Shimalactone Biosynthesis Involves Spontaneous Double Bicyclo‐Ring Formation with 8π‐6π Electrocyclization

Shimalactones A and B are neuritogenic polyketides possessing characteristic oxabicyclo[2.2.1]heptane and bicyclo[4.2.0]octadiene ring systems that are produced by the marine fungus Emericella variecolor GF10. We identified a candidate biosynthetic gene cluster and conducted heterologous expression analysis. Expression of ShmA polyketide synthase in Aspergillus oryzae resulted in the production of preshimalactone. Aspergillus oryzae and Saccharomyces cerevisiae transformants expressing ShmA and ShmB produced shimalactones A and B, thus suggesting that the double bicyclo‐ring formation reactions proceed non‐enzymatically from preshimalactone epoxide. DFT calculations strongly support the idea that oxabicyclo‐ring formation and 8π‐6π electrocyclization proceed spontaneously after opening of the preshimalactone epoxide ring through protonation. We confirmed the formation of preshimalactone epoxide in vitro, followed by its non‐enzymatic conversion to shimalactones in the dark.     

Facile synthetic procedure for and electrochemical properties of hexa(2-thienyl)benzenes directed toward electroactive materials

In the presence of RhCl₃·3H₂O and i-Pr₂NEt, the cyclotrimerization of di(2-thienyl)acetylenes proceeded smoothly to afford hexa(2-thienyl)benzenes. CV analysis of the hexa(2-thienyl)benzenes showed that they may be useful as electroactive materials.     

N+NO reaction on Rh(111) surfaces studied with fast near-edge X-ray absorption fine structure spectroscopy: role of NO dimer as an extrinsic precursor

We studied the mechanism of the N+NO reaction on Rh(111) surfaces by means of fast near-edge X-ray absorption fine structure spectroscopy. This reaction is important as a basis of NOx reduction reactions on platinum-group metal surfaces. Atomic nitrogen layers on Rh(111) were titrated with NO at various temperatures. N2O is exclusively formed and desorbs into the gas phase below 350 K. The consumption rate of atomic nitrogen exhibits strange temperature dependence between 100 and 350 K; the reaction proceeds slower with increasing temperature. Reaction kinetics analyses and isotope-controlled experiments have revealed that the surface N atoms do not react with chemisorbed NO molecules but with NO dimers weakly bound on top of the chemisorbed layer, which play a role as an extrinsic precursor. The present results may support the possibility that NO dimers participate in various NO-related synthetic, biochemical, and surface reactions as an intermediate.     

Total Syntheses of (−)-Strictosidine and Related Indole Alkaloid Glycosides

A collective synthesis of glycosylated monoterpenoid indole alkaloids is reported. A highly diastereoselective Pictet–Spengler reaction with α-cyanotryptamine and secologanin tetraacetate as substrates, followed by a reductive decyanation reaction, was developed for the synthesis of (−)-strictosidine, which is an important intermediate in biosynthesis. This two-step chemical method was established as an alternative to the biosynthetically employed strictosidine synthase. Furthermore, after carrying out chemical and computational studies, a transition state for induction of diastereoselectivity in our newly discovered Pictet–Spengler reaction is proposed. Having achieved the first enantioselective total synthesis of (−)-strictosidine in just 10 steps, subsequent bioinspired transformations resulted in the concise total syntheses of (−)-strictosamide, (−)-neonaucleoside A, (−)-cymoside, and (−)-3α-dihydrocadambine.     

Sequential One‐Pot Addition of Excess Aryl‐Grignard Reagents and Electrophiles to O‐Alkyl Thioformates

The sequential addition of aromatic Grignard reagents to O‐alkyl thioformates proceeded to completion within 30 s to give aryl benzylic sulfanes in good yields. This reaction may begin with the nucleophilic attack of the Grignard reagent onto the carbon atom of the O‐alkyl thioformates, followed by the elimination of ROMgBr to generate aromatic thioaldehydes, which then react with a second molecule of the Grignard reagent at the sulfur atom to form arylsulfanyl benzylic Grignard reagents. To confirm the generation of aromatic thioaldehydes, the reaction between O‐alkyl thioformates and phenyl Grignard reagent was carried out in the presence of cyclopentadiene. As a result, hetero‐Diels–Alder adducts of the thioaldehyde and the diene were formed. The treatment of a mixture of the thioformate and phenyl Grignard reagent with iodine gave 1,2‐bis(phenylsulfanyl)‐1,2‐diphenyl ethane as a product, which indicated the formation of arylsulfanyl benzylic Grignard reagents in the reaction mixture. When electrophiles were added to the Grignard reagents that were generated in situ, four‐component coupling products, that is, O‐alkyl thioformates, two molecules of Grignard reagents, and electrophiles, were obtained in moderate‐to‐good yields. The use of silyl chloride or allylic bromides gave the adducts within 5 min, whereas the reaction with benzylic halides required more than 30 min. The addition to carbonyl compounds was complete within 1 min and the use of lithium bromide as an additive enhanced the yields of the four‐component coupling products. Finally, oxiranes and imines also participated in the coupling reaction.     

