Challenge:Russian conquest of Constantinople

What would need to happen for the largest Orthodox Empire to take back the birthplace of there faith?

Rules
1)must happen between 1740 and 1870
2)Austria must remain antagonistic between both the Russians and the Turks.
3)No wanking the Russians
4)Russians can only conquer Constantinople, Thrice and nearby parts of Anatolia(allied nations can carve out parts of Anatolia thou)
 
Make Russia a bit better prepared for 1828-1829 war and Diebitsch can take Constantinople. To hold it against Austrians and especially Britons (who were actually a bigger problem for Russian designs in region) would be completely different thing, as Britons would have kittens as big as elephants.
 
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