Free radical-mediated vinyl amination: access to N,N-dialkyl enamines and their beta-stannyl and beta-thio derivatives

Selective and Functional-Group-Tolerant Photoalkylation of Imines by Energy-Transfer Photocatalysis

Basic amines show broad bioactivity and remain a promising source of new medicines. The direct photoalkylation of imines offers a promising strategy for complex amines. However, the lack of efficient imine photoreactivity hinders this reaction and remains a fundamental limitation in organic photochemistry. We report an efficient photoalkylation of imines that provides primary amines directly without protecting or leaving groups. The transformation effects C-H addition across N-H imines under energy-transfer photocatalysis by a ketone. Our method is distinguished from organometallic, metal-catalyzed, and photoredox approaches to imine alkylation by its lack of protecting groups and its broad scope, which includes unactivated alkanes, protic substrates, basic amines, heterocycles, and ketone imines. We highlight this scope through the condensation and alkylation of two pharmaceutical ketones, providing complex amines succinctly. Our mechanistic analysis supports a three-step process, involving hydrogen-atom transfer to an imine triplet excited state, intersystem crossing, and radical recombination, with photocatalytic enhancement through energy transfer. We further show that N-H imines are more photoreactive than N-substituted imines, a distinction partially explained by sterics and side reactions. To fully explain this distinction, we introduce the thermodynamic parameter excited-state hydrogen-atom affinity, which is highly effective at predicting the photoreactivity of imines.

Highly Stereoselective Synthesis of Tetrasubstituted Vinyl Selenides via Rhodium-Catalyzed [1,4]-Acyl Migration of Selenoesters and Diazo Compounds

Herein, we disclose a highly stereoselective Rh(II)-catalyzed 1,4-acyl rearrangement of selenium esters and α-diazo carbonyl compounds, which provides an efficient method for synthesizing tetrasubstituted vinyl selenides. Furthermore, this reaction also offers a synthetic tool for medium and large ring compounds.

One-Pot Synthesis of Primary and Secondary Aliphatic Amines via Mild and Selective sp3 C-H Imination

The direct replacement of sp3 C-H bonds with simple amine units (-NH2) remains synthetically challenging, although primary aliphatic amines are ubiquitous in medicinal chemistry and natural product synthesis. We report a mild and selective protocol for preparing primary and secondary aliphatic amines in a single pot, based on intermolecular sp3 C-H imination. The first C-H imination of diverse alkanes, this method shows useful site-selectivity within substrates bearing multiple sp3 C-H bonds. Furthermore, this reaction tolerates polar functional groups relevant for complex molecule synthesis, highlighted in the synthesis of amine pharmaceuticals and amination of natural products. We characterize a unique C-H imination mechanism based on radical rebound to an iminyl radical, supported by kinetic isotope effects, stereoablation, resubmission, and computational modeling. This work constitutes a selective method for complex amine synthesis and a new mechanistic platform for C-H amination.

Total synthesis of the Lycopodium alkaloid serratezomine A using free radical-mediated vinyl amination to prepare a β-stannyl enamine linchpin

Serratezomine A is a member of the structurally diverse class of compounds known as the Lycopodium alkaloids. The key supporting studies and successful total synthesis of serratezomine A are described in this account. Significant features of the synthesis include the first application of free radical mediated vinyl amination and Hwu's oxidative allylation in a total synthesis and an intramolecular lactonization via a transannular S(N)i reaction. Minimal use of protecting groups and the highly diastereoselective formation of a hindered, quaternary stereocenter using an umpolung allylation are also highlights from a strategy perspective. Observation of quaternary carbon epimerization via a retro-Mannich/Mannich sequence highlights the additional challenge presented by the axial alcohol at C8 in serratezomine A.

Development of an enantioselective route toward the Lycopodium alkaloids: total synthesis of lycopodine

Synthesis of a C(15)-desmethyl tricycle core of lycopodine has been accomplished. Key steps in the synthetic sequence include organocatalytic, intramolecular Michael addition of a keto sulfone and a tandem 1,3-sulfonyl shift/Mannich cyclization to construct the tricyclic core ring system. Synthetic work toward this natural product family led to the development of N-(p-dodecylphenylsulfonyl)-2-pyrrolidinecarboxamide, an organocatalyst which facilitates enantioselective, intramolecular Michael additions. A detailed mechanistic discussion is provided for both the intramolecular Michael addition and the sulfone rearrangement. Finally, the application of these discoveries to the enantioselective total synthesis of alkaloid lycopodine is described.

