There are only a few *A gateways to ATH. From boston, the most sensible appears to be LH through FRA on 747/AB3. Several more or less practical 2 stops are possible as well such as BOS-IAD-VIE-ATH on UA/OS, BOS-IAD-MUC-ATH on UA/LH, BOS-NYC/IAD-WAW-ATH on UA/US/LO, as well as a bunch of 2 stop options through FRA. Of course there's also BKK and SIN, but that's probably not what you had in mind.

By the way, when did LH end the BOS-MUC flight? Is it likely to resume in the spring?

Michael