The present invention relates to a compound of formula III, wherein, R is selected from the group consisting of C1-C4 alkyl, substituted or unsubstituted phenyl, and substituted or unsubstituted benzyl, preferably isopropyl; and two Ar groups may be the same or different, and are each independently selected from the group consisting of substituted or unsubstituted aryl groups, preferably substituted or unsubstituted phenyl, such as p-methoxyphenyl and the like, wherein the compound is preferably in a solid form.