7-Acetylbenzo[b]naphthol[1,8-f,g]thiazocin-8(7H)-one (sulfide-N-acetyllactam 3) and its 13-oxide (sulfoxide-N-acetyllactam4 , exhibiting polymorphy, 4a and 4b) have been prepared and their structures established by X-ray crystallography from diffractometer data. Compound 3 (C 1 9 H 1 3 NO 2 S) crystallizes in the monoclinic space groupP 2 1 /c with a = 10.716(3), b = 9.222(2), c = 16.077(5) , β = 106.68(2)°, V = 1521.9(7) 3 , Z = 4, D c = 1.394 g cm - 3 and μ(Mo Kα) = 0.22 mm - 1 . Compound 4a (C 1 9 H 1 3 NO 3 S) crystallizes in the monoclinic space group Pc with a = 7.821(1),b = 24.795(6), c = 8.342(1) , β = 101.15(1)°, V = 1587.2(4) 3 , Z = 4 (two molecules,4a and 4a , in the asymmetric unit), D c = 1.40 g cm - 3 and μ(Mo Kα) = 0.21 mm - 1 . Compound 4b also crystallizes in the monoclinic space group P2 1 /c with a = 11.791(1), b = 10.719(1), c = 25.193(2) , β = 96.02(1)°, V = 3166.4(4) 3 , Z = 8 (two molecules, 4b and 4b in the asymmetric unit), D c = 1.394 g cm - 3 and μ(Cu Kα) = 1.92 mm - 1 . The crystal structures were solved by direct methods and refined to R = 0.052 for 3, R = 0.088 for 4a, and R = 0.048 for 4b, by using 9413, 4759 and 5195 reflections, respectively. Owing to the cis-amide structure, compounds 3 and 4 assume a boat-like conformation with transannular sulfur-nitrogen close contact; r(S N) = 2.968(1) for 3 and 2.753(3)-2.832(2) for 4a and 4b . The out-of-ring position of the sulfinyl oxygen leads to a sulfurane-like trigonal bipyramidal geometry about sulfur in 4a and 4b ; θ(O S N) = 175.8(3)-178.7(2)°. The interatomic distances r(S O) = 1.483(2)-1.498(3) and r(O S N) = 4.245-4.314 are in agreement with an S O double bond and S N close contact rather than with an O-S-N type molecular structure observed in spirosulfuranes. The stereochemistries of 1,5-thiazocine and 1,5-thiazonine rings are compared.