Conjugate addition-rearrangement of N-benzylhydroxy-lamine to lactone
6 provides isoxazolidin-5-one 7 which was in turn mesylated at the hyd
roxy group and subjected to the next skeleton rearrangement to afford
3,5-disubstituted isoxazolidine 10. Standard transformation of 10 gave
negamycin lactone 4.