The only carriers that serve CAI from IST, as far as I can tell, are EgyptAir and Turkish--both which are *A.

It appears that you can redeem on EgyptAir for 12.5K, but it's not a transfer partner of SPG. TK charges 30K roundtrip, but I'm not sure if it allows one-way flights or not. I know nothing about the fees in either program.

ANA charges 20k for the distance between IST-CAI, and it's a transfer partner of Amex MR. Should you have an absurd number of MR points, than that could be worthwhile, but ANA charges (as far as I know) hefty fuel surcharges--which may or may not apply here.

Just buy the ticket. The taxes on united alone are $190. That'd mean you'd be spending 17.5k for $109 worth of ticket. Plus you'd be forgoing the miles earned.

You'd be redeeming at half a cent per mile. That's a horrendous value for United Miles.