The action of sodium nitrite–acetic acid on α- and β-pinene. Participation by a neighbouring geminal dinitro group in the base-catalysed epimerization of a secondary alcohol

Abstract

The reaction of (–)-α-pinene (1) with sodium nitrite–acetic acid gave principally (2S,4R)-2-nitromentha-1(6),8-diene (2) and smaller amounts of (2S,4R)-2-nitromenth-1(6)-en-8-yl acetate (3), (1R,4S)-2-endo-nitrato-6-endo-nitrobornane (4), (1S,4R)-3,3-dimethyl-2-endo-nitrato-6-exo-nitro-8,9-dinor-bprnane (5), and (–)-3-nitropin-2-ene (6). A similar reaction with (–)-β-pinene (8) afforded (4S)-7-nitromentha-1,8-diene (9) as the major product and (4S)-7-nitromenth-1-en-8-yl acetate (10), (1S,4S)-2-endo-nitrato-10-nitrobornane (11), and (1R,4S)-3,3-dimethyl-2-endo-nitrato-10-nitro-8,9-dinorbornane (12) in lower yields. Treatment of the nitro-nitrates (11) and (12) in basic medium gave the corresponding 10,10-dinitro alcohols (20) and (21) by intramolecular transfer of the nitro group with retention of configuration at C-2. In stronger alkaline conditions the dinitro endo-alcohol (21) produced unexpectedly the epimericexo-alcohol (22). The neighbouring participation of the 10,10-dinitro group is shown to be essential for the epimerization to take place. Plausible mechanisms for these reactions are proposed.