(3'r,4's,5s,5's,6'r)-3',4',5'-Tris-benzyloxy-6'-benzyloxymethyl-3-(4-ethylphenyl)-3',4',5',6'-tetrahydro-2-trimethylsilyl-spiro[thieno[2,3,f]isobenzofuran-5(7h),2'-[2h]pyran]