I'm not sure what you meant to say here.
i5 processors max out at x600K (with "x" as the series number), so a x700K would be an i7.
The 6000-series Intel processors are "Skylake". So the Skylake unlocked i5 is the 6600K.
The 7000-series Intel processors are "Kaby Lake". The Kaby Lake unlocked i5 is the 7600K.
Always refer to the motherboard's documentation, but it looks like generally the 200-series motherboards (which are made to pair with the 7000-series processors) should be able to accommodate last generation (6000-series) processors. Going the other direction though, namely putting a 100-series motherboard (meant to pair with a 6000-series processor) with a 7000-series processor, will generally require a motherboard BIOS update, which you can't do without a different processor in the board.
If you have a Kaby Lake build planned, the 7600K i5 with that motherboard (PRIME Z270-A) will be fine.
The site for that board (https://www.asus.com/us/Motherboards/PRIME-Z270-A/) also specifically mentions 6th generation ("Skylake") compatibility, so a 6600K will be fine too.
If you have an i7 planned instead, that will be fine too.