Reaction of (1,omega)-N-acylamino alcohols with Lawesson's reagent: Synthesis of sulfur-containing heterocycles

Lawesson's reagent [LR: 2,4-bis(p-methoxyphenyl)-1,3,2,4-dithiaphosphetane 2,4-disulfide] is shown to be a valuable reagent for the ready access of sulfur-containing heterocycles: thiazolines 2 starting from the (1,2)-N-acylamino alcohols 1 and benzothiazines 14 from [2-(N-acylamino)phenyl]alkanols 12. Treatment of (1,2)-N-acylamino secondary alcohols 1a-p with LR gave the thiazolines 1a-p via direct conversion of the alcohols to the respective thiols, and the subsequent thionation of the amide carbonyl, followed by cyclization with the elimination of hydrogen sulfide. However, reaction of (1,2)-N-acylamino tertiary alcohols 1q-u with LR yielded the dehydration products 5-7 and 9. Treatment of [2-(N-acylamino)phenyl]alkanols 12a-f with a molar equivalent of LR yielded the 3,1-benzothiazines 14a-f. In this reaction, the [2-(N-acylamino)phenyl]alkanethiols 13a-e were isolated when the corresponding alcohols 12a-e were treated with 0.5 equiv of LR. Further thionation of 13c with LR also gave 3,1-benzothiazine 14c.