Abstract
The enol ether 7 undergoes rapid conversion into a 2:3 mixture of cis aldol 9 and trans aldol 11 in the presence of copper(II) triflate (0.1 equiv) and water (1 equiv) in MeCN with no intermediate formation of the corresponding ketoaldehyde 12. In the presence of oxygen, slow oxygenation of the aldols takes place to give the peracetal 13 and trioxane 14 (93:7, 47% combined yield), and the methoxyaldehyde 8 in 12% yield. It is proposed that retroaldolisation to an enol or Cu(II) enolate which is oxidised to an enol radical is the key step. Oxygenation of trans aldol 11 with Cu(II) O 2 in the presence of MeOH gave the peracetal (37%), trioxane (2%), and the methoxyaldehydes 8 (4%) and 10 (4%). The cis aldol 9 reacted, by comparison, very slowly under the same conditions.
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