The 'groups' are chemical structures present in a compound. The student asks the Teaching Assistant (typically a graduate student who is required to do teach) a general question about how to carry out a multi step process to selectively transform specific regions of chemical compounds.

The punchline is that the Teaching Assistant immediately sees the intent of the question, and goes from the general to the specific and replies: [ you won't be able to synthesise LSD with that approach because ] "all of the precursors are also scheduled."