[allthegoodnamesargon]

This is a question regarding oxidative hydroboronation of alkenes.

As far as I'm aware, (+)car-3-ene followed by treatment of diborane and H₂O₂ leads to the formation of (-)-cis-caran-trans-4-ol. I think steric hindrance due to the methyl group is the reason why trans conformation is favored. My question is can cis-caran-cis-4-ol be formed from cis-caran-trans-4-ol ?

Also, what other side products are possible? I can't think of any because there's only one C=C bond to "attack" the empty p-orbital of Boron. Could it be possible that Boron can form single bonds with the more substituted carbon?

Image of (+)car-3-ene:

[OC pro]

After hydroboration you will get the trans product as you mention correctly. Now, you can invert using a Mitsunobu using a Carboxylate anion (typically benzoic acid is used) to build up the cis/cis benzoic acid ester. After hydrolysis you will end up with the alcohol with methyl group and alcohol on the same side.
See also at http://www.orgsyn.org/orgsyn/orgsyn/prepContent.asp?prep=cv9p0607
There is almost the same problem.

Edit: concerning your second question: the borane addition should work well. I think you will end up with a mixture of both diastereoisomers favoring the trans product of course. But they should be separable on silica gel (petrol ether with small amounts of diethyl ether, I suppose 10:1 will be a good start for TLC; the compound looks quite unpolar; stain it with Mo-stain)

[allthegoodnamesargon]

Thanks!