Random number generators tested on quantum Monte Carlo simulations

We have tested and compared several (pseudo) random number generators (RNGs) applied to a practical application, ground state energy calculations of molecules using variational and diffusion Monte Carlo metheds. A new multiple recursive generator with 8th‐order recursion (MRG8) and the Mersenne twister generator (MT19937) are tested and compared with the RANLUX generator with five luxury levels (RANLUX‐[0–4]). Both MRG8 and MT19937 are proven to give the same total energy as that evaluated with RANLUX‐4 (highest luxury level) within the statistical error bars with less computational cost to generate the sequence. We also tested the notorious implementation of linear congruential generator (LCG), RANDU, for comparison. © 2010 Wiley Periodicals, Inc. J Comput Chem, 2010     

Amphiphilic Hybrid π‐Conjugated Polymers Containing Polyhedral Oligomeric Silsesquioxanes

Amphiphilic hybrid π‐conjugated polymers that have polyhedral oligomeric silsesquioxanes on their side chains have been successfully synthesized by the Sonogashira–Hagihara polycondensation reaction. The obtained polymers were studied with ultraviolet‐visible absorption and photoluminescence spectra. In these polymers, the π‐conjugation length was extended along the poly(p‐phenylene‐ethynylene) backbone. Furthermore, the content of the POSS substituents can influence the aggregation behavior of the polymers and subsequent luminescent properties.

     

Step‐Growth Copolymerization Between an Immobilized Monomer and a Mobile Monomer in Metal–Organic Frameworks

The A–A/B–B step‐growth copolymerization between a monomer immobilized in the crystalline state and a monomer mobile in the solution state is demonstrated. One of the two monomers was immobilized as organic ligands of the metal–organic framework (MOF) and polymerized with the mobile guest monomer, resulting in the formation of linear polymers. The polymerization behavior was completely different from that of the solution polymerizations. In particular, the degrees of polymerization (DP) converged to a specific value depending on the MOF structures. The inevitable termination is caused not by imperfectness of the polymerization reaction, but by the selection of the two polymerization partners among the several adjacent immobilized monomers. This is fully supported by the Monte Carlo simulation on the basis of the polymerization mechanism. Precise immobilization of monomers in the supramolecular assemblies is a promising way for the controlled A–A/B–B step‐growth polymerization.     

Detailed Investigation of the Structural,Thermal, and Electronic Properties of Gold Isocyanide Complexes with Mechano‐Triggered Single‐Crystal‐to‐Single‐Crystal Phase Transitions

Mechano‐induced phase transitions in organic crystalline materials, which can alter their properties, have received much attention. However, most mechano‐responsive molecular crystals exhibit crystal‐to‐amorphous phase transitions, and the intermolecular interaction patterns in the daughter phase are difficult to characterize. We have investigated phenyl(phenylisocyanide)gold(I) ( 1 ) and phenyl(3,5‐dimethylphenylisocyanide)gold(I) ( 2 ) complexes, which exhibit a mechano‐triggered single‐crystal‐to‐single‐crystal phase transition. Previous reports of complexes 1 and 2 have focused on the relationships between the crystalline structures and photoluminescence properties; in this work we have focused on other aspects. The face index measurements of complexes 1 and 2 before and after the mechano‐induced phase transitions have indicated that they undergo non‐epitaxial phase transitions without a rigorous orientational relationship between the mother and daughter phases. Differential scanning calorimetry analyses revealed the phase transition of complex 1 to be enthalpically driven by the formation of new aurophilic interactions. In contrast, the phase transition of complex 2 was found to be entropically driven, with the closure of an empty void in the mother phase. Scanning electron microscopy observation showed that the degree of the charging effect of both complexes 1 and 2 was changed by the phase transitions, which suggests that the formation of the aurophilic interactions affords more effective conductive pathways. Moreover, flash‐photolysis time‐resolved microwave conductivity measurements revealed that complex 1 increased in conductivity after the phase change, whereas the conductivity of complex 2 decreased. These contrasting results were explained by the different patterns in the aurophilic interactions. Finally, an intriguing disappearing polymorphism of complex 2 has been reported, in which a polymorph form could not be obtained again after some period of time, even with repeated trials. The present studies provide us with a variety of hitherto unknown insights into mechano‐responsive molecular crystals, which help us to understand the phase transition behaviors upon mechanical stimulation and establish rational design principles.