I’m not sure which trans decalin alkene you’re referring to, but to answer your second question, opening of epoxides happens in a very specific way – it occurs so that both substituents will end up axial so the half-chair can relax into a boat. It’s called the Furst-Plattner rule. https://en.wikipedia.org/wiki/F%C3%BCrst-Plattner_Rule

Remember that when two adjacent substituents on a ring are “cis”, one is axial and the other is equatorial. Takes some time to get used to, but they are on the same side!

How did you connect the middle carbons ( the carbons that create the bridge) in a cis confirmation?

Glad to hear, thank you for stopping by, Komal.

I’m assuming you’re talking about the isomer of dimethyl decalin where there are methyl groups on the ring junctions. Yes, with cis decalin those methyl groups would have a dihedral angle of 60 degrees to each other (staggered) whereas in trans decalin they would be staggered.