Jennifer L. Stockdill

Rapid access to the heterocyclic core of the calyciphylline A and daphnicyclidin A-type Daphniphyllum alkaloids via tandem cyclization of a neutral aminyl radical.

A streamlined approach to the tertiary amine-containing core of the calyciphylline A and daphnicyclidin A-type Daphniphyllum alkaloids is presented. A known carvone derivative is converted into the core structure in only four synthetic operations, and it is well poised for further elaboration. The key enabling methodology is a radical cyclization cascade beginning with addition of a secondary, neutral aminyl radical to the β-position of an enone, followed by trapping with a pendant alkyne.

Real-Time, Selective Detection of Copper(II) Using Ionophore-Grafted Carbon-Fiber Microelectrodes

Rapid, selective detection of metals in complex samples remains an elusive goal that could provide critical analytical information for biological and environmental sciences and industrial waste management. Fast-scan cyclic voltammetry (FSCV) using carbon-fiber microelectrodes (CFMs) is an emerging technique for metal analysis with broad potential applicability because of its rapid response to changes in analyte concentration and minimal disturbance to the analysis medium. In this communication, we report the first effective application of covalently modified CFMs to achieve highly selective, subsecond Cu(II) measurements using FSCV. A two-part strategy is employed for maximum selectivity: Cu(II) binding is augmented by a covalently grafted ionophore, while binding of other metals is prevented by chemical blocking of nonselective surface adsorption sites. The resulting electrodes selectively detect Cu(II) in a complex medium comprising several interfering metals. Overall, this strategy represents a transformative innovation in real-time electrochemical detection of metal analytes.

A density-controlled scaffolding strategy for covalent functionalization of carbon-fiber microelectrodes

Trace metal detection is of great importance in environmental and biological systems. It is crucial to develop a portable and sensitive device that can determine levels of trace metals in real time. Recently, we described a method for ultrafast and sensitive detection of Cu(II) and Pb(II) in aqueous environmental samples using fast scan cyclic voltammetry (FSCV) at carbon-fiber microelectrodes (CFMs). However, the application of this technique to more complex samples is limited by analytical selectivity. In this paper, we describe a scaffolding strategy for covalent modification of CFMs as a platform for creating selective adsorption sites. We create a monolayer of acetylene-terminated scaffolds on CFMs through the electrochemical reduction of alkynyl aryl diazonium salts bearing sterically differentiated silyl groups, which control the density of the scaffolds. Desilylation reveals the alkyne for further functionalization via Cu(I)-catalyzed azide-alkyne cycloaddition (CuAAC). As a proof of principle, we optimized the conditions for azidomethyl ferrocene to be grafted with the alkynes. All the surface variations of CFMs are electrochemically verified. This innovative strategy provides the groundwork for a broadly applicable method to generate analyte-selective CFMs. The generalized approach offers the potential to attach azide-appended recognition groups to different electrodes in a high throughput manner. This technology will ultimately allow real-time ultra-selective FSCV analysis of metals in complex ecological and biological systems.

Exploiting the MeDbz Linker to Generate Protected or Unprotected C-Terminally Modified Peptides

C-terminally modified peptides are important targets for pharmaceutical and biochemical applications. Known methods for C-terminal diversification are limited mainly in terms of the scope of accessible modifications or by epimerization of the C-terminal amino acid. In this work, we present a broadly applicable approach that enables access to a variety of C-terminally functionalized peptides in either protected or unprotected form. This chemistry proceeds without epimerization of C-terminal Ala and tolerates nucleophiles of varying nucleophilicity. Finally, unprotected peptides bearing nucleophilic side chain groups can be selectively functionalized by strong nucleophiles, whereas macrocyclization is observed for weaker nucleophiles. The potential utility of this method is demonstrated through the divergent synthesis of the conotoxin conopressin G and GLP-1(7-36) and analogs.

Sequence Diversification by Divergent C-Terminal Elongation of Peptides

Sequence diversification at the C terminus is traditionally limited by significant epimerization of the C-terminal residue during its activation toward nucleophilic attack, thus mandating repetition of the peptide synthesis for each targeted variation. Here, we accomplish divergent C-terminal elongation of a single peptide substrate with concomitant resin cleavage via displacement of an N-acyl urea moiety. Sterically hindered amino acids such as Ile and Pro are well-tolerated in this approach, which proceeds reasonable conversion and no detectable epimerization of the starting peptides C-terminal amino acid.

Tin-Free Access to the ABC Core of the Calyciphylline A Alkaloids and Unexpected Formation of a D-Ring-Contracted Tetracyclic Core

A tin-free strategy for the successful cyclization of a variety of internal alkyne-containing N-chloroamine precursors to the ABC core via cyclization of a neutral aminyl radical is established. Deuterium labeling experiments confirm that the solvent is the primary source of the final H atom in the cyclization cascade. These conditions enabled a streamlined route to a β-ketoester intermediate poised for intramolecular Knoevenagel condensation to construct the seven-membered D-ring of calyciphylline A alkaloids. However, exposure to CsF in t-BuOH at elevated temperatures led to an unexpected decarboxylation to form a D-ring-contracted tetracyclic core.

Synthetic and Computational Study of Tin-Free Reductive Tandem Cyclizations of Neutral Aminyl Radicals

5- exo, 5- exo Cyclizations of conformationally unbiased propargylic aminyl radicals proceed with excellent yield, chemoselectivity, and diastereoselectivity under tin-free reductive cyclization conditions, regardless of the electronic environments and intermediate radical stabilization resulting from various olefin substituents. These conditions avoid the need for slow addition of initiator and reductant. By contrast, analogous 6- exo, 5- exo cyclizations require substituents capable of intermediate radical stabilization to avoid premature reduction products. These experimental results are corroborated by computations that further establish the reactivity of these aminyl radicals upon exposure to tin-free cyclization conditions.