Total synthesis of thapsigargin, a potent SERCA pump inhibitor

The enantioselective total synthesis of thapsigargin, a potent, selective inhibitor of the Ca2+ pump SERCA, is described. Starting from ketoalcohol 8, key steps involve regioselective introduction of the internal olefin at C4-C5, judicious protecting group choice to allow chelation-controlled reduction at C3, and chemoselective introduction of the angelate ester function at C3-O. A selective esterification approach completes the total synthesis in a total of 42 steps and 0.61% overall yield (88.6% average yield per step). [reaction: see text].     

Total synthesis of gymnocin-A

A highly convergent total synthesis of gymnocin-A, a cyctotoxic polyether marine natural product, has been achieved. The synthesis features Suzuki-Miyaura coupling of the ABCD and FGHIJKLMN rings, stereoselective introduction of the C17 hydroxyl group, ring-closure of the F ring, and a late-stage incorporation of a 2-methyl-2-butenal side chain.     

Total synthesis of (+)-brefeldin A

(+)-Brefeldin A was synthesized through an efficient route, which features (1) construction of the five-membered ring from a Crimmins aldol via tandem Li-I exchange and carbanion-mediated cyclization with concurrent removal of the chiral auxiliary, (2) introduction of the lower side chain (C10 to C16) via a Rh-catalyzed Michael addition of a vinyl boronic acid, (3) stereoselective reduction of the C7 ketone with SmI2, and (4) a 2-methyl-6-nitrobenzoic anhydride-mediated (Shiina) lactonization.     

Total synthesis of rutamycin B and oligomycin C

The asymmetric synthesis of the macrolide antibiotics (+)-rutamycin B (1) and (+)-oligomycin C (2) is described. The approach relied on the synthesis and coupling of the individual spiroketal fragments 3a and 3b with the C1-C17 polyproprionate fragment 4. The preparation of the spiroketal fragments was achieved using chiral (E)-crotylsilane bond construction methodology, which allowed the introduction of the stereogenic centers prior to spiroketalization. The present work details the synthesis of the C19-C28 and C29-C34 subunits as well as their convergent assembly through an alkylation reaction of the lithiated N,N-dimethylhydrazones 6 and 8 to afford the individual linear spiroketal intermediates 5a and 5b, respectively. After functional group adjustment, these advanced intermediates were cyclized to their respective spiroketal-coupling partners 40 and 41. The requisite polypropionate fragment was assembled in a convergent manner using asymmetric crotylation methodology for the introduction of six of the nine-stereogenic centers. The use of three consecutive crotylation reactions was used for the construction of the C3-C12 subunit 32. A Mukaiyama-type aldol reaction of 35 with the chiral alpha-methyl aldehyde 39 was used for the introduction of the C12-C13 stereocenters. This anti aldol finished the construction of the C3-C17 advanced intermediate 36. A two-carbon homologation completed the construction of the polypropionate fragment 38. The completion of the synthesis of the two macrolide antibiotics was accomplished by the union of two principal fragments that was achieved with an intermolecular palladium-(0) catalyzed cross-coupling reaction between the terminal vinylstannanes of the individual spiroketals 3a and 3b and the polypropionate fragment 4. The individual carboxylic acids 46 and 47 were cyclized to their respective macrocyclic lactones 48 and 49 under Yamaguchi reaction conditions. Deprotection of these macrolides completed the synthesis of the rutamycin B and oligomycin C.     

Function-oriented synthesis: studies aimed at the synthesis and mode of action of 1alpha-alkyldaphnane analogues

[reaction: see text] An efficient synthetic route to the ABC tricyclic core of 1alpha-alkyldaphnanes has been developed. The conformational bias imparted by the C6-C9 oxo-bridge of BC-ring system 12 was used to elaborate the ABC-ring system precursor including the introduction of the beta-C5 hydroxyl group. A completely diastereoselective palladium-catalyzed enyne cyclization was then employed to establish the A-ring with a C1 appendage.     

Total synthesis of (+)-zaragozic acid C

A total synthesis of (+)-zaragozic acid C is described. Key features of the synthesis are the use of a double Sharpless asymmetric dihydroxylation reaction of diene 6 to control stereochemistry at four contiguous stereocenters from C3 to C6; the introduction of the C1-side chain by reaction between the anion derived from the dithiane monosulfoxide 27 and the core aldehyde 12; a high yielding, acid-mediated simultaneous acetonide deprotection-dithiane removal-ketalization procedure leading exclusively to the 2, 8-dioxabicyclo[3.2.1]octane core 34; and a novel triple oxidation procedure allowing installation of the tricarboxylic acid.     

Total synthesis of (+)-Brefeldin A

The total synthesis of (+)-brefeldin A has been accomplished via 15 linear steps in a 7.9% overall yield from the known Weinreb amide 6. The key parts of this approach include the stereoselective construction of the cis-disubstituted hydroxycyclopentane skeleton and the direct introduction of the C1-C3 acrylate moiety using a new variant of a trans-vinylogous acyl anion equivalent.     

