A compound represented by the following formula (I):wherein R1 represents a C1-C19 alkyl, R2 represents a C1-C4 alkyl and Ac represents an acetyl, the compound represented by the formula (I) is optionally contained with up to 10 wt. % of a compound represented by the following formula (II):having a chemical purity of 97 wt. % or higher, wherein in the case where the compound represented by the formula (II) is contained with the compound represented by formula (I), the chemical purity of the mixture of the compound represented by the formula (I) and the compound represented by the formula (II) is 97 wt. % or higher.
