(1S,4R,4'S,4aS,5S,6S,8aR)-4'-benzyloxycarbonylamino-6-triethylsilyloxy-1,3',4',4,4a,6,7,8a-octahydrospiro[furan-2'(5H),5(8H)-1,4-methanonaphthalene]-5',8-dione