Homologation Strategy for the Generation of 1-Chloroalkyl Radicals from Organoboranes.

The generation of 1-bromo and 1-chloroalkyl radicals from organoboranes has been investigated. The direct approach involving the hydroboration of halogenated alkenes is impeded by partial dehalogenation taking place during the hydroboration process. An indirect method involving the generation of B-(1-chloroalkyl)catecholborane by homologation of B-alkylcatecholborane with dichloromethyllithium was developed. A reaction sequence involving a hydroboration reaction, a Matteson homologation, and a radical allylation process has been performed as a one-pot process that takes advantage of three different reactivities of organoboron species. Starting from styrene derivatives, it was possible to prepare B-(1-chloro-2-arylpropyl)catecholboranes that are excellent precursors to 1-chloro-2-arylpropyl radicals. A concise approach for the synthesis of an optically active α-methylene-γ-lactone from p-chlorostyrene has been developed on the basis of a two-step sequence involving an enantioselective hydroboration-homologation-cyclization reaction followed by a hydrolysis-lactonization process.

Repairing the thiol-ene coupling reaction.

Thiol-ene coupling (TEC) reactions emerged as one of the most useful processes for coupling different molecular units under reaction mild conditions. However, TEC reactions involving weak CH bonds (allylic and benzylic fragments) are difficult to run and often low yielding. Mechanistic studies demonstrate that hydrogen-atom transfer processes at allylic and benzylic positions are responsible for the lack of efficiency of the radical-chain process. These competing reactions cannot be prevented, but reported herein is a method to repair the chain process by running the reaction in the presence of triethylborane and catechol. Under these reaction conditions, a unique repair mechanism leads to an efficient chain reaction, which is demonstrated with a broad range of anomeric O-allyl sugar derivatives including mono-, di-, and tetrasaccharides bearing various functionalities and protecting groups.

Modular Synthesis of Highly Substituted Pyridines via Enolate α-Alkenylation.

A novel methodology for the synthesis of highly substituted pyridines based on the palladium-catalyzed enolate α-alkenylation of ketones is presented; the formation of aromatic compounds is a new direction for this catalytic C-C bond forming reaction. In the key step, a protected ß-haloalkenylaldehyde participates in α-alkenylation with a ketone to afford a 1,5-dicarbonyl surrogate, which then undergoes cyclization/double elimination to the corresponding pyridine product, all in one pot. The ß-haloalkenylaldehyde starting materials can be obtained from the corresponding methylene ketone via Vilsmeier haloformylation. Using this concise route, a variety of highly substituted pyridines were synthesized in three steps from commercially available compounds.