Discussion
Consider the reaction sequence shown below:
TsCl = p-toluenesulfonyl chloride
The oxidant X used in step 1 is
Answer: B
The oxidant X is OsO4.
Step 1 converts the ring-fusion alkene into a 1,2-diol, and the drawing of the tosylate shows the OTs and OH on the same face (both wedged): a cis diol. Tosyl chloride then esterifies only the secondary OH, since the tertiary OH is too hindered.
Osmium tetroxide adds to an alkene through a cyclic osmate ester, delivering both oxygens to the same face, so it gives exactly this cis diol.
m-CPBA followed by NaOH makes an epoxide and then opens it with hydroxide from the back side, giving a trans diol. NaIO4 cleaves 1,2-diols rather than forming them, and CrO3 oxidises alcohols and allylic positions rather than dihydroxylating the alkene.
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