You can try YYZ-PEK-SIN, with AC from YYZ-PEK and CA from PEK-SIN. Or YYZ-PVG-SIN, with AC from YYZ-PVG and CA from PVG-SIN.

If you are not a fan of CA, you might want to consider YYZ-NRT with AC, NRT-SIN with NH.

Alternatively, UA also offers flights to SIN from ORD (with layover in HKG).

For non-Star Alliance airlines, you can go for CX, YYZ-HKG-SIN (my favourite option). Or BR, YYZ-TPE-SIN.

I would say booking on separate itineraries sometimes are indeed cheaper