Enantioselective Preparation of the C1−C11 Fragment of Apoptolidin

Bromine-Promoted One-Pot Furo[b]annulation and α-C(sp2)-Thiomethylation Cascade of (E)-3-Styrylquinoxalin-2(1H)-ones with Dimethyl Sulfoxide

A new process has been developed for the bromine-promoted sequential (sp²)C = (sp²)C bond functionalization of (E)-3-styrylquinoxalin-2(1H)-ones and furo[b]annulation via the 5-exo-cyclization in dimethyl sulfoxide (DMSO). The reaction represents a novel strategy for the synthesis of 2-aryl-3-(methylthio)furo[2,3-b]quinoxalines and involves 3-(1,2-dibromo-2-arylethyl)quinoxalin-2(1H)-ones and 2-arylfuro[2,3-b]quinoxalines as key intermediates. Furthermore, DMSO was converted to dimethyl sulfide in situ, which served as the methylthiolation reagent in the reaction. This protocol constitutes an efficient and convenient method for the annulation and methylthiolation of (E)-3-styrylquinoxalin-2(1H)-ones bearing a wide range of functional groups in high yields at room temperature.

The Hydrazine-O2 Redox Couple as a Platform for Organocatalytic Oxidation: Benzo[c]cinnoline-Catalyzed Oxidation of Alkyl Halides to Aldehydes

An organocatalytic oxidation platform that capitalizes on the capacity of hydrazines to undergo rapid autoxidation to diazenes is described. Commercially available benzo[c]cinnoline is shown to catalyze the oxidation of alkyl halides to aldehydes in a novel mechanistic paradigm involving nucleophilic attack, prototropic shift, and hydrolysis. The hydrolysis and reoxidation events occur readily with only adventitious oxygen and water. A survey of the scope of viable substrates is shown along with mechanistic and computational studies that give insight into this mode of catalysis.

Study of the total synthesis of (-)-exiguolide

In this article, we disclose the various routes and strategies we had to explore before finally achieving the total synthesis of (-)-exiguolide ((-)-1). Two first types of approaches were set, both relying on the Trost's domino ene-yne coupling/oxa-Michael reaction that we choose for its ability to control the geometry of the methylacrylate-bearing tetrahydropyrane ring B. In our first approach, we expected to assemble the two main fragments (C14-C21 and C1-C13) by creating the C13-C14 bond through a palladium(0)-catalyzed cross-coupling, but this step failed, unfortunately. In the second approach, which was more linear, we created the C16-C17 bond through condensation of a lithium acetylide on a Weinreb amide, and we assembled the C1-C5 and C6-C21 subunits through Trost's domino ene-yne coupling/oxa-Michael reaction. These two approaches served us to design an ameliorated third strategy, which finally led to the total synthesis of (-)-exiguolide.

A stereoselective synthesis of (+)-herboxidiene/GEX1A

A stereoselective synthesis of (+)-herboxidiene is described. The convergent synthesis utilized a Suzuki cross-coupling reaction to assemble the key segments. The synthesis of the functionalized tetrahydropyran ring utilized an Achmatowicz reaction as the key step. The synthesis of the C10-C19 segment was accomplished using Brown's crotylboration, asymmetric alkylation, and a stereoselective allylic chlorination reactions.

Total synthesis of 7,11-cyclobotryococca-5,12,26-triene using an oxidative radical cyclization as a key step

An efficient total synthesis of the novel botryococcene-related hydrocarbon 7,11-cyclobotryococca-5,12,26-triene is reported that uses, as a key step, an oxidative radical cyclization of a 4-pentenyl malonate for the synthesis of a [3.3.0]-bicyclic gamma-lactone.

Total synthesis of apoptolidin A

A highly convergent, enantioselective total synthesis of the potent antitumor agent apoptolidin A has been completed. The key transformations include highly selective glycosylations to attach the C27 disaccharide and the C9 6'-deoxy-l-glucose, a cross-metathesis to incorporate the C1-C10 trienoate unit, and a Yamaguchi macrolactonization to complete the macrocycle. Twelve stereocenters in the polypropionate segments and sugar units were established through diastereoselective chlorotitanium enolate aldol reactions.

Evolution of the total synthesis of (-)-okilactomycin exploiting a tandem oxy-cope rearrangement/oxidation, a Petasis-Ferrier union/rearrangement, and ring-closing metathesis

An effective, asymmetric total synthesis of the antitumor antibiotic (-)-okilactomycin (1), as well as assignment of the absolute configuration, has been achieved exploiting a convergent strategy. Highlights of the synthesis include a diastereoselective oxy-Cope rearrangement/oxidation sequence to install the C(1) and C(13) stereogenic centers, a Petasis-Ferrier union/rearrangement to construct the highly functionalized tetrahydropyranone inscribed within the 13-membered macrocycle ring, employing for the first time a sterically demanding acetal, an intramolecular chemoselective acylation to access an embedded bicyclic lactone, and an efficient ring-closing metathesis (RCM) reaction to generate the macrocyclic ring.

Synthesis and evaluation of the cytotoxicity of apoptolidinones A and D

Apoptolidins A-D are microbial secondary metabolites shown to be selectively cytotoxic against several cancer cell lines and noncytotoxic against normal cells. Total syntheses of apoptolidinones A and D are reported. The efficient synthetic strategy leading to the apoptolidinones features construction of the common 20-membered macrolactone by an intramolecular Suzuki reaction and stereocontrolled aldol reactions establishing the C19/C20 and C22/C23 stereocenters. In contrast to apoptolidin A, the aglycones apoptolidinone A and D were shown to be noncytotoxic when evaluated against human lung cancer cells (H292).

