Bakthan Singaram

Reductions of Aliphatic and Aromatic Nitriles to Primary Amines with Diisopropylaminoborane

Diisopropylaminoborane [BH(2)N(iPr)(2)] in the presence of a catalytic amount of lithium borohydride (LiBH(4)) reduces a large variety of aliphatic and aromatic nitriles in excellent yields. BH(2)N(iPr)(2) can be prepared by two methods: first by reacting diisopropylamineborane [(iPr)(2)N:BH(3)] with 1.1 equiv of n-butyllithium (n-BuLi) followed by methyl iodide (MeI), or reacting iPrN:BH(3) with 1 equiv of n-BuLi followed by trimethylsilyl chloride (TMSCl). BH(2)N(iPr)(2) prepared with MeI was found to reduce benzonitriles to the corresponding benzylamines at ambient temperatures, whereas diisopropylaminoborane prepared with TMSCl does not reduce nitriles unless a catalytic amount of a lithium ion source, such as LiBH(4) or lithium tetraphenylborate (LiBPh(4)), is added to the reaction. The reductions of benzonitriles with one or more electron-withdrawing groups on the aromatic ring generally occur much faster with higher yields. For example, 2,4-dichlorobenzonitrile was successfully reduced to 2,4-dichlorobenzylamine in 99% yield after 5 h at 25 degrees C. On the other hand, benzonitriles containing electron-donating groups on the aromatic ring require refluxing in tetrahydrofuran (THF) for complete reduction. For instance, 4-methoxybenzonitrile was successfully reduced to 4-methoxybenzylamine in 80% yield. Aliphatic nitriles can also be reduced by the BH(2)N(iPr)(2)/cat. LiBH(4) reducing system. Benzyl cyanide was reduced to phenethylamine in 83% yield. BH(2)N(iPr)(2) can also reduce nitriles in the presence of unconjugated alkenes and alkynes such as the reduction of 2-hexynenitrile to hex-5-yn-1-amine in 80% yield. Unfortunately, selective reduction of a nitrile in the presence of an aldehyde is not possible as aldehydes are reduced along with the nitrile. However, selective reduction of the nitrile group at 25 degrees C in the presence of an ester is possible as long as the nitrile group is activated by an electron-withdrawing substituent. It should be pointed out that lithium aminoborohydrides (LABs) do not reduce nitriles under ambient conditions and behave as bases with aliphatic nitriles as well as nitriles containing acidic alpha-protons. Consequently, both LABs and BH(2)N(iPr)(2) are complementary to each other and offer methods for the selective reductions of multifunctional compounds.

Recognition of phospho sugars and nucleotides with an array of boronic acid appended bipyridinium salts

The solution-phase sensor array of three cationic bis-boronic acid appended benzyl viologens (BBV) and the anionic fluorescent dye, 8-hydroxypyrene-1,3,6-trisulfonic acid trisodium salt (HPTS), is able to discriminate among five phospho sugars, four nucleotides and three neutral saccharides in aqueous buffered solution at low mM concentrations. Linear discriminant analysis, principal component analysis, and hierachical cluster analysis studies showed the discrimination limit (lowest analyte concentration where the discrimination is still 100%) to be 4mM. Calculated K(b) and F(max)/F(0) values from binding curves of the three BBVs with 1-12 were also used to perform multi-variate analyses with very good discrimination results.

Single Cell “Glucose Nanosensor” Verifies Elevated Glucose Levels in Individual Cancer Cells

Because the transition from oxidative phosphorylation to anaerobic glycolytic metabolism is a hallmark of cancer progression, approaches to identify single living cancer cells by their unique glucose metabolic signature would be useful. Here, we present nanopipettes specifically developed to measure glucose levels in single cells with temporal and spatial resolution, and we use this technology to verify the hypothesis that individual cancer cells can indeed display higher intracellular glucose levels. The nanopipettes were functionalized as glucose nanosensors by immobilizing glucose oxidase (GOx) covalently to the tip so that the interaction of glucose with GOx resulted in a catalytic oxidation of β-d-glucose to d-gluconic acid, which was measured as a change in impedance due to drop in pH of the medium at the nanopipette tip. Calibration studies showed a direct relationship between impedance changes at the tip and glucose concentration in solution. The glucose nanosensor quantified single cell intracellular glucose levels in human fibroblasts and the metastatic breast cancer lines MDA-MB-231 and MCF7 and revealed that the cancer cells expressed reproducible and reliable increases in glucose levels compared to the nonmalignant cells. Nanopipettes allow repeated sampling of the same cell, as cells remain viable during and after measurements. Therefore, nanopipette-based glucose sensors provide an approach to compare changes in glucose levels with changes in proliferative or metastatic state. The platform has great promise for mechanistic investigations, as a diagnostic tool to distinguish cancer cells from nonmalignant cells in heterogeneous tissue biopsies, as well as a tool for monitoring cancer progression in situ.

