An efficient synthetic strategy to obtain 1-chloro-Cs-trishomocubane and 1-chloro-D3-trishomocubane is described. 1-Chloro-Cs-trishomocubane is synthesized by a regioselective Diels–Alder reaction, and B3PW91/6-31G(d,p) calculations offer a plausible explanation of the reaction mechanism. Surprisingly, 1-chloro-Cs-trishomocubane does not undergo an acid-catalyzed rearrangement to form 1-chloro-D3-trishomocubane and was obtained by chlorosulfation of Cookson's diketone. A possible mechanism of the reaction involving the formation of Cs- and D3-trishomocubane nonclassical cations was proposed on the basis of a mechanistic [B3PW91/6-31G(d,p) and MP2/cc-pVDZ] study.