Total Synthesis of 19-Nordigitoxigenin, An Antiaroside Y Aglycon.

The first total synthesis of 19-nordigitoxigenin, an aglycon of antiroside Y, has been achieved. The key steps of our synthesis are (i) construction of the 19-norsteroid ring system via a Mizoroki-Heck reaction between a bromoanisole corresponding to the A-ring and cyclic alkene incorporating the CD-rings, followed by a Friedel-Crafts-type cyclodehydration, and (ii) incorporation of the butenolide moiety at C17 via a silyl-tethered radical cyclization and subsequent ozone oxidation.

Structural verification of petromyzestrosterol by total syntheses of both C14-epimers of its 3-O-methyl derivative.

The structure of petromyzestrosterol, a pheromonal steroid of the sea lamprey, was verified by total syntheses of both C14-epimers of its 3-O-methyl derivative. The key features of our synthesis involve (1) a highly stereoselective Mizoroki-Heck reaction to unite the A- and CD-ring segments and (2) Friedel-Crafts-type cyclodehydration to construct the B-ring. Petromyzestrosterol is concluded to bear an α-configured C14 hydroxy group based on a comparison of NMR data of both the synthesized C14-epimers of the 3-O-methyl derivative with those of the natural petromyzestrosterol. The downfield shifts of C9 and C12 via the γ-gauche effect in the 14ß-isomer would enable the structural elucidation of C14 in the 14-hydroxy estrogenic steroids.

Total Synthesis of the Cardiotonic Steroid (+)-Cannogenol.

The total synthesis of (+)-cannogenol, an aglycon common to various biologically important cardiotonic glycosides, has been achieved. Synthesis of the versatile intermediate involves Mizoroki-Heck and intramolecular Diels-Alder reactions from the enantiomerically pure CD-ring segment, newly prepared in a multidecagram scale this time. Total synthesis by the site-selective transformations of the versatile intermediate demonstrated the applicability of our synthetic approach.

Total synthesis and biological evaluation of oscillatoxins D, E, and F.

Oscillatoxins (OTXs) and aplysiatoxins are biosynthetically related polyketides produced by marine cyanobacteria. We previously developed a synthetic route to phenolic O-methyl analogs of OTX-D and 30-methyl-OTX-D during collective synthesis of these natural products. According to our synthetic strategy, we achieved total synthesis of OTX-D, 30-methyl-OTX-D, OTX-E, and OTX-F by deprotecting the O-methyl group in an earlier intermediate, and determined their biological activities. Although OTX-D and 30-methyl-OTX-D have been reported to show antileukemic activity against L1210 cell line, we found that their cytotoxicity in vitro against this cell line is relatively weak (IC50: 29-52 µm). In contrast, OTX-F demonstrated cell line-selective antiproliferative activity against DMS-114 lung cancer cells, which implies that OTXs target as yet unknown target molecules as part of this unique activity.

Studies toward the Synthesis of Chartelline C.

Chartelline C, a marine alkaloid, possesses a unique core scaffold including indolenine ß-lactam and imidazole moieties linked by an unsaturated 10-membered ring. A new synthetic approach for the construction of the indolenine ß-lactam was planned, based on the inherent reactivity of chartelline A with NaOMe, triggered by bromination of bromoenamide, and proceeding through an N-acyl imine. However, the N-acyl imine intermediate was not observed. Instead, the corresponding bromoindolenine was obtained, which led to the desired indolenine ß-lactam in 92% yield.

Biomimetic Synthesis of Chaxine and its Related Compounds.

The highly oxidized natural products chaxine B and BB have been synthesized from ergosterol in eight steps according to a route inspired by their proposed biosynthesis; key steps were an oxidative cascade from a furan intermediate to an enol ester using m-chloroperbenzoic acids (MCPBA), followed by diastereoselective epoxidation and acyloxy migration. This concise synthesis resulted in the revision of the structures of chaxine B and its naturally occurring analogs and syntheses of the unnatural analogues of these natural products for biological investigations.

Biosynthesis of Indole Diterpene Lolitrems: Radical-Induced Cyclization of an Epoxyalcohol Affording a Characteristic Lolitremane Skeleton.

Lolitrems are tremorgenic indole diterpenes that exhibit a unique 5/6 bicyclic system of the indole moiety. Although genetic analysis has indicated that the prenyltransferase LtmE and the cytochrome P450 LtmJ are involved in the construction of this unique structure, the detailed mechanism remains to be elucidated. Herein, we report the reconstitution of the biosynthetic pathway for lolitrems employing a recently established genome-editing technique for the expression host Aspergillus oryzae. Heterologous expression and bioconversion of the various intermediates revealed that LtmJ catalyzes multistep oxidation to furnish the lolitrem core. We also isolated the key reaction intermediate with an epoxyalcohol moiety. This observation allowed us to establish the mechanism of radical-induced cyclization, which was firmly supported by density functional theory calculations and a model experiment with a synthetic analogue.

