Spongistatin synthetic studies. An efficient, second-generation construction of an advanced ABCD intermediate

[reaction: see text] A short, efficient, and stereocontrolled synthesis of (-)-4, an advanced ABCD subunit of the spongistatins, has been achieved. Central to the synthetic strategy is the multicomponent linchpin union of silyl dithianes with epoxides to access both the AB and CD fragments. Fragment coupling was then achieved via an efficient stereoselective aldol reaction. The linear sequence required 22 steps and proceeded in 4.0% overall yield.     

Zooxanthellamide Cs: vasoconstrictive polyhydroxylated macrolides with the largest lactone ring size from a marine dinoflagellate of Symbiodinium sp

Zooxanthellamide Cs (ZAD-Cs), C(128)H(220)N(2)O(53)S(2) (ca. 2.7 kDa), was obtained from a cultured marine dinoflagellate of the genus Symbiodinium as an inseparable isomeric mixture of polyhydroxylated 61- to 66-membered macrolides. The chemical structures of the components were clarified by detailed 2D NMR analysis to be the macrolactonized analogues of zooxanthellamide A (ZAD-A), which had been previously isolated from the same microalgae. Chemical lability of ZAD-Cs suggests that ZAD-A is an artifact derived from ZAD-Cs during the isolation steps. Three of the components possess the largest (63-, 64-, and 66-membered) ring sizes found to date among the natural macrolides. ZAD-Cs exhibited higher vasoconstrictive activity than that of the zooxanthellatoxins, the first vasoconstrictive macrolides from Symbiodinium sp. The structure-activity relationship suggests that the huge macrolactone structure is important for biological activity. The relationship between the structures of the polyol metabolites and the phylogenetic systematics of Symbiodinium sp. is also discussed.     

Reaction pathway and stereoselectivity of asymmetric synthesis of chrysanthemate with the aratani C1-symmetric salicylaldimine-copper catalyst

The reaction pathway and the mechanism of asymmetric induction in the synthesis of (+)-trans-(1R,3R)-chrysanthemic acid methyl ester from methyl diazoacetate and 2,5-dimethyl-2,4-hexadiene in the presence of a C(1)-chiral salicylaldimine Cu(I) complex has been probed with the aid of hybrid density functional calculations. The key finding is that the alkoxycarbonyl carbene complex intermediate is intrinsically chiral and that the intramolecular hydrogen bonding in the carbene complex transmits the chirality information from the side chain to the carbene complex. Molecular orbital backgrounds of the structure of the carbene complex and the transition state of the cyclopropanation have been elucidated.     

Competitive potentiometric determination of binding constants between α-cyclodextrin and 1-alkanols

The binding constants of 1-alkanols with α-cyclodextrin (α-CD) were determined by indirect competitive potentiometry, although potentiometry is usually inapplicable to nonionic compounds. This novel method utilizes competitive binding of 1-alkanol and octyltrimethylammonium bromide (OTAB) to α-CD in aqueous media, where the concentration of free OTAB depending on the competitive binding is monitored with an OTAB-selective electrode. Therefore, the concentration of 1-alkanol is indirectly estimated from the observed electromotive force. The binding constants of 1-propapnol, 1-butanol, 1-hexanol, and 1-octanol with α-CD, obtained by this method, are close to the literature values. The implications and limitations of this indirect competitive potentiometry were discussed.     

Large-scale separation of metallic and semiconducting single-walled carbon nanotubes

In the applications of single-walled carbon nanotubes (SWNTs), it is extremely important to separate semiconducting and metallic SWNTs. Although several methods have been reported for the separation, only low yields have been achieved at great expense. We show a separation method involving a dispersion-centrifugation process in a tetrahydrofuran solution of amine, which makes metallic SWNTs highly concentrated to 87% in a simple way.     

The structure of enopeptins A and B, novel depsipeptide antibiotics

The structures of enopeptins A and B, novel depsipeptide antibiotics with 1,3,5,7,9-decapentaene-1,10-dicarboxylic acid and 2-amino-3-hydroxycyclopent-2-enone in a side chain, were determined by chemical and spectroscopic means.     

Side chain-directed assembly of triangular molecular panels into a tetrahedron vs. open cone

Despite their structural similarity, triangular tetradentate ligands 2b and 2c experience different assembly pathways on complexation with (en)Pd(NO3)2 to give M8L4 tetrahedral (3) and open cone (4) structures, respectively, due to steric restriction by side chains at the corner or on the edge of the ligands.     

Syntheses and reactions of functional polymers. XCIII. Syntheses of polymers containing the N-(4-hydroxy-3-nitrophenyl)succinimide structure in the chain and the use of these activated polymer esters of N-blocked amino acids or peptides

Polymers containing the N-(4-hydroxy-3-nitrophenyl)succinimide residue were designed in order to achieve acyl activation of a reacting carboxylic acid in the solid phase. These polymers were prepared through the following three routes: (a) styrene was allowed to copolymerize with N-(4-hydroxy-3-nitrophenyl)- or N-(4-acetoxy-3-nitrophenyl)maleimide, (b) styrene was copolymerized with N-(4-acetoxyphenyl)maleimide in the presence of divinylbenzene (DVB), and the copolymer obtained was hydrolyzed and nitrated, (c) a copolymer of maleic anhydride and styrene was reacted with p-aminophenol, followed by nitration. The polymers prepared by routes b and c were converted to the activated polymer esters of N-blocked amino acids and peptides by using dicyclohexylcarbodiimide (DCC). The acylated polymers thus obtained were treated with amino acid esters and found to give peptides quantitatively without racemization.     

Improved assay for catechol-O-methyltransferase activity utilizing norepinephrine as an enzymatic substrate and reversed-phase high-performance liquid chromatography with fluorescence detection

We have previously established a rapid catechol-O-methyltransferase (COMT) assay using norepinephrine (NE) as a natural substrate and flow-injection analysis. In this study, the method is improved for screening of COMT inhibitors or activators using reversed-phase high-performance liquid chromatographic separation with fluorescence detection. The excess substrate, NE, was removed by the addition of borate in the eluent for HPLC to make an ionic complex with NE, which was eluted faster than the enzymatic product, normetanephrine. The method had good precision and accuracy, and was able to assay one sample in 5 min, showing the usability for screening of COMT inhibitors or activators.