Last Friday I tweeted this thought (with some typos). I think Romans is best understood if interpreted in a series of loops:
5:12-21 interprets 1:18-31
6:1-8:1 interprets 2:1-3:31
8:2-11:36 interprets 4:1-5:11 (with a small connection between 5:1-12 and 8:18-39)
12:1-16:27 interprets 6:1-8:1 (maybe 1:18-31 and 5:12-21 as well)
What do you think of reading Romans with these looping arguments in mind? Does passage 2 explain passage 1? Can you see key words and concepts, thematic connections, and the unfolding process?
the archives near Emmaus 
How does Romans 1:1-17 fit here?
I think as an introductory key the whole thing: Paul’s Gospel about Jesus, the Son of David, foreseen by the prophets, raised by the Spirit, available to Jew and Gentile, vindicating God.