The intramolecular Diels–Alder (IMDA) reaction of furanose tethered 1,3,9-decatrienes (4a–4r) was investigated under thermal conditions and in the presence of a Lewis acid. The stereoselectivity was determined by establishing the structures of adducts through single crystal X-ray diffraction and 1H NMR spectroscopy. It was found that contrary to expectations, the thermal IMDA reaction of (3E) and (3Z)- 1,3,9-decatrienes proceeded with nearly equal rate and furnished IMDA adducts (6–25) with moderate stereoselectivity. In some cases, rearranged products (9, 12, 17 and 24) arising out of a 1,5-sigmatropic shift, cis–trans isomerization followed by IMDA reaction were formed. In contrast, a Lewis acid promoted IMDA reaction afforded only one adduct albeit in lower yields. Not surprisingly, cis-boat transition states were favored over trans-boat transition states. Experimental results were corroborated with transition state modeling of these reactions by applying density functional theory based electronic structure calculations.