Synthesis of Bridged Bicyclic Amines by Intramolecular Amination of Remote C–H Bonds: Synergistic Activation by Light and Heat

Skeletal Editing Strategies Driven by Total Synthesis

ConspectusSingle-atom skeletal editing strategies that precisely modify the core frameworks of molecules have the potential to streamline and accelerate organic synthesis by enabling conceptually simple, but otherwise synthetically challenging, retrosynthetic disconnections. In contrast to broader skeletal remodeling and rearrangement strategies, these methodologies more specifically target single-atom changes with high selectivity, even within complex molecules such as natural products or pharmaceuticals. For the past several years, our laboratory has developed several skeletal editing methodologies, including single-atom ring contractions, expansions, and transpositions of both saturated and unsaturated heterocycles, as well as other carbon scaffolds. This Account details the evolution of "skeletal editing logic" within the context of our extensive work on natural product total synthesis.Early work in the Sarpong group leveraged metal-mediated C-C bond cleavage of in situ-generated strained intermediates to accomplish total syntheses of natural products, such as the icetexane diterpenoids and cyathane diterpenes. Continuing our focus on leveraging C-C bond cleavage through "break-it-to-make-it" strategies, we then developed carvone remodeling strategies to access a variety of terpenoids (including longiborneol sesquiterpenoids, phomactins, and xishacorenes) from hydroxylated pinene derivatives. In applying this skeletal remodeling and C-C cleavage framework to alkaloid natural products, such as the preparaherquimides and lycodine-type alkaloids, we recognized that single-atom changes to the saturated nitrogen-containing rings within these natural products would enable the direct conversion between distinct but structurally related natural product families. Thus, we began developing methods that selectively modify the core frameworks of N-heterocycles; this focus led to our work on the deconstructive fluorination and diversification of piperidines and ultimately to our recent body of work on direct, single-atom core framework modifications (single-atom skeletal editing). In the context of saturated heterocycles, we developed photomediated enantioselective ring contractions of α-acylated motifs and reductive ring contractions of cyclic hydroxylamines. For unsaturated heterocycles, we have developed ring contractions of azines (e.g., pyrimidine to pyrazole), 15N isotopic labeling of azines, and phototranspositions of indazoles to benzimidazoles. To direct our focus on reaction development, a cheminformatic analysis of heteroaromatic skeletal edits served to quantitatively inform which transformations would most significantly expand the accessible chemical space. Apart from heterocycles, we also reported single-nitrogen insertion through the reductive amination of carbonyl C-C bonds. Ultimately, the goal of this research is to develop mild and selective skeletal editing methodologies that can be applied to total synthesis and organic synthesis more generally. While recent total syntheses from our group have targeted simplified retrosyntheses through single-atom skeletal editing logic (e.g., daphenylline and harringtonolide), multiple steps were still required to achieve the formal desired "edit". As such, the continued development of truly single-step, mild, and selective reactions that can edit the cores of highly complex molecules remains highly desirable.

Remote C-H Amination and Alkylation of Camphor at C8 through Hydrogen-Atom Abstraction

Camphor continues to serve as a versatile chiral building block for chemical synthesis. We have developed a novel method to functionalize the camphor skeleton at C8 using an intramolecular hydrogen atom abstraction. The key advance involves the use of a camphor-derived aminonitrile, which is converted to the corresponding nitrogen-centered radical under photoredox conditions to effect the 1,5-hydrogen atom transfer at C8. The resulting carbon-centered radical at C8 was utilized in a C-H amination to access topologically complex proline derivatives. Furthermore, the total synthesis of several sesquiterpenoids was accomplished by engaging the radical generated at C8 in alkylation reactions.

Enantioselective Syntheses of 2-Azabicyclo[2.2.1]heptanes via Brønsted Acid Catalyzed Ring-Opening of meso-Epoxides

A chiral phosphoric acid-catalyzed ring-opening of meso-epoxides was developed. A range of 2-azabicyclo[2.2.1]heptanes were obtained in high yields with excellent enantioselectivities. In addition, the hydroxyl and amide groups in the products provided handles for further derivatization.

2-Oxa-5-azabicyclo[2.2.1]heptane as a Platform for Functional Diversity: Synthesis of Backbone-Constrained γ-Amino Acid Analogues

We communicate a versatile synthetic approach to C-3 disubstituted 2-oxa-5-azabicyclo[2.2.1]heptanes as carbon-atom bridged morpholines, starting with 4R-hydroxy-l-proline as a chiron. Attaching an acetic acid moiety on the C-3 carbon of the 2-oxa-5-azabicyclo[2.2.1]heptane core reveals the framework of an embedded γ-amino butyric acid (GABA). Variations in the nature of the substituent on the tertiary C-3 atom with different alkyls or aryls led to backbone-constrained analogues of the U.S. Food and Drug Administration-approved drugs baclofen and pregabalin.

Asymmetric Synthesis of Enantiopure Pyrrolidines by C(sp3 )-H Amination of Hydrocarbons

The asymmetric synthesis of enantiopure pyrrolidines is reported via a streamlined strategy relying on two sequential C-H functionalizations of simple hydrocarbons. The first step is a regio- and stereoselective catalytic nitrene C-H insertion. Then, a subsequent diastereoselective cyclization involving a 1,5-hydrogen atom transfer (HAT) from a N-centered radical leads to the formation of pyrrolidines that can then be converted to their free NH-derivatives.

Bicyclic Piperidines via [2 + 2] Photocycloaddition

A synthetic strategy to fused bicyclic piperidines-building blocks for medicinal chemistry-is developed. The key step was an intramolecular [2 + 2]-photocyclization. The photochemical step was performed on a gram scale. Crystallographic analysis of the obtained compounds revealed that they occupy a novel chemical space and can be considered as elongated analogues of 3-substituted piperidines.

Manganese-Catalyzed N-F Bond Activation for Hydroamination and Carboamination of Alkenes

A visible-light-promoted method for generating amidyl radicals from N-fluorosulfonamides via a manganese-catalyzed N-F bond activation strategy is reported. This protocol employs a simple manganese complex, Mn₂(CO)₁₀, as the precatalyst and a cheap silane, (MeO)₃SiH, as both the hydrogen-atom donor and the F-atom acceptor, enabling intramolecular/intermolecular hydroaminations of alkenes, two-component carboamination of alkenes, and even three-component carboamination of alkenes. A wide range of valuable aliphatic sulfonamides can be readily prepared using these practical reactions.