1,1,2,3,4,4,6-Heptamethyl-1,2,3,4-tetrahydronaphthalene, a novel naphthalenic compound, is useful as an intermediate for the preparation of 5,6,7,8-tetrahydro-3,5,5,6,7,8,8-heptamethyl-2-naphthalenecarbaldehyde. It is prepared by a process consisting in the addition of an olefin of formula ##STR1## wherein R.sup.1 and R.sup.2 represent different substituents and each defines a hydrogen atom or a methyl radical, with p-cymene. 4,4-Dimethyl-2-pentene [compound (III): R.sup.1 =CH.sub.3 ; R.sup.2 =H] is obtained by the co-metathesis reaction of an olefin of formula ##STR2## wherein R.sup.1 and R.sup.2 represent identical substituents designating each a hydrogen atom or a methyl radical, with an olefin of formula ##STR3## wherein R.sup.3 and R.sup.4, identical or different, designate each a hydrogen atom or a methyl radical in the presence of an appropriate catalyst consisting of Re.sub.2 O.sub.7 on an inert solid carrier, or of WCl.sub.6 /(C.sub.2 H.sub.5).sub.2 O/Bu.sub.4 Sn.