摘要 |
<p>A compound of formula <CHEM> wherein R and R<1> may be the same or different and each is hydrogen or alkyl provided that at least one of R or R<1> is alkyl and R<2> is alkyl, R<3>OCOCH2 or R<3>OCOCH2CH2 where R<3> is alkyl, or <CHEM></p> |