NEW MATERIAL:The compound of formula I (X is halogen, OH or H; R is H or OH-protecting group; Ar is aryl).

EXAMPLE: ( 2Z,6Z,10Z,14Z )-16-benzyloxy-8-p-trisulfonyl-2,6,10,14-tetramethylhexadeca-2,6,10,14-tetraen-1-ol.

USE: A synthetic intermediate of natural polyprenols useful as pharmaceuticals and foods, etc.

PREPARATION: The objective 20C cis-isoprenoid of formula I ' can be prepared by (1) anionizing the α-position of the sulfone of formula II (THPO is tetrahydropyranyloxy) with n-butyl lithium, (2) reacting with the cis-isoprenoid of formula III (Hal is halogen), and (3) removing the OH-protecting group of the resultant compound of formula IV. The obtained cis-isoprenoid of formula I ' can be converted to another 20C cis-isoprenoid of formula I " by conventional halogenation treatment.