N-heterocyclic Carbene-catalyzed Transformations Of Silyl Enol Ether

The past decade has witnessed tremendous progress of N-heterocyclic carbene (NHC) catalysis. In particular, NHCs exhibit highly reactivity toward the activation of silicon-based reagents. Butenolides are a common structural subunit found in natural products and biologically active compounds. Functionalized olefines analogues are considered to be versatile building blocks in organic synthesis. In this paper, N-heterocyclic carbene-mediated transformations of silyl enol ether was mainly studied, including three parts as follows:1. A N-heterocyclic carbene-catalysed vinylogous Mukaiyama aldol reaction of ketones was developed. Under the catalysis of 5 mol% NHC catalyst, a-ketoesters and a-trifluoromethyl ketones reacted with 2-(trimethysilyloxy)furan efficiently to produce y-substituted butenolides containing adjacent quaternary and tertiary carbon centers in high yields with good diastereoselectivities. Possible reaction pathes about silicon activation were proposed. Two possible open-chain transition-state models were also proposed. The efficient, transition-metal free conditions and simple procedure offers a novel protocol for the diastereoselective preparation of γ-substituted butenolides.2. N-heterocyclic carbenes have been utilized as highly efficient nucleophilic organocatalysts to mediate vinylogous Mukaiyama Michael reaction of 2-(trimethylsilyloxy)furan with enones to afford y-substituted butenolides in 44-99% yields with 3:1-32:1 diastereoselectivities. Based on the pioneering work on NHCs-catalysed transformations of silylated reagents, a plausible mechanism was proposed and the reaction mechanism was further studied by NMR experiments. The mild and transition-metal free conditions, high yields and excellent diastereoselectivities provide a novel approach for the synthesis of y-substituted butenolides.3. N-heterocyclic carbenes (NHCs) have been utilized as highly efficient organocatalysts to catalyse Peterson olefination reaction of aldehydes with trimethylsilylketene ethyl trimethylsilyl acetal to produce the corresponding functionalized olefines in 34-93% yields with excellent stereoselectivities. The extremely mild condition, simple procedure and excellent stereoselectivities provide a novel protocol for the synthesis of these important building blocks.In summary, a new method for synthesis of y-substituted butenolides and functionalized olefines based on organocatalyst-NHCs was developed. This method has many advantages such as simple procedure, mild condition, high yield and selectivity, environmental friendly and so on.