TechPowerUp posted a review on the Intel's Coffee Lake CPUs Likely Compatible With LGA 1151 Motherboards A quote from the article:

Another interesting tidbit to have dropped from recent SiSotware leaks on Intel's upcoming Coffee Lake CPUs is that these could be backwards compatible with LGA 1151 motherboards that rock the 200 series chipset (and perhaps even the 100 series.) This last tidbit seems to be a bit of a stretch, even if it does end up being somewhat of a motherboard manufacturer's choice whether or not to issue updated, supporting BIOSes for the most recent Intel processors on their older boards. But why lose so many sales of motherboards equipped with Intel's upcoming, complimentary 300 series chipsets?

 Intel's Coffee Lake CPUs Likely Compatible With LGA 1151 Motherboards @ techPowerUp