Asymmetric Synthesis of the Aromatic Fragment of Sespendole.

Sespendole is an indole sesquiterpene alkaloid bearing two isoprenyl groups, one of which is highly oxidized. Herein, we disclose an eight-step synthesis of the aromatic fragment of sespendole in an optically pure form, starting from 4-bromo-2-fluoronitrobenzene. The key steps were a Claisen rearrangement at room temperature for introduction of the prenyl group and a coupling between the dianion generated from prenylated bromo-N-tosylanilide and a chiral epoxy aldehyde.

Toward a Synthesis of Fawcettimine-Type Lycopodium Alkaloids: Stereocontrolled Synthesis of a Functionalized Azaspirocycle Precursor.

The stereocontrolled synthesis of a new azaspirocycle precursor of the fawcettimine-type Lycopodium alkaloids is described. Our approach provides an efficient entry to the azaspirocycle via a cascade Wacker-allylation sequence followed by a highly stereoselective Claisen rearrangement. This azaspirocycle, bearing all of the requisite functionality with pivotal stereogenic centers, is considered to be a versatile precursor useful for the fawcettimine-type Lycopodium alkaloids.

De Novo Synthesis of Possible Candidates for the Inagami-Tamura Endogenous Digitalis-like Factor.

De novo synthesis of possible candidates for the Inagami-Tamura endogenous digitalis-like factor (EDLF) was achieved to validate a previously proposed structure. Our synthetic approach involves a highly regio- and diastereoselective Mizoroki-Heck reaction and a Friedel-Crafts-type cyclodehydration to construct steroidal tetracycle 14 as a versatile common intermediate leading to seven 2,14ß-dihydroxyestradiol analogues 1a-c, 2a-c, and 3 as possible candidates. By comparing the potency of inhibitory activity against Na+/K+-ATPase between the synthesized candidates and the EDLF, it was found that the proposed structure is not likely to be a true structure of the Inagami-Tamura EDLF.

Inhibition of veratridine-induced delayed inactivation of the voltage-sensitive sodium channel by synthetic analogs of crambescin B.

Crambescin B carboxylic acid, a synthetic analog of crambescin B, was recently found to inhibit the voltage-sensitive sodium channels (VSSC) in a cell-based assay using neuroblastoma Neuro 2A cells. In the present study, whole-cell patch-clamp recordings were conducted with three heterologously expressed VSSC subtypes, Nav1.2, Nav1.6 and Nav1.7, in a human embryonic kidney cell line HEK293T to further characterize the inhibition of VSSC by crambescin B carboxylic acid. Contrary to the previous observation, crambescin B carboxylic acid did not inhibit peak current evoked by depolarization from the holding potential of -100mV to the test potential of -10mV in the absence or presence of veratridine (VTD). In the presence of VTD, however, crambescin B carboxylic acid diminished VTD-induced sustained and tail currents through the three VSSC subtypes in a dose-dependent manner, whereas TTX inhibited both the peak current and the VTD-induced sustained and tail currents through all subtypes of VSSC tested. We thus concluded that crambescin B carboxylic acid does not block VSSC in a similar manner to TTX but modulate the action of VTD, thereby causing an apparent block of VSSC in the cell-based assay.

Asymmetric synthesis of crambescin A-C carboxylic acids and their inhibitory activity on voltage-gated sodium channels.

Synthesis of both enantiomers of crambescin B carboxylic acid is described. A cis-enyne starting material was epoxidized under the conditions of Katsuki asymmetric epoxidation to give 95% ee of the epoxide, which was transformed to crambescin B carboxylic acid via bromocation-triggered cascade cyclization as the key step. Enantiomerically pure crambescin A and C carboxylic acids were also synthesized from the product of the cascade reaction. Structure-activity relationship (SAR) studies against voltage-gated sodium channel (VGSC) inhibition using those synthetic compounds revealed that the natural enantiomer of crambescin B carboxylic acid was most active and comparable to tetrodotoxin, and the unalkylated cyclic guanidinium structure is indispensible, while the carboxylate moiety is not important. The absolute stereochemistry of crambescin A was determined by a comparison of the methyl ester derived from natural crambescin A with that derived from the stereochemically defined crambescin A carboxylic acid synthesized in this study.

Synthesis of crambescin B carboxylic acid, a potent inhibitor of voltage-gated sodium channels.

The stereocontrolled synthesis of a racemic carboxylic acid of crambescin B, a marine alkaloid, is described. The synthesis features two highly stereoselective reactions: (I) palladium-catalyzed hydroxymethylation of an alkynyl aziridine having an N-guanidino group and (II) cascade bromocyclization providing a spiro-hemiaminal structure. The cell-based colorimetric assay showed that the synthesized carboxylic acid exhibited a potent inhibitory activity on voltage-gated sodium channels.

