UCLA

General base-catalyzed double-Michael reactions of allenes with various dinucleophiles are described. The reactions are facilitated most efficiently by a catalytic amount of trimethylphosphine, affording six different types of benzannulated five-membered heterocycles: benzimidazolines, benzoxazolines, benzothiazolines, 1,3-benzodioxoles, 1,3-benzoxathioles, and 1,3-benzodithioles. This atom-economical reaction is operationally simple and provides the product heterocycles in good to excellent yields. Careful mechanistic studies unveiled the phosphine-triggered general base catalysis pathway. Furthermore, the double-Michael reaction can serve as an alternative method for the selective mono-ketalization of beta-diketones.

We have employed a novel acid-mediated nitro-Nazarov reaction of conjugated nitrodienes to synthesize a variety of five-membered cyclic nitronates via a proposed nitro-allyl cation intermediate. An array of nitronates with varying alkyl and aryl substituents was isolated in good to excellent yields. To verify the existence of the nitro-allyl cation intermediate, we synthesized nitrodienes tethered to an aromatic ring in hopes of trapping the cationic nitro-Nazarov reaction intermediate by electrophilic aromatic substitution. We were pleased that tetracyclic nitronates were constructed through an interrupted nitro-Nazarov cyclization reaction, which generated two new rings and an all carbon quaternary center. A five-membered cyclic nitronate was then subjected to a [3+2] dipolar cycloaddition with acrylates to obtain highly functionalized nitroso acetals. Reduction conditions were also performed on the cyclic nitronate to obtain a dihydroisoxazole and a four-membered cyclic nitrone.

Progress toward synthesizing alpha-aryl vinylboronic acids was fruitless. Transmetalation with trimethyl borate and aryl vinylaluminum intermediates did not transpire as hoped. The substrates that were successful based on thin layer chromatography (TLC) proved difficult to purify due to boronic acids being highly soluble in water. As of now, the condition in synthesizing and purifying alpha-aryl vinylboronic acids is still pending.