A linchpin carbacyclization approach for the synthesis of carbanucleosides

A convenient synthesis of carbanucleosides, with both enantiomers equally accessible, is reported. The key step is a tandem linchpin cyclization process to give access to substituted carbafuranose derivatives having the correct relative stereochemistry for subsequent nucleobase introduction with inversion of configuration at C1. This was illustrated by the synthesis of 2',3'-dideoxycarbathymidine via a convergent nucleobase introduction and of 2',3'-dideoxy-6'-hydroxycarbauridine via a linear nucleobase introduction. Both methods relied on Mitsunobu chemistry, and the first example of the Mukaiyama modification of the Mitsunobu reaction involving nucleobases as nucleophiles is reported.     

Enantioselective total synthesis of (+)-leucascandrolide A macrolactone

[reaction: see text] The enantioselective synthesis of the (+)-leucascandrolide A macrolactone has been achieved in 20 linear steps from 1,3-propanediol. The key steps in the synthesis are a reductive cleavage of bicyclic ketal 5 to establish the C15 stereogenic center and a diastereoselective aldol of the boron enolate of methyl ketone 3 to aldehyde 4 in preparation for a heteroconjugate addition for the introduction of the C3 stereocenter.     

Total synthesis of (-)-reveromycin a

The asymmetric total synthesis of (-)-reveromycin A is described. The key steps involved a Lewis acid catalyzed inverse electron demand hetero-Diels-Alder reaction followed by hydroboration/oxidation to afford the spiroketal core 4 in a highly stereoselective manner and introduction of the C18 hemisuccinate by high-pressure acylation.     

Tether-directed selective synthesis of fulleropyrrolidine bisadducts

Selective synthesis of C60 bisadducts has been achieved by using the Prato 1,3-dipolar cycloaddition of tethered bis-azomethine ylides. New bis(benzaldehydes) 1-4 tethered by a rigid linker were prepared and used to direct the second cycloaddition of azomethine ylide to C60. Equatorial, trans-4, trans-3, trans-2, and trans-1 bisadducts have been selectively prepared with this approach. However, the introduction of chiral centers in the pyrrolidine rings in the course of the reaction complicated the chemistry, as a number of stereoisomers theoretically could be formed. The structure determination of the isomeric bisadducts was made based on spectroscopic data and theoretical calculations. To our best knowledge, this represents the first example of a systematic study on tether-directed selective synthesis of C60 fulleropyrrolidine bisadducts.     

Effect of synthesis conditions of polyallylamine-beads-glucose (PAA-Glu) on boron adsorption

In this study, we assessed the boron adsorption characteristic of our synthesized adsorbent: polyallylamine-beads-glucose (PAA-Glu) by using the adsorption amount and adsorption site availability (ASA), and determined the optimum conditions for PAA-Glu synthesis. ASA is our proposed indicator and it expresses the percentage of the experimental equilibrium adsorption in relation to the theoretical equilibrium adsorption and indicates the availability of adsorption sites (hydroxyl groups) on adsorbents. We investigated the effects of the degree of cross-linking (20, 40 and 60 %), the introduction amount of α-D-glucose (40, 60 and 80 %), and the introduction temperature (30–150 °C) as regards PAA-Glu on the boron adsorption amount. The boron adsorption amount for PAA-Glu (20 % cross-linked, 30 °C) was superior to that for another degree of cross-linked PAA-Glu. However, the ASA for PAA-Glu (60 % cross-linked, 30 °C) exhibited the best value in this synthesized PAA-Glu. Furthermore, we examined the introduction temperature at which glucose was introduced to PAA-Glu (20 % cross-linked) in the 30–150 °C range, and we confirmed that the optimum temperature range for the synthesis of PAA-Glu was 60–80 °C. We found that controlling the space between the main chains of the polymer by using a spacer such as a cross-linker allowed boron molecules to come and go and led to an improvement in the boron adsorption amount and ASA.     

Apoptolidinone A: synthesis of the apoptolidin A aglycone

An efficient stereocontrolled synthesis of apoptolidinone A, the aglycone of apoptolidin A is described. The synthetic strategy relies on a cross coupling between C11/C12 of a northern half (C1-C11) and a southern part (C12-C28) followed by a ring-size selective macrolactonization. Key steps for the introduction of the southern half stereocenters are a stereoselective aldol reaction, a substrate controlled dihydroxylation and a chelation-controlled Grignard/aldehyde addition. The conjugated triene of the northern half was built up successively by E-selective Wittig reactions. L-Malic acid was chosen as the chiral pool source for the C8/C9 stereocenters. The final cleavage of the silyl ethers and the conversion of the C21 methyl ketal into the hemiketal was achieved by HF.pyridine.     