Structural study on a naturally occurring terphenyl quinone.

Two terphenyl quinones were synthesized for a structural study on a naturally occurring biologically active terphenyl quinone. 3-Methoxy-5,6-diphenylcyclohexa-3,5-dien-1,2-dione, a possible structure proposed by our analysis of the NMR spectra, was synthesized by Suzuki-Miyaura coupling and subsequent oxidation of the resulting substituted phenol, although not being identical to the natural product. Recently isolated 3-methoxy-2,5-diphenylcyclohexa-2,5-dien-1,4-dione was synthesized from a commercially available 2,5-diphenyl-1,4-benzoquinone in three steps in a good overall yield, and its NMR spectra were identical to those of the natural product.

Deprotection of the methoxymethyl group on 3-spiro-2-oxindole under basic conditions.

Deprotection of a methoxymethyl (MOM) group on an oxindole nitrogen under basic conditions is demonstrated. The mechanisms of both the deprotection and the formation of N-methyloxindole were revealed by using deuterated NaOMe-MeOH in mechanistic studies.

Pharmacological evaluation of a novel cyclic phosphatidic acid derivative 3-S-cyclic phosphatidic acid (3-S-cPA).

Cyclic phosphatidic acid (cPA) is a naturally occurring phospholipid mediator possessing cyclic phosphate ring, which is necessary for its specific biological activities. To stabilize cyclic phosphate ring of cPA, we synthesized a series of cPA derivatives. We have shown that racemic 3-S-cPA, with a phosphate oxygen atom replaced with a sulfur atom at the sn-3, was a more effective autotaxin (ATX) inhibitor than cPA. In this study, we showed that racemic 3-S-cPA also had potent biological activities such as inhibition of cancer cell migration, suppression of the nociceptive reflex, and attenuation of ischemia-induced delayed neuronal cell death in the hippocampal CA1. Moreover, we synthesized both enantiomers of palmitoleoyl derivative of 3-S-cPA, and found that the chirality of 3-S-cPA is not involved in ATX inhibition. Based on these findings, racemic 3-S-cPA is suggested as an effective therapeutic compound like cPA.

The antitumor agent doxorubicin binds to Fanconi anemia group F protein.

Doxorubicin, a commonly used cancer chemotherapy agent, elicits several potent biological effects, including synergistic-antitumor activity in combination with cisplatin. However, the mechanism of this synergism remains obscure. Here, we employed an improved T7 phage display screening method to identify Fanconi anemia group F protein (FANCF) as a doxorubicin-binding protein. The FANCF-doxorubicin interaction was confirmed by pull-down assay and SPR analysis. FANCF is a component of the Fanconi anemia complex, which monoubiquitinates D2 protein of Fanconi anemia group as a cellular response against DNA cross-linkers such as cisplatin. We observed that the monoubiquitination was inhibited by doxorubicin treatment.

Exploration of the binding proteins of perfluorooctane sulfonate by a T7 phage display screen.

Perfluorooctane sulfonate (PFOS) is a pollutant widely found throughout nature and is toxic to animals. We created a PFOS analogue on a polyethylene glycol polyacrylamide copolymer and isolated peptides that preferentially bound the PFOS analogue using a T7 phage display system. Bioinformatic analysis using the FASTAskan program on the RELIC bioinformatics server showed several human proteins that likely bound PFOS. Among them, we confirmed binding between PFOS and a recombinant soluble form of monocyte differentiation antigen CD14 (sCD14) by a surface plasmon biosensor. Furthermore, PFOS inhibited TNF-α production induced by the sCD14 in mouse macrophage RAW264.7 cells.

Determination of the absolute structure of (+)-akaterpin.

We describe the total synthesis and structural determination of (+)-akaterpin (1), an inhibitor of phosphatidylinositol-specific phospholipase C (PI-PLC). The key features of the synthetic strategy include the resolution of ß,γ-unsaturated ketone (±)-2a with chiral sulfoximine 6. The absolute stereochemistry was determined by comparison of the specific optical rotation data of (+)-1 and (-)-1 with that of natural akaterpin.

Efficient synthesis of 3-O-thia-cPA and preliminary analysis of its biological activity toward autotaxin.

The efficient synthesis of 3-O-thia-cPAs (4a-d), sulfur analogues of cyclic phosphatidic acid (cPA), has been achieved. The key step of the synthesis is an intramolecular Arbuzov reaction to construct the cyclic thiophosphate moiety. The present synthetic route enables the synthesis of 4a-d in only four steps from the commercially available glycidol. Preliminary biological experiments showed that 4a-d exhibited a similar inhibitory effect on autotaxin (ATX) as original cPA.