For stepwise pathways, we considered those beginning with a C-C bond formation between two 1,3-hexadiene molecules at the terminus of the diene moieties and generating a diallyl intermediate, rac-8 or meso-8. Either of the two intermediates contains two stereocenters, but the meso-8 isomer is achiral due to the presence of an inversion center in the structure. Other stepwise pathways involving bond formation at internal sites of the diene moiety are likely to be unfavorable due to the generation of isolated (i.e., not in conjugation with a double bond) radical sites; these pathways are not considered. In the second step, a rotation about the first formed C-C bond followed by a radical recombination or a H-transfer leads to a cycloaddition or ene product. and erythro-4 can be formed from it, though erythro-4 was not observed experimentally.