you can transfer through marriott, but you will be slashing your points by 1/3 in the process (marriott transfers to TK @ 3:1). The better solution is to book through United or Singapore who are also in Star Alliance (although I'm not seeing TK flights through SG, and their Lufthansa flights are more than United). Looks like UA doesn't show near the amount of seats you can direct through TK, but you can get there for close to the same price in coach and biz isn't that bad,