I second the recommendation to grab an earlier flight. If you miss the SFO-FRA flight there's not a lot of other options. The LH flight is about an hour later, but it's often pretty full. The next UA departure is not til evening.

If you have status you can do a SDC for free if the fare class is available. You can also just ask nicely if there are any delays in the system at all.

As others have noted, DO NOT exit the terminal. The airside transfer is just ever so slightly subtle. At the far end of the domestic terminal, there's a junction...left leads out of the secure area by the elite security and right leads to a bridge to the international terminal.