Synthesis and mass spectra of 4-O-acetyl-1,5-anhydro-2,3,6-tri-O-ethyl-d-glucitol and the positional isomers of 4-O-acetyl-1,5-anhydro-di-O-ethyl-O-methyl-d-glucitol and 4-O-acetyl-1,5-anhydro-O-ethyl-di-O-methyl-d-glucitol