A simple and reproducible procedure for the synthesis of bioisosteres of coumarin—1,2-benzoxathiine 2,2-dioxide is presented. The developed method is based on the intramolecular aldol cyclization of derivatives of mesylsalicyl aldehydes in the presence of strong organic bases, where best results were obtained with 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU). It has been shown that depending on the nature of the substituent in the aromatic ring, intermediate aldol adducts (3,4-dihydro-1,2-benzoxathiin-4-ol 2,2-dioxides) in different ratios with title compound are formed. Dehydration of the intermediate aldols with POCl3 led to full conversion into 1,2-benzoxathiine 2,2-dioxide derivatives. The scaffold of 1,2-benzoxathiine 2,2-dioxide is unequivocally proven by a single-crystal X-ray structure.