Singapore Airlines usually makes premium cabin awards available only to their own KrisFlyer members. You can transfer points from Chase, American Express, and Citi into the KrisFlyer program, so their miles are relatively easy to get.
If you want to fly Singapore’s business or first class on more than an intra-Asia flight you aren’t going to be able to use miles from United, Aeroplan, or other Star Alliance programs — most of the time.
Six years ago there was a brief window where Singapore Airlines awards were bookable by partners but there hasn’t been anything like it since.