Acid-Catalyzed Rearrangement of [m.n.2]Propellanones

The acid-catalyzed rearrangement of [m.n.2]propellanones (m = 3-5, n = 3, 4) was studied to ascertain the effect of ring size on the mode of rearrangement. Propellanones 9 and 10, containing a five-membered cycloalkanone ring, did not rearrange. Propellanones containing a six- or seven-membered cycloalkanone ring (11-14) rearranged smoothly to cyclopentanones 17a, 18, and 19 in a nonnucleophilic medium. In the presence of a nucleophile, the course of the rearrangement depended on whether the third cycloalkane ring contained three, four, or five carbon atoms. Thus propellanones 11 and 14 rearranged to (1S*,5R*,6S*)-tricyclo[4.3.2.0^1,6]undecane derivatives 20a,b and (1S*,6R*,7S*)-tricyclo[5.3.2.0^1,6]dodecane derivatives 22a and 23 by an unusual 1,2 alkyl shift of the central propellane bond followed by nucleophilic attack. The structures of 20a,b, 22a, and 23 were established by chemical transformations (Scheme II). The stereochemistry of methyl substituents on the cyclobutane ring of dimethyl[5.3.2]propellanones 15 and 16 influenced the course of rearrangement in the presence of a nucleophile.