Shimano Di2 Minimum Junction Setup
According to Slowtwitch Shimano needs one junction A in the system otherwise it won't work.

You can skip junction B. 

It may cause some interesting wiring.

The 6700 series (EW6770) junction may make things smaller/easier. 

This is the same answer for all current e-tube di2 systems. (6700,6800,9000)