Synthesis of trialkylsiloxy‐substituted and other 5,6‐dihydro‐4H‐1,2‐oxazines by hetero diels‐alder reactions of nitroso alkenes — Preparative scope and diastereoselectivity

A variety of 5,6‐dihydro‐4H‐1,2‐oxazines 3 and 4 is prepared in good yields from silyl enol ethers 1 and nitroso alkenes 2a or 2b, respectively. A systematic variation of substituents reveals preparative scope and limitations of this hetero Diels‐Alder reaction with inverse electron demand. The cycloaddition is rather sensitive with regard to steric effects – in particular, large groups at C‐1 of the silyl enol ethers 1 completely prevent the reaction. On the other hand, excellent diastereo‐selectivities are observed employing appropriately substituted olefins. The presented method for the synthesis of 1,2‐oxazines also displays high regio‐ and periselectivity as is demonstrated by transformations 1r→3r and 13→14.