A New Stereoselective Synthesis of Grandisol

Grandisol (10) is stereoselectively synthesized in 9 steps, starting from commercially available 3‐methyl‐3‐buten‐1‐ol (1). The key step of the synthesis is the Cu(I)‐catalyzed [2 + 2]‐photocycloaddition of 3 to 4a or 4b. Moreover, solvent effects on the endo/exo product ratio of the [2 + 2]‐photocycloaddition are investigated.