties of these new esters against members of the resident flora of the human skin were determined and compared with those of erythromycin A. The number and the stereochemistry of the double bonds seem to play a crucial role in the expression of the in vitro antibacterial activity.     

Total synthesis of microtubule-stabilizing agent (-)-laulimalide.

An enantioselective first total synthesis of laulimalide (1) is described. Laulimalide, a remarkably potent antitumor macrolide, has been isolated from the Indonesian sponge Hyattella sp. and the Okinawan sponge Fasciospongia rimosa. Laulimalide represents a new class of antitumor agents with significant clinical potential. The synthesis is convergent and involved the assembly of C(3)-C(16) segment 4 and C(17)-C(28) segment 5 by Julia olefination. The sensitive C(2)-C(3) cis-olefin functionality was installed by Yamaguchi macrolactonization of a hydroxy alkynic acid followed by hydrogenation of the resulting alkynoic lactone over Lindlar's catalyst. Initial attempts of intramolecular Still's variant of Horner-Emmons olefination between the C(19)-phosphonocetate and C(3)-aldehyde provided a 1:2 mixture of cis- and trans-macrolactones. The trans-isomer was photoisomerized to a mixture of cis- and trans-isomers. The other key steps involved ring-closing olefin metathesis to construct both dihydropyran units, stereoselective anomeric alkylation to functionalize the dihydropyran ring, stereoselective reduction of the resulting alkynyl ketone to set the C(20)-hydroxyl stereochemistry, and a novel Julia olefination protocol for the installation of the C(13)-exo-methylene unit. The sensitive epoxide at C(16)-C(17) was introduced in a highly stereoselective manner by Sharpless epoxidation at the final stage of the synthesis.     

Stereoselective synthesis of the C1-C10 fragment of rhizoxin D

A novel approach towards the synthesis of the C1-C10 fragment of the biologically active antimitotic agent rhizoxin D is described. The synthesis involves a stereoselective Michael addition reaction of lithium diallyl cuprate with an α,β-unsaturated six membered lactone.     

Stereoselective synthesis of the cytotoxic macrolide aspergillide B

A total, stereoselective synthesis of the cytotoxic macrolide aspergillide B has been performed. A cross metathesis and a C-glycosidation via a Mukaiyama-type aldol reaction were key features of the synthesis. The macrocyclic lactone ring was created by means of the Yamaguchi procedure.     

Improved synthesis of epothilone B employing alkylation of an alkyne for assembly of subunits

[formula: see text] A strategy for assembling the two principal modules of epothilone B was developed that merges an allylic bromide with a terminal acetylene to fabricate the C10-C11 bond of the macrocycle. The resulting alkyne was semihydrogenated to give a seco ester previously employed in our total synthesis of epothilone B. This new approach affords a more efficient route to the naturally occurring macrolide and to its 9,10-dehydro analogue.     

Stereoselective synthesis of the C(1)-C(11) fragment of peloruside A

A highly stereoselective synthesis of the C(1)-C(11) fragment 4 of peloruside A has been accomplished via a stereoselective double allylboration and an intramolecular epoxide opening to provide the functionally dense C(3)-C(11) segment 14. A glycolate aldol reaction was then employed to introduce the remaining stereocenters at C(2)-C(3). [reaction: see text]     

Total synthesis of macroviracin D (BA-2836-4)

The first total synthesis of the complex glycolipid macroviracin D (BA-2836-4) (1) is described. This antivirally active metabolite isolated from the mycelium extracts of Streptomyces sp. BA-2836 incorporates a unique 46-membered macrodilactone motif decorated with glycosylated fatty acid appendices. Compound 1 consists of three identical subunits which are closely related to one of the segments found in cycloviracin B(1) (2), another antiviral glycoconjugate previously synthesized in our laboratory. Key steps of the synthesis route to 1 involve the stereoselective, ligand-controlled addition of the functionalized diorganozinc derivative 9 to aldehydes 8 a, b, a series of beta-selective glycosidation reactions using appropriately protected trichloroacetimidate donors, and three esterifications via the Yamaguchi method; one of them is performed intramolecularly to forge the macrocyclic lactone ring of the target in 89 % isolated yield. This total synthesis also firmly establishes the absolute configuration of the subunits of compound 1 as 3R,17S,23R.     

