Diastereoselective allylstannane additions to (S)-5,6-dihydro-2H-5-phenyloxazin-2-one. A concise synthesis of (S)-beta-methylisoleucine.

The addition of allyl stannanes to (S)-4,5-dihydro-5-phenyl-2H-oxazinone (3) was achieved under Brønsted acid catalysis to give 2-allylmorpholinones with high diastereoselectivity (up to dr 14.2:1). The product of dimethylallyltributylstannane addition to 3 was converted to l-beta-methylisoleucine, an alpha-amino acid residue found in the complex, biologically active marine-derived peptides polytheonamides A and B, and polydiscamides A-C.