.pi.-Selective dichlorocyclopropanation and epoxidation of 9-chloro-1,4,5,8-tetrahydro-4a,6a-methanonaphthalene. Controlled synthesis of the C-9 epimers of (1a.alpha.,2a.alpha.,6a.alpha.,7a.alpha.)-1,8,8-trichloro-1a,2,3,6,7,7a-hexahydro-2a,6a-methanocyclopropa[b]naphthalene