Microwave-promoted syntheses of quinazolines and dihydroquinazolines from 2-aminoarylalkanone O-phenyl oximes

A wide range of biologically active compounds contain the quinazoline ring system. A new free-radical-based method of making functionalized quinazolines is described, which relies on microwave-promoted reactions of O-phenyl oximes with aldehydes. A small set of 2-aminoaryl alkanone O-phenyl oximes was prepared and shown to produce dihydroquinazolines when mixed with an aldehyde in toluene and subjected to microwave heating. When ZnCl(2) was included in the reaction mixture, fully aromatic quinazolines were produced in high yields by a rapid and convenient process. The method worked well with alkyl, aryl, and heterocyclic aldehydes and for a variety of substituents in the benzenic part of the molecule. Similar reactions employing ketones instead of aldehydes were less efficient. Although some dihydroquinazolines did form, they were accompanied by several byproducts. Surprisingly, in each case, one of the byproducts was a quinoline derivative, and a plausible mechanism to account for this rearrangement is proposed.

Total synthesis of the lycopodium alkaloid (+)-serratezomine A

The first total synthesis of (+)-serratezomine A is described. Key aspects of the synthesis include (a) the first deployment of free-radical-mediated vinyl amination (an intramolecular alkyne aminostannation) in a complex target synthesis, (b) the use of a beta-stannyl enamine as the lynchpin for convergent assembly of the natural product backbone, (c) the use of an oxidative allylation promoted by cerium(IV) (CAN) to establish the all-carbon quaternary chiral center with the proper configuration, and (d) an intramolecular substitution reaction to form the sensitive bridging lactone. Overall, 15 steps (longest linear sequence) are required to prepare the natural product from a commercially available aldehyde, and assembly of the contiguous array of six stereocenters is accomplished with high stereocontrol.

Copper-catalyzed intramolecular N-vinylation of sulfonamides: general and efficient synthesis of heterocyclic enamines and macrolactams

With the catalysis of CuI/N,N'-dimethylethylenediamine, intramolecular C-N coupling between sulfonamides and vinyl halides was successfully implemented, leading to the efficient synthesis of 5-, 6-, 7-, and even 8-membered heterocyclic enamines in both exo and endo modes. The bicyclic enamines thus formed provided a convenient entry to the corresponding 9- to 12-membered lactams by oxidative C=C bond cleavage.

Synthesis of the ABC- and D-ring systems of the indole alkaloid ambiguine G

A glycine Schiff base Michael addition is used in sequence with free radical-mediated aryl amination to ultimately arrive at the ambiguine G ABC-tricycle convergently. Additionally, thermal Diels-Alder cycloaddition of beta-chloromethacrolein with Cohen's diene is used to diastereoselectively construct the D-cyclohexane ring bearing a neopentyl chloride.

An unprecedented radical ring closure on a pyridine nitrogen

An unprecedented radical ring-closure onto the pyridine nitrogen was observed when certain types of substituents were present around the pyridine nucleus.

The first azacyclopentenyl carbinyl radical isomerizations (ACCRI): independent use of steric and electronic (polarization) effects as gating elements

The first examples of the azacyclopentenyl carbinyl radical isomerization are described within a series of enantiomerically enriched 2-substituted indolines, a substructure found extensively in both heterocyclic and natural product chemistry. The isomerization was identified by the varying loss of enantiomeric enrichment (ee) of imines during aryl radical cyclizations to azomethine nitrogen. Independent modification of the steric and electronic nature of the ring substituents revealed the full spectrum of sensitivity to these variables and ultimately defined the use of these effects as gating elements. An example is also given in which a 1,4-amino group transfer is effected via the isomerization mechanism. Analogies are drawn between the title isomerization and the azacyclopropyl carbinyl radical isomerization that has been studied in both chemical and biological contexts.

Cyclizative radical carbonylations of azaenynes by TTMSS and hexanethiol leading to α-silyl- and thiomethylene lactams. Insights into the E/Z stereoselectivities

Free-radical mediated cyclizative carbonylations of azaenynes were carried out using TTMSS as a radical mediator to compare the efficiency and the stereochemistry with those using tributyltin hydride. Using a substrate concentration of 0.1 M, the reactions gave good yields of alpha-silylmethylene lactams having four to seven-membered rings. The observed E-diastereoselectivity of the resulting vinylsilane moiety is in sharp contrast to the Z-selectivity observed during the analogous carbonylation using tributyltin hydride. When hexanethiol was used as the radical mediator, alpha-thiomethylene lactams were formed with E-favoring stereoselectivity again. Ab initio and DFT molecular orbital calculations on the stability of E and Z products were carried out for a set of five-membered methylene lactams bearing SnH3, SiH3, and SMe groups. The distinct thermodynamic preference for the Z-isomer was only predicted for the Sn-bearing lactam. A steric effect due to the bulky (TMS)3Si group is proposed for the E-selectivity observed in the TTMSS-mediated reaction.