Laterally coupled silicon-germanium modulator for passive waveguide systems

We model a laterally coupled Franz-Keldysh add-drop ring modulator designed to overcome the C-band indirect absorption of silicon-germanium. Although our concept is based on loss-sensitive interferometry, it utilizes the same highly absorptive germanium-rich compositions geared toward complementary metal-oxide semiconductor (CMOS) photodetectors and electroabsorption modulators. The proposed device can be integrated with passive waveguide networks in which the carrier plasma modulation mechanism is ineffective. In addition, unlike previous silicon-germanium modulator schemes, complex butt-coupling between the passive transport and the active silicon-germanium waveguides is not required. Instead, the optical mode remains guided within the transport waveguide, minimizing transition losses.     

Dialkylaluminum hydride-promoted cyclodimerization of silylated 1,3-enynes via skeletal rearrangement

The dialkylaluminum hydride-promoted reaction of 1-silylalk-3-en-1-ynes gave symmetrical 1,2,3,5-tetrasubstituted benzenes as single regioisomers. The novel cyclodimerization via skeletal rearrangement can be rationalized by an unprecedented mechanism involving sequential hydroalumination, alkene isomerization, carboalumination, carbon-carbon bond cleavage, and retro-hydroalumination.     

Synthesis of key fragments of amphidinolide Q--a cytotoxic 12-membered macrolide

β-hydroxy aldehyde and alkyl ketone moieties were effectively synthesized as key intermediates of amphidinolide Q, a cytotoxic macrolide from the cultured dinoflagellate Amphidinium sp.. The asymmetric center of the former derivative was produced by Sharpless asymmetric epoxidation, followed by E-selective 1,4-addition to give the sp2 methyl group. Derivatization of the L-ascorbic acid derivative by Evans asymmetric alkylation and Peterson olefination provided the latter intermediate. The coupling reaction of the segments was examined.     

Solvation structure and stability of [(CH3)2NH]m(NH3)n-H hypervalent clusters: ionization potentials and switching of hydrogen-atom localized site

Ionization potentials (IPs) of [(CH(3))(2)NH](m)(NH(3))(n)-H hypervalent radical clusters produced by an ArF excimer laser photolysis of dimethylamine (DMA)-ammonia mixed clusters are determined by the photoionization threshold measurements. The IPs of the DMA(1)(NH(3))(n)-H hypervalent radicals decrease rapidly with the number of ammonia up to n=4, and then its decrease rate becomes much slower for n ≥ 5. This trend is very similar to that found for NH(4)(NH(3))(n) clusters. The calculated results on the stable structures and IP as well as the observed IP for DMA(1)(NH(3))(n)-H indicate that the hydrogen atom-localized site is the NH(3) moiety for n=1, while the doubly coordinated DMA-H is favorable for n=2-4, and then 4-fold-coordinated NH(4) is again more stable for n ≥ 5. These changes are consistent with the results on the femtosecond pump-probe experiments of DMA(n)-H clusters. Switching of the hydrogen atom-localized site is ascribed to the instability of DMA-H against a hydrogen-atom dissociation.     

Ring-opening reaction of unactivated 3-arylaziridine-2-carboxylates with nitrile reagents

Ring-opening reactions of unactivated 3-arylaziridine-2-carboxylates with nitrile reagents, using trans-1-benzyl-3-(3,4-methylenedioxyphenyl)aziridine-2-carboxylate as a typical aziridine substrate, were examined. Formation of azomethine ylide by C2-C3 bond cleavage was observed when the aziridine was treated with trimethylsilyl cyanide under thermal conditions. On the other hand the use of bromine cyanide (BrCN) and diethylaluminum cyanide (Et₂AlCN) led to N-C3 bond cleavage and the stereospecificity was found to be dependent on the reagent used. Additional aluminum-catalyzed ring-opening reactions disclosed that the potential cationic character of the C3 benzylic position and stereochemical requirements of substituents in the arylaziridine system control the reactivity. Furthermore, the synthetic utility of the ring-opening reaction was demonstrated not only by application to the cyclization of a ring-opened cyanopropanoate to an isoquinoline skeleton but also by the extension of other carbon nucleophile from nitrile (C1) to a ketene acetal (C2).     

Asymmetric epoxidation of α-substituted acroleins catalyzed by diphenylprolinol silyl ether

Asymmetric epoxidation of α-substituted acroleins with hydrogen peroxide has been catalyzed by diphenylprolinol diphenylmethylsilyl ether to afford α-substituted-β,β-unsubstituted-α,β-epoxy aldehyde with excellent enantioselectivity and the generation of a chiral quaternary carbon center. The method was applied to a short synthesis of (R)-methyl palmoxirate.     

Functional multineuron calcium imaging for systems pharmacology

Functional multineuron calcium imaging (fMCI) is a large-scale technique used to access brain function on a single-neuron scale. It detects the activity of individual neurons by imaging action potential-evoked transient calcium influxes into their cell bodies. fMCI has recently been used as a high-throughput research tool to examine how neuronal activity is altered in animal models of brain diseases, for example stroke, Alzheimer’s disease, and epilepsy, and to estimate how pharmacological agents act on normal and abnormal states of neuronal networks. It offers unique opportunities to discover the mechanisms underlying neurological disorders and new therapeutic targets.     

Guanidine chemistry

Guanidines are categorized as strong organobases; however, their catalytic utility in organic synthesis has not been discussed thoroughly. The author's group has extensively and systematically studied their potential ability focusing on: 1) modified guanidines as chiral auxiliaries; 2) guanidinium ylides for aziridine formation; 3) the affinity of bisguanidine for proton and metal salts; and 4) the potential chirality of bisguanidine. Under the first topic, a variety of chiral guanidines was designed by the introduction of chirality on the three guanidinyl nitrogens, and the modified guanidines prepared using our original methods were found to be effective not only in catalytic but also in stoichiometric asymmetric syntheses. Under the second topic, the reaction of guanidinium salts carrying a glycinate function with aromatic or unsaturated aldehydes under basic conditions unexpectedly afforded aziridine-2-carboxylates, which were available as useful building blocks in organic synthesis due to their convertibility to functionalized amino acid derivatives in the ring-opening reaction, together with urea compounds recyclable to the starting guanidinium salts. The introduction of a chiral template to the guanidinium salt allowed us to expand the cyclic aziridination reaction to an asymmetric version. Under the third topic, effective complexabilty of bisguanidines with either proton or metal ions in water was observed, suggesting their possible application to the removal of toxic substances from polluted water and recovery of rare elements as material sources. Under the final topic, monomethylation or monoethylation of bisguanidine afforded a chiral product via asymmetric crystallization, indicating that bisguanidines have a potential chiral character due to the plane asymmetry.