Asymmetric Total Synthesis of ent‐Pyripyropene A

An asymmetric total synthesis of ent‐pyripyropene A was achieved by a convergent synthetic route. We used our originally developed Ti^III‐catalyzed radical cyclization to construct an AB‐ring portion that consisted of a trans‐decalin skeleton with five contiguous stereogenic centers. The coupling between the AB‐ring and the DE‐ring portions, and a subsequent C‐ring cyclization, led to the total synthesis of ent‐pyripyropene A. An evaluation of the insecticidal activity of ent‐pyripyropene A against two aphid species revealed that ent‐pyripyropene A was 35–175 times less active than naturally occurring pyripyropene A. This result indicated that the biological target of pyripyropene A recognizes the absolute configuration of pyripyropene A.     

Solid‐Phase Combinatorial Synthesis and Biological Evaluation of Destruxin E Analogues

The solid‐phase combinatorial synthesis of cyclodepsipeptide destruxin E has been demonstrated. The combinatorial synthesis of cyclization precursors 8 was achieved by using a split and pool method on SynPhase Lanterns. The products were successfully macrolactonized in parallel in the solution phase by using 2‐methyl‐6‐nitrobenzoic anhydride and 4‐(dimethylamino)pyridine N‐oxide to afford macrolactones 9 , and the subsequent formation of an epoxide in the side chain gave 18 member destruxin E analogues 6 . Biological evaluation of analogues 6 indicated that the N‐MeAla residue was crucial to the induction of morphological changes in osteoclast‐like multinuclear cells (OCLs). Based on structure–activity relationships, azido‐containing analogues 15 were then designed for use as a molecular probe. The synthesis and biological evaluation of analogues 15 revealed that 15 b , in which the Ile residue was replaced with a Lys(N₃) residue, induced morphological changes in OCLs at a sufficient concentration, and modification around the Ile residue would be tolerated for attachment of a chemical tag toward the target identification of destruxin E ( 1 ).     

Mapping of the primary mannose binding site of pradimicin A

Pradimicin A (PRM-A) is an actinomycete-derived antibiotic with the lectin-like property of being able to recognize D-mannopyranoside (Man) in the presence of Ca(2+) ion. PRM-A and its derivatives have been attracting a great deal of attention as the only family of natural carbohydrate receptors with nonpeptidic skeleton and, more recently, as conceptually novel drug candidates for human immunodeficiency virus (HIV). Despite its scientific interest and potential therapeutic importance, understanding how PRM-A recognizes Man has been severely limited. Conventional interaction analysis of PRM-A with Man in solution has been frustrated by aggregation of PRM-A and the three-component equilibrium consisting of the [PRM-A(2)/Ca(2+)], [PRM-A(2)/Ca(2+)/Man(2)], [PRM-A(2)/Ca(2+)/Man(4)] complexes, and their mixed oligomers. In this Article, we demonstrate the interaction analysis of PRM-A with methyl α-D-mannopyranoside (Man-OMe) in the solid state, which benefits from aggregate-forming propensity of PRM-A and eliminates the problem associated with the complicated equilibrium in solution. Isothermal titration calorimetry (ITC) analysis and coprecipitation experiments revealed that the primary Man binding of PRM-A is markedly tighter than the secondary one, leading to preparation of the solid aggregate solely composed of the [PRM-A(2)/Ca(2+)/Man-OMe(2)] complex. The simple 1:1 complexes of biosynthetically (13)C-enriched PRM-As and [(13)C(6)]Man-OMe facilitated the analysis of the primary Man binding of PRM-A by two-dimensional dipolar-assisted rotational resonance (2D-DARR), which clearly identified that the cavity consisted of D-alanine moiety and ABC rings of PRM-A is the Man binding site. Interestingly, the proposed Man binding site of PRM-A seems to resemble the typical architecture of artificial carbohydrate receptors.     

(S)-stereoisomer of telomestatin as a potent G-quadruplex binder and telomerase inhibitor

Total synthesis of the (S)-stereoisomer of telomestatin (1) was accomplished. (S)-Telomestatin exhibited potency four times that of the natural product, (R)-telomestatin, which was the most potent telomerase inhibitor previously reported. In the circular dichroism spectral analysis of the complexes possessing randomly structured single-stranded d[TTAGGG](4) oligonucleotide, (S)-telomestatin, like (R)-telomestatin, induced an antiparallel G-quadruplex structure. The melting temperature (T(m)) value of the (S)-isomer complex was greater than that of the (R)-telomestatin complex. Therefore, it is concluded that the stereochemistry of the thiazoline of telomestatin is important to the binding ability of a G-quadruplex binder, and (S)-telomestatin as a G-quadruplex binder is more potent than the natural product.     

Efficient synthesis of [¹¹C]ramelteon as a positron emission tomography probe for imaging melatonin receptors involved in circadian rhythms

Ramelteon (TAK-375) is a novel melatonin receptor agonist that is used for clinical treatment of insomnia. The present report describes radiolabeling of ramelteon with the short-lived positron-emitter 11C (T(1/2)=20.4 min) by 2 methods. One method was [11C]methylation of an acetoamide precursor and the other was [11C]acylation of the corresponding amine precursor. First, [11C]methylation method showed the low reproducibility together with the production of many kinds of side products from which the [11C-methyl]Ramelteon was separated with chemical purity of <28% and radiochemical purity of >98%. Whereas, the [11C]acylation method showed high efficiency and reproducibility with a good radiochemical yield (22-43%, decay corrected), high chemical and radiochemical purities (>99% each), and high specific activity (43-162 GBq/μmol) (n=5) after HPLC purification. [11C]Ramelteon is a potential positron emission tomography (PET) probe for imaging the melatonin receptor.     

Total synthesis of optically active lycopladine A by utilizing diastereoselective protection of carbonyl group in a 1,3-cyclohexanedione derivative

We successfully synthesized two enantiomers of bicyclic enones, (7R,7aR)- and (7S,7aS)-9, from the hemiacetal 2a, which we first synthesized from the symmetrical diketone 1a via diastereoselective carbon-oxygen bond formation between one of the carbonyl groups and the chiral alcohol on the C2 side chain in a 2,2-disubstituted 1,3-cycloalkanedione derivative. We also report the total synthesis of natural (+)-lycopladine A [(+)-6] from (7R,7aR)-9 and the formal synthesis of unnatural (-)-lycopladine A [(-)-6] from (7S,7aS)-9.     

The synthesis of 2-ethoxy-6,11-dimethyl-7,9-bisdehydroaza[14]annulene and its benzannelated derivatives

Title azaannulenes were synthesized starting from annulenones, and the ¹H-NMR spectra are discussed in connection with tropicity of these annulenes.