To obtain a new compound useful as an intermediate for producing triethylsilyl ether analogs of CD ring fragment which are raw materials of vitamin D3 analogs expected as being effective for osteoporosis treatment.

This compound is a compound of formula I (R1 is a lower alkyl), for instance, [1aS-(1aα,3aβ,6aα,6bα)]-[octahydro-3a-methyl-2H-indeno[4,5- b]oxiren-4-ylidene]acetic acid ethyl ester. The compound of formula I is obtained, for instance, in the case of the above-illustrated compound, by reacting a compound of formula II with triethylphosphonoacetate in the presence of sodium ethoxide.