NEW MATERIAL:The compound of formula I (only one of the dotted lines is C-C double bond; R is 1W5C alkyl).

EXAMPLE: Ethyl α-monocyclofarnesylacetate.

USE: Useful as an agent for imparting or modifying fragrance, a synthetic raw material of other perfume, pharmaceuticals, etc., or their intermediate. It has fragrance of floral note and having excellent durability. Excellent effect can be obtained by adding to foods, drinks, soap, detergent, cosmetics, sanitary goods, pharmaceuticals, and various other materials.

PROCESS: The monocyclofarnesyl of formula 1 can be prepared by reacting the compound of formula 5 with MgX of formula 4 in the absence or presence of e.g. an organic solvent, reacting the reaction product with a compound including the novel compound of formula 3, and reacting the product with the orthoacetic acid ester of formula 2 in the presence of an acid (e.g. acetic acid, propionic acid, etc.).