The Synthesis and Reactivity of 1-Nitro-1,3-dienes and the Synthesis of Cyclic β-Silylalkenyl Triflates Through Highly Reactive Vinyl Cation Intermediates
- Author(s): Lee, Craig
- Advisor(s): Kwon, Ohyun
- et al.
The synthesis and reactivity of substituted conjugated nitrodienes was investigated. We have prepared β-alkoxymethyl substituted nitrodienes via electrophilic nitration of 1,3-butadienes to employ in a tandem 6π-electrocyclization/[3 + 2] dipolar cycloaddition sequence with dipolarophiles to generate bicyclic nitroso acetals. Additionally, a series of β-alkenyl tethered nitrodienes was synthesized via a trimethylsilyl chloride (TMSCl) assisted organocuprate addition/elimination into 2-bis(methylthio)nitroethylene to give β-ethylthio nitroalkenes. A palladium catalyzed desulfitative cross-coupling of the β-ethylthio nitroalkenes with vinyl boronic acids gave a variety of acyclic β-substituted conjugated nitrodienes. This highly modular methodology allowed for access to a wide range of acyclic nitrodienes that could be exposed to 6π-electrocyclization/intramolecular [3 + 2] dipolar cycloaddition conditions to provide many multicyclic nitroso acetals. Disappointingly, after screening a multitude of reaction conditions and substituient effects, the substituted nitrodienes failed to give nitroso acetal products.
Formation of highly reactive cyclic vinyl cations and the subsequent trapping by a triflate ion is described. Generation of 6-(trimethylsilyl)hex-5-yn-1-ols through organolithium addition to benzyl- or alkenyl- aldehydes gave a multitude of benzylic and allylic alcohols. Treatment of these alcohols with triflic acid induced stabilized benzylic or allylic cation formation which could be intercepted by a tethered alkyne to provide a vinyl cation intermediate. By taking advantage of the high reactivity of alkenyl cations, typically dissociative anions such as triflate ions can trap the cation, providing 1-(trimethylsilyl)-1-cyclohexenyl-2-triflates. This reactivity can also be extended to produce 3-(trimethylsilyl)-1,4-dihydronaphthalen-2-yl triflates. Using the methodology, the total synthesis of the aryl tetralin lactone β-Apopicropodophyllin was completed, hinging on a key Diels–Alder/retro-Dielsï¿½ï¿½–Alder reaction trapping of an intermediate cyclic alkyne.