Iodo-cyclization of N-homoallyl thioamides was carried out in the presence of Et3N to form 2,4-diaryl-5,6-dihydro-4H-1,3-thiazines in good to high yields. The reaction of thioamides bearing 1-naphthyl or 2-methoxyphenyl group at the carbon atom adjacent to the nitrogen atom showed high diastereoselectivity. The relative stereochemistry was confirmed by X-ray molecular structure analyses. The obtained thiazines were converted to 6-alkylidene-1,3-thiazines by treating with pyrrolidine.