Acylation of nitronates affords α-acyloxyoxime derivatives via an umpolung functionalization of the α-position. This transformation involves generation of hitherto unknown N-acyloxy, N-oxyenamines and their fast [3,3]-sigmatropic rearrangement driven by the cleavage of the weak N-O bond. The reaction has a broad scope, and it is regioselective in the case of nitronates possessing nonsymmetrically substituted α-positions. Application to the formal total synthesis of clausenamide and cis-clausenamide is presented.
A novel efficient regioselective method for the carbon chain activation of aliphatic nitro compounds is described using a tandem double acylation/[3,3]-rearrangement sequence.
Sequential acylation-silylation of nitroalkanes leads to Osilylated α-acyloxyoximes in high yields. The first step of the reaction involves deprotonation of nitro compound with sodium hydride promoted by DBU or alcohol/15-crown-5 system followed by treatment with acyl chloride. In situ generated acyl nitronate is further silylated by silyl triflate that triggers hetero-Claisen [3,3]-rearrangement within N-acyloxyen-amine moiety furnishing orthogonally protected oxime derivatives. The procedure has large substrate scope for both nitroalkanes and acylating agents (acyl chlorides, chloroformates) and allows tuning of reaction conditions depending on the particular type of substrate. Application of obtained oxime derivatives in organic synthesis is demonstrated.
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