CASE SQL statement oracle database

my requirement is if first condition is null then check for the second condition if second condition is null as well then third condition in oracle database, so I am trying to achieve this by case statement as shown below but getting an error as missing expression/missing keyword.

CASE
     WHEN (TRUNC( OTA_EVENTS.COURSE_START_DATE) BETWEEN PER_ALL_ASSIGNMENTS_F.EFFECTIVE_START_DATE AND PER_ALL_ASSIGNMENTS_F.EFFECTIVE_END_DATE ) IS NULL  
     then  (TRUNC(OTA_DELEGATE_BOOKINGS.Date_Booking_Placed) BETWEEN PER_ALL_ASSIGNMENTS_F.EFFECTIVE_START_DATE AND PER_ALL_ASSIGNMENTS_F.EFFECTIVE_END_DATE) 
     ELSE (TRUNC(OTA_DELEGATE_BOOKINGS.DATE_STATUS_CHANGED) BETWEEN PER_ALL_ASSIGNMENTS_F.EFFECTIVE_START_DATE AND PER_ALL_ASSIGNMENTS_F.EFFECTIVE_END_DATE) 
END

Please help if there is any other way to achieve this