A series of phthalic anhydrides underwent a 1,3-dipolar cycloaddition reaction with N-benzylazomethine ylide, formed in situ from N-(methoxymethyl)-N-(trimethylsilylmethyl)benzylamine and a catalytic amount of trifluoroacetic acid, to produce unstable spiro(isobenzofuran-1,5′-oxazolidin)-3-ones. The spiro-fused oxazolidines were reduced with sodium borohydride to afford 1(3H)-isobenzofuranones, which were generally isolated in moderate to high overall yields.