Charge Transport in Sequence-Defined Conjugated Oligomers.

A major challenge in synthetic polymers lies in understanding how primary monomer sequence affects materials properties. In this work, we show that charge transport in single molecule junctions of conjugated oligomers critically depends on the primary sequence of monomers. A series of sequence-defined oligomers ranging from two to seven units was synthesized by an iterative approach based on the van Leusen reaction, providing conjugated oligomers with backbones consisting of para-linked phenylenes connected to oxazole, imidazole, or nitro-substituted pyrrole.

Expanding the limits of the second genetic code with ribozymes.

The site-specific incorporation of noncanonical monomers into polypeptides through genetic code reprogramming permits synthesis of bio-based products that extend beyond natural limits. To better enable such efforts, flexizymes (transfer RNA (tRNA) synthetase-like ribozymes that recognize synthetic leaving groups) have been used to expand the scope of chemical substrates for ribosome-directed polymerization. The development of design rules for flexizyme-catalyzed acylation should allow scalable and rational expansion of genetic code reprogramming.

On-Demand Complex Peptide Synthesis: An Aspirational (and Elusive?) Goal for Peptide Synthesis.

Peptide synthesis is a truly interdisciplinary tool, familiar to a broad group of scientists who do not otherwise overlap scientifically. For this reason, some may perceive even complex peptide synthesis to be a "solved problem", while others might argue that immense opportunity remains untapped or simply inaccessible. At the extreme of complexity, what might a concise assessment of the state-of-the-art in peptide synthesis look like? As one of the most practiced forms of synthetic chemistry by chemists and non-chemists alike, what restrictions remain that constrain access to chemical space? Using popular terminology, what forms of peptide synthesis are appropriately termed "on-demand"? The purpose of this Perspective is to appraise synthetic access to complex peptides, particularly those containing unnatural α-amino amides.

A one-pot amidation of primary nitroalkanes.

It has been over a half-century since Kornblum demonstrated the conversion of a primary nitroalkane to a carboxylic acid; addition of an amine results in carboxylic acid formation as well. We describe the formation of amides from terminal nitroalkanes in a two-step, one-pot reaction involving tandem halogenation/umpolung amide synthesis (UmAS).

Enantioselective Synthesis of D-α-Amino Amides from Aliphatic Aldehydes.

Peptides consisting of D-amino amides are highly represented among both biologically active natural products and non-natural small molecules used in therapeutic development. Chemical synthesis of D-amino amides most often involves approaches based on enzymatic resolution or fractional recrystallization of their diastereomeric amine salts, techniques that produce an equal amount of the L-amino acid. Enantioselective synthesis, however, promises selective and general access to a specific α-amino amide, and may enable efficient peptide synthesis regardless of the availability of the corresponding α-amino acid.

Enantioselective Addition of Bromonitromethane to Aliphatic -Boc Aldimines Using a Homogeneous Bifunctional Chiral Organocatalyst.

This report details the enantioselective synthesis of β-amino-α-bromo nitroalkanes with β-alkyl substituents, using homogeneous catalysis to prepare either antipode. Use of a bifunctional Brønsted base/acid catalyst allows equal access to either enantiomer of the products, enabling the use of Umpolung Amide Synthesis (UmAS) to prepare the corresponding L- or D-α-amino amide bearing alkyl side chains - overall, in only 4 steps from aldehyde. The approach also addresses an underlying incompatibility between bromonitromethane and solid hydroxide bases.

Umpolung amide synthesis using substoichiometric N-iodosuccinimide (NIS) and oxygen as a terminal oxidant.

Umpolung Amide Synthesis (UmAS) provides direct access to amides from an α-bromo nitroalkane and an amine. Based on its mechanistic bifurcation after convergent C-N bond formation, depending on the absence or presence of oxygen, UmAS using substoichiometric N-iodosuccinimide (NIS) under aerobic conditions has been developed. In combination with the enantioselective preparation of α-bromo nitroalkane donors, this protocol realizes the goal of enantioselective α-amino amide and peptide synthesis based solely on catalytic methods.