The reaction of 0-ethylphthalidium tetrafluoroborate (1) with excess NaOCH3 in CHBOH gives ortho ester 4 as the major product. The mechanism of formation of this solvent-derived material is explored. A plausible intermediate, 1-ethoxyisobenzofuran (8), has been prepared and isolated in solution; 8 reacts readily with: eg, CH30D to give the mixed ortho ester 5 containing ca. one deuterium/mole. This indicates that 8 reacts by rate-determining ...