Christopher D. Thomas

A Tandem Isomerization/Prins Strategy: Iridium(III)/Brønsted Acid Cooperative Catalysis†

Working together: A mild and efficient isomerization/protonation sequence generates pyran-fused indoles by cooperative catalysis between cationic iridium(III) and Bi(OTf)3 . Three distinct cyclization manifolds lead to the corresponding bioactive scaffolds in good yields. In addition, N-substituted indoles can be synthesized enantioselectively in the presence of a chiral phosphate.

Total Synthesis of Dolabelide C: A Phosphate-Mediated Approach

The first synthesis of dolabelide C (1), a cytotoxic marine macrolide, is reported utilizing a phosphate tether-mediated approach. Bicyclic phosphates (S,S,S(P))-5 and (R,R,R(P))-5 serve as the central building blocks for the construction of two major 1,3-anti-diol subunits in 1 through selective cleavage pathways, regioselective olefin reduction, and cross-metathesis. Overall, phosphate-mediated processes provided copious amounts of both major subunits allowing for a detailed RCM macrocyclization study to the 24-membered macrolactone 1.

A Multifaceted Phosphate Tether: Application to the C15-C30 Subunit of Dolabelides A-D

Construction of the C15-C30 subunit of dolabelide utilizing a temporary phosphate tether is described. Two routes are reported that make use of the orthogonal protecting- and leaving-group properties innate to phosphate esters. One route relies on a selective terminal oxidation, while a second utilizes a CM/selective hydrogenation sequence. Both routes depend on a highly regio- and diastereoselective cuprate addition to set the requisite stereochemistry at C22.

Assembly of Four Diverse Heterocyclic Libraries Enabled by Prins Cyclization, Au-Catalyzed Enyne Cycloisomerization, and Automated Amide Synthesis

We describe a unified synthetic strategy for efficient assembly of four new heterocyclic libraries. The synthesis began by creating a range of structurally diverse pyrrolidinones or piperidinones. Such compounds were obtained in a simple one-flask operation starting with readily available amines, ketoesters, and unsaturated anhydrides. The use of tetrahydropyran-containing ketoesters, which were rapidly assembled by our Prins cyclization protocol, enabled efficient fusion of pyran and piperidinone cores. A newly developed Au(I)-catalyzed cycloisomerization of alkyne-containing enamides further expanded heterocyclic diversity by providing rapid entry into a wide range of bicyclic and tricyclic dienamides. The final stage of the process entailed diversification of each of the initially produced carboxylic acids using a fully automated platform for amide synthesis, which delivered 1872 compounds in high diastereomeric and chemical purity.

Superconducting tunnel junction base electrode planarization

We have investigated the planarizing effects of multilayered superconducting thin films. Atomic force microscopy measurements indicate that coating a standard niobium base electrode with alternating layers of aluminum and niobium significantly reduces the film’s overall surface roughness. Planarized films such as these were used as the base electrodes of superconducting tunnel junctions that show vastly improved leakage characteristics over conventional junctions fabricated under the same conditions.

Growth and in situ x‐ray characterization of YBa2Cu3Ox/LaAlO3 superlattices

We utilized a miniature sputtering cell with mylar windows and multitarget sputtering guns in order to study the growth of YBCO/LaAlO3 superlattices on SrTiO3 substrates. Computer modeling of the diffraction spectra enabled us to determine the interface width between the layers. After deposition of the superlattices, the substrate temperature was increased to study any interdiffusion.

Intrinsically damped multilayered (stacked) Nb/Al-AlNx/Nb superconducting tunnel junctions

Abstract Single and stacked Nb/Al-AlNx/Nb superconducting tunnel junctions with both hysteretic (underdamped) and non-hysteretic (overdamped) current-voltage relationships have been produced utilizing reactively sputtered aluminum nitride tunnel barriers. Standard multilayer deposition and lithographic processing techniques, compatible with existing Nb/Al-AlOx/Nb fabrication techniques, are used in fabrication. The degree of damping in the junctions is controlled through the deposition parameters. Critical current dependence on applied magnetic field indicates that the overdamped junctions have a distributed Josephson coupling and are not simple microshorts. The shorter deposition time to grow reactively sputtered AlNx barriers makes this system a promising alternative to fabricate stacked Josephson junctions.

Role of engineered materials in superconducting tunnel-junction x-ray detectors: suppression of quasi-particle recombination losses via a phononic bandgap

While much progress has been made towards improved energy reso1utvn in superconducting tunnel junction (STJ) detectors recently, results are still more than an order of magnitude worse than the theoretical limit. Several factors have been identified as contributing to degradation of energy resolution in STJ devices: recombination losses, parasitic quasiparticle trapping and quasiparticle diffusion into current leads. In addition, STJ detectors tend to have poor photon capture efficiency. Semiconducting detectors achieve their near theoretical energy resolutions and high efficiencies via doping and/or applying an external field to a pure substance. These methods are ineffective for STJ detectors, therefore such alternatives as engineered materials, consisting of multiple materials artificially patterned on the microscopic level, should be considered. The most common engineered structures in use are quasiparticle trapping configurations, which alleviate lead diffusion and detection efficiency problems. We have previously proposed a multilayered approach which addresses parasitic trapping, along with diffusion and efficiency. We now propose the possibility of an engineered structure which will alleviate quasiparticle recombination losses via the existence of a phononic band gap that overlaps the 2i energy of phonons produced during recombination of quasiparticles. We will present a 1D Kronig-Penny model for phonons normally incident to the layers of a multilayered superconducting tunnel junction as an idealized example

X-ray photon detection with multilayered Josephson junctions

Abstract Superconducting tunnel junctions can be configured as high resolution X-ray spectrometers however, because they are thin, STJs suffer from inherent photoabsorption inefficiency. A common approach to enhance quantum efficiency is to couple the junctions to superconducting absorbers and employ quasiparticle traps. An alternative approach is to use vertical stacks of tunnel junctions as detectors. These multilayered superconducting tunnel junctions (MSTJs) need not be thin and may have additional advantages including increased signal to noise ratio. We report on the fabrication of multilayered tunnel junctions and successful X-ray detection by an MSTJ.

High energy resolution x‐ray detection based on a coupled Fiske cavity and Josephson junction oscillator

We describe the use of a miniature microwave Fiske cavity coupled to a Josephson junction oscillator to monitor the dynamic quasiparticle population created in the surrounding superconductor by incident x rays. An expression is obtained for the phase velocities of the TM modes supported by this structure. The design has the advantage of being inherently thick—sufficient to completely absorb low energy x rays and is potentially scalable to a pixilated device with a pixel size 50 μm in linear dimension. The device represents an alternative strategy to implement the dynamic microwave absorption approach suggested by Gulian and Van Vechten and subsequent variations.