Nitro ketones in organic synthesis : a new, short synthesis of racemic trans‐2‐methyl‐1,7‐dioxaspiro[5.5]undecane, trans, trans‐ and trans, cis‐2,8‐dimethyl‐1,7‐dioxaspiro[5.5]undecane by Henry reaction

Abstract

The synthesis of spiroketals from nitro ketones is described. Racemic trans-2-Methyl-1,7-dioxaspiro[5.5]undecane (1a), and 2,8-dimethyl-1,7-dioxaspiro[5.5]undecane (1b) are synthesized as a mixture of trans, trans and trans, cis isomers in a ratio of 6.5:3.5 (separable by flash chromatography) by nitroaldol reaction (Henry reaction) of 6-nitro-2-hexanone (2) with THP-protected hydroxy aldehydes 3 on Amberlyst A21. Subsequent dehydration of the obtained nitro alkanols 4 with Ac2O/DMAP affords the nitro alkenes 5 which can be directly converted into 1 by sodium tetrahydridoborate reduction, and refluxing of the reaction mixture after acidification with 2 N HCI.