Enantioselective synthesis of antiinfluenza compound A-315675

Drug discovery efforts at Abbott Laboratories have led to the identification of influenza neuraminidase inhibitor A-315675 (1) as a candidate for development as an antiinfluenza drug. A convergent, stereoselective synthesis of this highly functionalized pyrrolidine is reported that utilizes pyrrolinone 2 as the key intermediate. The C5, C6 stereochemistry was established through a diastereoselective condensation of chiral imine compound 3 with silyloxypyrrole 4 to give pyrrolinone 2. The stereochemical outcome of this reaction depended critically on the choice of the imine functional group (FG), with tritylsulfenyl and (R)-toluenesulfinyl providing the desired products in good yields as crystalline intermediates. Conversion of pyrrolinone 2 into 1 was accomplished in seven subsequent steps, including Michael addition of cis-1-propenylcuprate at C4 and introduction of a cyano group as a carboxylic acid equivalent at C2.     

Total synthesis of zaragozic acid C by an aldol-based strategy

A total synthesis of zaragozic acid C by a convergent strategy is described in which the key features include (1) the simultaneous creation of the C4 and C5 quaternary stereogenic centers by a Sn(OTf)₂-promoted aldol coupling reaction between an α-keto ester and a silyl ketene thioacetal derived from l- and d-tartaric acids, respectively, (2) the direct introduction of lithium acetylide as the C1 side chain equivalent onto the fully functionalized aldehyde, and (3) construction of the bicyclic core structure by acid-catalyzed internal ketalization under kinetically controlled conditions.     

Stereocontrolled synthesis of (-)-macrolactin A

The total synthesis of (-)-macrolactin A, a 24-membered macrolide, has been achieved using a newly developed 1,3-diol synthon for the introduction of two key stereogenic centers. The synthon was derived from sequential use of the Noyori asymmetric reduction followed by chiral sulfoxide methodology. Tellurium-derived cuprate organometallics offered an efficient and highly stereoselective means for installation of the C8 Z/E-diene, while the C15 E/E-segment was derived from a Julia-Lythgoe olefination. Yamaguchi lactonization was used to secure the macrocycle in a convergent approach with the longest linear sequence of 19 steps from Noyori alcohol 6.     

Total synthesis of the Amaryllidaceae alkaloid clivonine

Two syntheses of the Amaryllidaceae alkaloid clivonine (1) are described. Both employ previously reported 7-arylhydrindane 6 as an intermediate but differ in the method employed for subsequent introduction of what becomes the ring-B lactone carbonyl carbon (C7). The synthesis featuring a Bischler-Napieralski reaction for this transformation constitutes the first asymmetric synthesis of natural (+)-clivonine. Crystal structures for compounds (±)-13, (±)-16, (-)-20 and (±)-28 are also reported.     

Total synthesis of ningalin D

A concise (nine-step) and effective (19% overall yield) total synthesis of ningalin D (1a) is disclosed and is based on a key 1,2,4,5-tetrazine --> 1,2-diazine --> pyrrole Diels-Alder strategy to assemble a fully substituted pyrrole core central to its structure. Additional highlights of the synthesis include a double Dieckmann condensation to introduce the C and D aryl rings enlisting substituents judiciously placed on the dienophile and intrinsic to the widely used tetrazine 2, a highly effective Suzuki coupling of the resulting C and D phenol triflates for introduction of the sterically demanding F and G aryl rings, and an unusually effective formal oxidative decarboxylation reaction cascade initiated by a Curtius rearrangement to directly provide the biphenylene quinone methide found imbedded in the structure of ningalin D. The cytotoxic and multidrug resistance (MDR) reversal activity of ningalin D, its derivatives, and the key synthetic intermediates are detailed.     

Stereoselective synthesis of the C94-C104 fragment of symbiodinolide

Stereoselective synthesis of the C94-C104 fragment of symbiodinolide which is a polyol marine natural product with a molecular weight of 2860 has been accomplished. The synthetic route features Achmatowicz rearrangement and RuO₄-catalyzed dihydroxylation for the construction of the tetrahydropyran moiety and the dithiane addition to the aldehyde for the introduction of the side chain.     

Towards the synthesis of massadine: a unified strategy for the stereoselective synthesis of the carbocyclic C,D-ring subunit

Massadine is a hexacyclic marine natural product, which belongs to the family of pyrrole-imidazole alkaloids. Herein, we describe a unified approach to the C,D-ring subunit of this sponge metabolite based on the exploitation of a norbornene scaffold for the stereocontrolled construction of massadine's carbon skeleton. Highlights of the sequence presented include the application of a stereospecific norbornyl rearrangement for facile introduction of an oxygen at the C7-position within the norbornene nucleus, a highly regioselective and end group differentiating ozonolytic scission of a C-C double bond, and an oxidative decarboxylation reaction for the installation of the hindered secondary C2-alcohol function. Furthermore, the iterative assembly of the two guanidine entities as well as the implementation of the spirocyclic junction between the C- and the D-rings are described. Collectively, these key transformations permit an entry to an appropriately functionalized carbon framework, which will serve as a starting point for our efforts toward the completion of the synthesis of massadine.