Short synthesis of the C16-C28 polyketide fragment of apoptolidin A aglycone

Starting from (E,E)-1-[(1R)-(phenylethyl)oxy]-2-methylpenta-1,3-diene and triethylsilyl enol ether of butanone rapid access to Koert's advanced C10-C28 polyketide fragment of apoptolidin A is now possible.

Apoptolidin A: total synthesis and partially glycosylated analogues

The total synthesis of apoptolidin A is described employing an early glycosylation strategy. Strategic disconnections were chosen between C11-C12 (cross-coupling) and C19O-C1 (macrocyclization). The cis-selective glycosylation at C9-OH was achieved with the new SIBA protective group at O2/O3 of the L-glucose residue. Auxiliary substitutents at the 2-position of the 2-deoxy sugars were applied to form selectively the glycosidic linkages of the C27 disaccharide. The cross-coupling of the glycosylated northern half with the glycosylated southern half was achieved with CuI-thiophene carboxylate. The macrocyclization of a trihydroxy carboxylic acid produced the 20-membered macrolide selectively. H2SiF6 was suitable for the final deprotection of the silyl ethers and the conversion of the C21 methylketal into the hemiketal. The synthetic flexibility of the approach was proven by the synthesis of some glycovariants.

Lactol-directed osmylation. Stereodivergent synthesis of four C-19,20 apoptolidin diols from a single allylic hemiacetal

A synthetic approach to prepare four Apoptolidin C-19,20 diastereomeric diol derivatives was developed. Two diastereomers were obtained from the (Z)-form, which is converted to the (E)-form, followed by dihydroxylation to deliver two more diastereomers. The (E)-allylic hemiacetal and methoxyacetal showed opposite diastereoselectivity.

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.

Apoptolidins B and C: isolation, structure determination, and biological activity

[reaction: see text] Apoptolidin (1) is a promising new therapeutic lead that exhibits remarkable selectivity against cancer cells relative to normal cells. We report the isolation, characterization, solution structure, stability, and biological activity of two new members of this family: apoptolidins B (2) and C (3). These new agents are found to have antiproliferative activity on par with or better than apoptolidin itself in an assay with H292 lung cancer cells.

Correlation of F0F1-ATPase inhibition and antiproliferative activity of apoptolidin analogues

[structure: see text] Apoptolidin (1) exhibits potent and highly selective apoptosis inducing activity against sensitive cancer cell lines and is hypothesized to act by inhibition of mitochondrial F(0)F(1)-ATP synthase. A series of apoptolidin derivatives, including a new intermolecular Diels-Alder adduct, were analyzed for antiproliferative activity in E1A-transformed rat fibroblasts. Potent F(0)F(1)-ATPase inhibition was not a sufficient determinant of antiproliferative activity for several analogues, suggesting the existence of a secondary biological target or more complex mode of action for apoptolidin.

Apoptolidin: Induction of Apoptosis by a Natural Product

Apoptolidin is a natural product that selectively induces apoptosis in several cancer cell lines. Apoptosis, programmed cell death, is a biological key pathway for regulating homeostasis and morphogenesis. Apoptotic misregulations are connected with several diseases, in particular cancer. The extrinsic way to apoptosis leads through death ligands and death receptors to the activiation of the caspase cascade, which results in proteolytic degradation of the cell architecture. The intrinsic pathway transmits signals of internal cellular damage to the mitochondrion, which loses its structural integrity, and forms an apoptosome that initiates the caspase cascade. Compounds which regulate apoptosis are of high medical significance. Many natural products regulate apoptotic pathways, and apoptolidin is one of them. The known synthetic routes to apoptolidin are described and compared in this Review. Selected further natural products which regulate apoptosis are introduced briefly.

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.

Enantioselective Synthesis of Apoptolidin Sugars

[structure: see text] The de novo synthesis of the C9 and C27 sugar subunits (2) and (3), respectively, of the potent antitumor agent, apoptolidin, has been accomplished. A titanium tetrachloride-mediated asymmetric anti glycolate aldol addition was utilized to establish the 4' and 5' stereogenic centers of each of the three monosaccharides. Elaboration of the aldol adducts efficiently provided the three sugar units. A beta-selective glycosidation completed the construction of the C27 disaccharide.

Synthesis of the C(1)-C(11) Polyene Fragment of Apoptolidin with a New Sulfur Dioxide-Based Organic Chemistry

A new sulfur dioxide-based organic chemistry has been developed as a novel approach for the stereoselective synthesis of polyene fragments based on our one-pot, four-component synthesis of polyfunctional epsilon-alkanesulfonyl-gamma,delta-unsaturated ketones. The flexibility and efficiency of this methodology are illustrated by the preparation of (+)-methyl (2E,4E,6E,8R,9S)-9-{[(tert-butyl)dimethylsilyl]oxy}-2,4,6,8-tetramethyl-11-(triethylsilyl)undeca-2,4,6-trien-10-ynoate, a synthetic intermediate of Nicolaou and co-workers, that corresponds to the C(1)-C(11) fragment of apoptolidin, which was used by the authors in their total synthesis of this promising anticancer agent.