Models for diastereoselectivity in nitronate additions
Dave Evans taught me that you should generally analyze these types of problems by drawing out allyl conformations.



I've drawn the nitronate to minimize 1,3-allylic strain. Now, the top face of the nitronate is largely blocked by the t-butyl carboxylate. I strongly suspect that having a bulking alkyl group there is important to getting good selectivity.

This means that the electrophile will react on the bottom face, and that will lead to the desired stereochemistry depicted in 7 with the nitro group down on the ring.