Efficient synthetic routes for the preparation of rhinovirus protease inhibitors of formula I, as well as key intermediates useful in those synthetic routes. The sound-field production apparatus makes it possible to reproduce an impressive sound field that gives a better feeling of presence on the scene as a sound field in a movie theater or the like does by creating a sound-field space having an effect of sound-image broadening. The bidirectional switching element is connected to ends of the bipolar memory element, and has a bidirectional switching characteristic.