A semisynthetic strategy to access novel kauroxane or beyeroxane compounds from ent‑kauranal or ent‑beyerenal, respectively, is described. A Prins cyclization was used as a key step reaction to synthesize diterpene-tetrahydropyran hybrid compounds with high stereoselectivity by using 3-buten-1-ol as oxane precursor and indium trichloride as the Lewis acid catalyst. The absolute configurations of new compounds were determined by mechanistic means and supported with DFT calculations, as well as considering the stereoselectivity behavior associated with the chiral purity of diterpenes and their conformational restrictions. The structures were elucidated by their physical and spectroscopic data. Finally, the antiproliferative activity of the target molecules was done, being moderately bioactive.