Synthesis of α-Acyloxynitrones and Reactivity towards Samarium Diiodide

Upon treatment of fructose-derived nitrone 1 with samarium diiodide, with the aim of inducing its umpolung and coupling with electrophiles, an unexpected β-elimination of the benzyloxy group at C-1 was observed, yielding nitrone 2. The mechanism and scope of this transformation were investigated, from nitrone 1 and from five other α-alkoxy or α-acyloxy nitrones. This paper describes the preparation of new nitrones 3 and 4 from d-fructose; their synthesis demonstrates that acyl-protected nitrones can be prepared efficiently by intramolecular N-alkylation of sugar-derived oximes, a strategy previously developed only for ether- or ketal-protected cyclic nitrones. Nitrones 3 and 4 underwent fast β-elimination upon treatment with SmI₂. Contrastingly, no β-elimination occurred with nitrones 5–7, suggesting a specific behavior of endocyclic keto nitrones.

The synthesis of novel acyl-protected d-fructose-derived keto nitrones is described. The latter reacted readily with SmI₂ to regioselectively eliminate one of their α-substituents through formation of an original samarium–enaminolate intermediate. However, the efficiency and selectivity of this process does not extend to non-endocyclic α-heterosubstituted nitrones.

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