The invention relates to improved methods of preparing dihydrothienopyrimidines of formula 1, and intermediates thereof, (I) wherein X is SO or SO2, preferably SO, and wherein RA, R1, R2, R3, R4 and R5 have the meanings given in the description. The methods according to this invention are more suitable for large-scale synthesis of said compounds than prior methods because the new synthetic process avoids distillation and chromatographic purification between steps and results in a higher overall yield of the desired product.