Evaluation of the Efficiency of the Macrolactonization Using MNBA in the Synthesis of Erythromycin A Aglycon

Various intermediates for the synthesis of erythronolide A, an aglycon of erythromycin A, are prepared from the corresponding seco‐acids using 2‐methyl‐6‐nitrobenzoic anhydride (MNBA) in the presence of 4‐(dimethylamino)pyridine (DMAP) with or without triethylamine. The efficiency of the MNBA lactonization is assessed by studying this method and comparing the results with those of the other established macrocyclization protocols. It has been finally concluded that (i) the conformationally appropriate substrate for the monomeric cyclization gave the desired lactone in excellent yield under mild reaction conditions in the presence of MNBA and DMAP, (ii) the highly‐strained substrate for the cyclization also afforded the monomeric lactone in relatively good yield at 100°C in toluene, and (iii) the seco‐acid having stable linear conformation, which preferred dimerizing more than forming the monomeric lactone, provided the corresponding diolide in high yield with the constant ratio of the monomer to dimeric lactone (approximately 1/5). © 2009 The Japan Chemical Journal Forum and Wiley Periodicals, Inc. Chem Rec 9: 305–320; 2009: Published online in Wiley InterScience ( www.interscience.wiley.com ) DOI 10.1002/tcr.200900017     

Modular synthesis of the C9-C27 degradation product of aflastatin A via alkyne-epoxide cross-couplings

A modular approach to the synthesis of complex polyketide natural products is demonstrated for the synthesis of the C9-C27 degradation product from aflastatin A. The product of the cross-coupling of C23-C27 terminal alkyne with C17-C22 epoxide underwent functionalization of the resulting internal alkyne, which was then coupled similarly with C9-C16 epoxide. This synthesis concluded with regio- and stereoselective addition of methyl onto the internal alkyne followed by stereoselective hydroboration-oxidation.     

Total synthesis of (-)-bafilomycin A(1)

A highly stereoselective total synthesis of (-)-bafilomycin A(1), the naturally occurring enantiomer of this potent vacuolar ATPase inhibitor, is described. The synthesis features the highly stereoselective aldol reaction of methyl ketone 8b and aldehyde 60c and a Suzuki cross-coupling reaction of the highly functionalized advanced intermediates 12 and 39. Vinyl iodide 12 was synthesized by a 14-step sequence starting from the readily available beta-alkoxy aldehyde 14, while the vinylboronic acid component 39 was synthesized by a nine-step sequence from beta-hydroxy-alpha-methyl butyrate 44 via a sequence involving the alpha-methoxypropargylation of chiral aldehyde 49 with the alpha-methoxypropargylstannane reagent 54. Syntheses of fragments 12 and 39 also feature diastereoselective double asymmetric crotylboration reactions to set several of the critical stereocenters. The Suzuki cross-coupling of 12 and 39 provided seco ester 40, which following conversion to the seco acid underwent smooth macrolactonization to give 41. The success of the macrocyclization required that C(7)-OH be unprotected. The Mukaiyama aldol reaction between aldehyde 60c and the TMS enol ether generated from 8b provided aldol 65 with high diastereoselectivity. Finally, all silicon protecting groups were removed by treatment of the penultimate intermediate 65 with TAS-F (tris(dimethylamino)sulfonium difluorotrimethylsilicate), thereby completing the total synthesis of (-)-bafilomycin A(1).     

Total syntheses of epothilones B and D

Total syntheses of the microtubule stabilizing antitumor drugs epothilone B and D are described, starting from optically pure (S)-malic acid and methyl (R)-3-hydroxy-2-methylpropionate. The synthesis is highly convergent by coupling the three fragments C1-C6 (fragment D), C7-C10 (fragment C), and C11-C21 (fragment B). Key steps are two stereoselective Wittig type olefinations to generate the 12,13- and 16,17-double bonds, an enantioselective Mukaiyama aldol addition to synthesize fragment D, and a sulfone anion allyl iodide alkylation to connect fragments B and C. Finally fragment D was attached to the B + C fragment via aldol addition.     

