This invention relates to the field of organic chemistry, particularly to application of substituted 5-methylidene-1,3-dioxane-4,6-dione as a synthetic equivalent of an aldehyde. The method is intended for synthesis of aldehydes as well as in situ synthesis of them followed by convenrsion of them to 2,4-dioxaspiro[5.5]undecan-1,5,9-tiones or Schiff bases.