The Minimum Connection Time at LAX for an international to domestic connection from SQ to UA is 2 hours. (This is the standard I-D at LAX; there's no airline-specific override in place). SQ won't even sell you this ticket.

The connection wouldn't be airside, BTW; you end up landside after you exit customs. You can go back through security at TBIT and then walk to T7, as opposed to taking the inter-terminal shuttle, which is what I think you meant.