Model studies of colloidal silica precipitation using biosilica extracts from<Emphasis Type="Italic"> Equisetum telmateia</Emphasis>

Structural materials containing silicon are produced in single celled organisms through to higher plants and animals. Hydrated amorphous silica is a colloidal mineral of infinite functionality that is formed into structures with microscopic and macroscopic form. Proteins and proteoglycans are suggested to play a critical role in the catalysis of silica polycondensation and in structure direction during the formation of these magnificent structures. This article extends knowledge on the effect of protein containing biosilica extracts from Equisetum telmateia on the kinetics of silica formation and structure regulation. Utilising potassium silicon catecholate as the source of soluble silicon, bioextracts obtained from plant silica by dissolution of the siliceous phase with aqueous HF following extensive acid digestion of the plant cell wall were found to modify the kinetic rate constants for the formation of small silicic acid oligomers under circumneutral pH conditions and to modify the solubility of silicic acid in solution. Addition of the bioextracts at ca. 1 wt% to the reaction medium reduced the sizes and range of sizes of the fundamental silica particles formed and led to the formation of crystalline polymorphs of silica under conditions of ca. neutral pH, room temperature and in the absence of multiply charged cations, conditions assumed to be relevant to the biological mineralization environment. The ability of biological organisms to regulate the formation of silica structures with prevention of crystallinity is discussed as are the implications of this study in terms of the generation of new materials with specific form and function for industrial application.     

Intramolecular cyclization of delta-iminoacetylenes: a new entry to pyrazino[1,2-a]indoles

[reaction: see text] The synthesis of the pyrazino[1,2-a]indole nucleus was achieved by intramolecular cyclization of several 2-carbonyl-1-propargylindoles in the presence of ammonia. The reaction conditions were optimized using microwave heating and a pool of catalysts. Cyclization of 1-alkynylindole-2-carbaldehydes was easily accomplished under standard heating conditions, whereas microwave heating contributed to reduced reaction times and improved overall yields. Moreover, a fine-tuning of the microwave irradiation time made possible the selective synthesis of both pyrazino[1,2-a]indole isomers. TiCl4 proved the catalytic system of choice to achieve pyrazinoindoles in satisfactory yields starting from 1-alkynyl-2-acetylindoles and 1-alkynyl-2-benzoylindole derivatives. Also in these cases, microwave heating contributed to faster reactions and improved yields. The uncatalyzed versus catalyzed reaction mechanism is discussed.     

The palladium-catalysed reductive addition of aryl iodides to propargyl alcohols: a route to /gg,/gg-diaryl allylic alcohols

The reaction of aryl iodides with ethynyl and arylethynyl,dialky $\text{l}$ carbinols in the presence of the tri- or dialkylammonium formate-palladium reagent provides a convenient route to γ,γ-diarylallylic alcohols. In the presence of arylethynyl,alkylcarbinols a lack of regioselectivity was observed and mixtures of ß,γ-, γ,γ-diarylallylic alcohols, and α,ß-unsat$\text{u}$ rated ketones were obtained.     

2,3-Disubstituted pyrrolo[2,3-b]quinoxalines via aminopalladation-reductive elimination

2,3-Disubstituted pyrrolo[2,3-b]quinoxalines have been prepared in good to high yield through the reaction of 2-alkynyl-3-trifluoroacetamidoquinoxalines with aryl and vinyl halides or triflates in the presence of Pd(PPh₃)₄ and K₂CO₃ in MeCN at 100 °C.     

Sequential alkylation/cyclization/isomerization of ethyl 3-(o-trifluoroacetamidoaryl)-1-propargylic esters: a new route to 2-acyl- and 2-ethoxycarbonyl-3-alkenyl indoles

The reaction of ethyl 3-(o-trifluoroacetamidoaryl)-1-propargylic esters with ethyl iodoacetate or α-bromoacetophenone in the presence of K₂CO₃ in DMSO provides a new simple access to 2-acyl- and 2-ethoxycarbonyl-3-alkenyl indoles.