Rapid small intestinal permeability assay based on riboflavin and lactulose detected by bis-boronic acid appended benzyl viologens

Background Although organoboronic acids are efficient high-throughput sugar sensors, they have not been pursued for gut permeability studies. A modification of the lactulose/mannitol assay is described by which small intestinal permeability is assessed at the time of urine collection using a lactulose/riboflavin ratio. Methods Volunteers ingested 50 mg riboflavin and either 5 g mannitol or 10 g lactulose. Urine was collected for 6 hrs. Riboflavin was assayed by autofluorescence. Riboflavin was removed by C18 solid phase extraction. Lactulose and mannitol were then assayed using 1,1′-bis(2-boronobenzyl)-4,4′-bipyridinium (4,4′oBBV) coupled to the fluorophore HPTS. Results The temporal profile over 6 hrs for riboflavin paralleled mannitol. Riboflavin recovery in urine was 11.1 ± 1.9 % (mean ± SEM, n = 7), similar to mannitol. There was selective binding of 4,4′oBBV to lactulose, likely involving cooperativity between the fructose and galactose moieties. Lower limits of detection and quantification were 90 and 364 μM. The lactulose assay was insensitive to other permeability probes (e.g., sucrose, sucralose) while tolerating glucose or lactose. This assay can be adapted to automated systems. Stability of 4,4′oBBV exceeds 4 years. Conclusions Riboflavin measured by autofluorescence combined with lactulose measured with 4,4′oBBV represents a useful new chemistry for rapid measurement of intestinal permeability with excellent stability, cost and throughput benefits.

Binary reducing agents containing dichloroindium hydride for the selective, partial, or tandem reductions of bifunctional compounds consisting of halo-nitriles, halo-esters and halo-carboxylic acids

The selective, partial, or tandem reductions of bifunctional compounds containing primary alkyl or benzyl bromides can generate a variety of different products using a mixture of dichloroindium hydride (HInCl2) and an additional hydride, such as borane–tetrahydrofuran (BH3u2006:u2006THF) or diisobutylaluminum hydride (DIBAL-H). Binary metal hydride systems, containing HInCl2 and BH3u2006:u2006THF, are readily generated from anhydrous indium trichloride (InCl3) and sodium borohydride (NaBH4) in THF. Dichloroindium hydride can reduce primary halides and in the presence of another hydride, either generated in situ or added to the single-pot reaction, can perform tandem reductions of a variety of bifunctional bromides. Together, the hydrides reduce carbon–halogen bonds as well as an electrophilic group, such as a nitrile, an ester, or a carboxylic acid. The reduction of 4-(bromomethyl)benzonitrile using the HInCl2 and BH3u2006:u2006THF binary metal hydride system affords 4-methylbenzylamine in an excellent yield under ambient conditions and short reaction times. By using the binary metal hydride system consisting of HInCl2 and DIBAL-H, the tandem reduction of 4-(bromomethyl)benzonitrile was achieved affording para-tolualdehyde in an excellent yield, also under ambient conditions. The reduction of 4-(bromomethyl)phenyl acetic acid by HInCl2 selectively reduced the carbon–halogen bond generating para-tolylacetic acid. When using either of the binary hydride systems, both functional groups were reduced generating 4-methylphenethyl alcohol. Methyl 4-(bromomethyl) benzoate underwent a selective or tandem reduction to generate [4-(bromomethyl)phenyl]methanol, methyl 4-methyl benzoate or para-tolylmethanol depending on the hydride system used. Consequently, a single bifunctional compound was transformed into a variety of different compounds by the simple manipulation of the binary hydride system used without the need for protecting groups, and in most cases, by a one-pot procedure.