FORMATION OF 2-(1-HYDROXYALKYL)-3-OXAZOLINES FROM THE REACTION OF ACYLOINS AND AMMONIA PRECURSORS UNDER MILD CONDITIONS

摘要

Studies on the reaction between acetoin and an ammonia precursor under mild conditions revealed that two major products were formed, one of which was tetramethylpyrazine (TMP) and the other an unknown compound. By spectral elucidation (MS, IR, NMR), this unknown compound has been identified as 2,4,5-trimethyl-2-(1-hydroxyethyl)-3-oxazoline (OXZ). Two homologs of OXZ were also prepared. A 3-week storage study demonstrated that OXZ was readily formed, after which its formation declined. In contrast, TMP formation gradually increased before it leveled out in 15-20 days. At the end of this experiment, only TMP remained as the product, while OXZ was not found. Mechanistically, it is proposed that condensation of acetoin and ammonia generates alpha-hydroxyimine or alpha-amino ketone by tautomerism, which may lead to product formation via two pathways. One of the pathways is to form TMP via self-condensation of the alpha-amino ketone, dehydration, and dehydrogenation, which is well-known. The other pathway may be that the alpha-hydroxyimine condenses with a second molecule of acetoin and then is cyclized by Schiff base formation to OXZ. These storage results also indicate that the second pathway may be reversible, so that OXZ initially formed is able to be converted back to alpha-hydroxyimine, which, in turn, was tautomerized to alpha-amino ketone, so that the first pathway to form TMP could be followed.