Do you expect the Arduino to wake up from power down mode by a signal on the I2C bus? This only works if the Arduino is an I2C slave. The ADXL345 is not able to be an I2C master, so your setup won't work. Correct me if I got something wrong.
What's possible, is to connect the INT1 or INT2 of the ADXL345 to either INT0 or INT1 of the Arduino. The interrupt pins are able to wake an ATmega chip from power down sleep mode.