Contact your local ticket office to see if your preferred itinerary is possible to book, and in the affirmative whether you like the fare.

Anyway, I wouldn't hesitate to book the later TK1844 with only 1h10min at IST. Most likely you will make it. In the event of missing TK79 to SFO you most likely will be put on the next available TK flight to SFO (in business class) (same day or the following morning?) rather than via BOS or BOS (unless you specifically ask for this, but no guarantee that it will be granted). There is no reason to take the risk of loosing business class seats into the equation.

Anyway for flights 5 months out there is a significant risk of schedule changes or cancellations (TK is notorious for sch. changes and cancellations) which may play in your favour later (any cancellation or sch. change more than 15min will alllow you to rebook -7 / + 30 days and even to change destination and origin).