Synthesis of a 3,4-Dihydro-1,3-oxazin-2-ones Skeleton via an Intermolecular [4 + 2] Process of <i>N</i>-Acyliminium Ions with Ynamides/Terminal Alkynes

Abstract

An approach to access functionalized 3,4-dihydro-1,3-oxazin-2-ones has been developed by reacting semicyclic <i>N</i>,<i>O</i>-acetals <b>5</b> and <b>6</b> with ynamides <b>7</b> or terminal alkynes <b>8</b> in a one-pot fashion. The reaction went through a formal [4 + 2] cycloaddition process to generate a number of functionalized 3,4-dihydro-1,3-oxazin-2-ones <b>9a-9ak</b> and <b>10a-10bc</b> in yields of 34-97%. In addition, the utility of this transformation was demonstrated by the synthesis of (±)-sedamine <b>13</b>.