Synthesis of a novel diene from a cyclobutane precursor: an entry to 2,9-disubstituted [2]diadamantanes

In the gas-phase thermolysis (350 °C) of heptacyclic cyclobutane 2, n=1, (the [2+2] dimer of noradamantene) two isomeric dienes (the symmetric 5, n=1 and the asymmetric 6a) and the reduction product [2]diadamantane (7-H) are formed. Computations suggest that at 350 °C a diradical intermediate (8, n=1, derived from C–C bond cleavage of 2, n=1) gives rise to 5, n=1. Diradical 8, n=1 is also likely to abstract allylic hydrogens from 5, n=1 affording 7-H, and also to isomerize 5, n=1 to give the diene 6b from which 6a can result as the more stable diene of a Cope rearrangement. The gas-phase thermolysis of 2, n=1, in the presence of iodine affords 7-H at 350 °C and the new compound 2,9-diiodo-[2]diadamantane (7-I2) at 150 °C. 7-I2 can be easily prepared at room temperature by the iodination of 2, n=1 in CH2Cl2 and affords in a synthetically useful way diene 5, n=1 by reduction with Na/Hg at room temperature.