As noted by StarG, just make sure your HKG-SIN sector has first. There are a few 744 flights a day on the route with first which are typically timed to connect with the LAX flights, so just make sure you are on one of those.

As for SQ, I believe the only route to LAX with first is via NRT. The nonstop and flight via TPE (if still 777) only have business as well.