Novel approach to the zaragozic acids. Enantioselective total synthesis of 6,7-dideoxysqualestatin H5

The total synthesis of 6,7-dideoxysqualestatin H5 (3) has been completed by a concise approach that features the stereoselective intramolecular vinylogous aldol reaction of the furoic ester 25a to give 30 or its trimethylsilyl ether derivative 34, which possess the requisite absolute stereochemistry at C(3)-C(5) of 3. Compound 34 was reduced to the saturated bislactone 39, and the C(1) side chain subunit 47 was introduced leading to a mixture of the hemiacetals 48 and the corresponding ketone 49. When this mixture was stirred with methanolic acid, transketalization occurred to give a mixture of 50 and the spirocyclic methyl acetals 51a,b. Oxidation of the primary alcohol group in 50 followed by saponification of the two remaining ester groups gave 3. The longest linear sequence in the synthesis commences with commercially available erythronolactone (26) and requires 17 chemical steps with only 10 isolated intermediates.     

Total synthesis of (−)-dictyostatin, a microtubule-stabilising anticancer macrolide of marine sponge origin

An efficient convergent synthesis of the anticancer marine macrolide (−)-dictyostatin is described that proceeds in 4.6% yield over 27 steps. Most of the stereocentres were configured using substrate control, making use of a common building block to install the C12-C14 and C20-C22 stereotriads, with a lactate boron aldol reaction employed to construct a C4-C10 β-ketophosphonate as utilised in the pivotal Still-Gennari HWE coupling step with a fully elaborated C11-C26 aldehyde. Following introduction of the (2Z,4E)-dienoate, a modified Yamaguchi macrolactonisation and deprotection delivered the requisite 22-membered macrocyclic lactone.     

Total synthesis and evaluation of cytostatin, its C10-C11 diastereomers, and additional key analogues: impact on PP2A inhibition

The total synthesis of cytostatin, an antitumor agent belonging to the fostriecin family of natural products, is described in full detail. The convergent approach relied on a key epoxide-opening reaction to join the two stereotriad units and a single-step late-stage stereoselective installation of the sensitive (Z,Z,E)-triene through a beta-chelation-controlled nucleophilic addition. The synthetic route provided rapid access to the C4-C6 stereoisomers of the cytostatin lactone, which were prepared and used to define the C4-C6 relative stereochemistry of the natural product. In addition to the natural product, each of the C10-C11 diastereomers of cytostatin was divergently prepared (11 steps from key convergence step) by this route and used to unequivocally confirm the relative and absolute stereochemistry of cytostatin. Each of the cytostatin diastereomers exhibited a reduced activity toward inhibition of PP2A (>100-fold), demonstrating the importance of the presence and stereochemistry of the C10-methyl and C11-hydroxy groups for potent PP2A inhibition. Extensions of the studies provided dephosphocytostatin, sulfocytostatin (a key analogue related to the natural product sultriecin), 11-deshydroxycytostatin, and an analogue lacking the entire C12-C18 (Z,Z,E)-triene segment, which were used to define the magnitude of the C9-phosphate (>4000-fold), C11-alcohol (250-fold), and triene (220-fold) contribution to PP2A inhibition. A model of cytostatin bound to the active site of PP2A is presented, compared to that of fostriecin, which is also presented in detail for the first time, and used to provide insights into the role of the key substituents. Notably, the alpha,beta unsaturated lactone of cytostatin, like that of fostriecin, is projected to serve as a key electrophile, providing a covalent adduct with Cys269 unique to PP2A, contributing to its potency (> or =200-fold for fostriecin) and accounting for its selectivity.     

Studies on a urea-directed Stork-Crabtree hydrogenation. Synthesis of the C1-C9 subunit of (+)-zincophorin

A detailed account on the stereoselective synthesis of the C1-C9 subunit of (+)-zincophorin is described here. This approach features the first application of a stereoselective inverse electron demand hetero-[4 + 2] cycloaddition of chiral allenamides in natural product synthesis. The C1-C9 subunit matches Cossy's intermediate, thereby constituting a formal total synthesis. In addition, details of an unusual urea-directed Stork-Crabtree hydrogenation observed during these efforts are also disclosed here.