ok guys so this is a kind of question that a lot of undergraduates are going
to see in your organic 1 or organic 2 classes and this is a retrosynthetic
analysis problem, so a very common kind of question is
gonna give you a molecule like this and say: build this from some given starting
material. so I'm telling you we want to build this compound from alcohols
of five or fewer carbons so if this seems confusing you're gonna wanna go
ahead and watch my tutorial on retrosynthetic analysis.
ok so whenever we're doing doing retrosynthetic analysis we want to ask ourselves two questions
number one: what kind of molecule am i trying to make? number two: what kind of
reactions do I know that make those kinds of molecules? looking at the target
molecule this is an alcohol and so we want to go through our database of
reactions and say what reactions do I know that make alcohols? now if we are in
an organic 1 course or maybe an organic 2 course we know that Grignard
reactions make alcohols so a Grignard reaction is going to be very good
approach because number one: it forms alcohols and also makes new carbon
carbon bonds so it takes
pieces, takes small molecules, and combines them so when we think retro
synthetically we're going to pull apart this molecule in ways that make sense
because we will know reactions that will put them together in the way that we
just pulled them apart. so when we do a Grignard reaction we know that the
nucleophilic carbon, the carbon that is attached to the magnesium is always going
to form a new sigma bond with a carbonyl carbon. and the carbonyl carbon, the
carbonyl becomes an oxyanion which is protonated, and so what we understand is
that any bond that is stemming from the carbon that was the carbonyl carbon can
have been the bond that was formed. in other words this carbon right here was
the carbonyl carbon, right, because the carbon that was the carbonyl carbon
will remain attached to that oxygen atom and so that means that either of these
two bonds, either of these two bonds can have been a bond that was formed in a
Grignard reaction. and so let's again look at our restraints though in order
to choose the correct bond, that means we can either fragment here or
here, but if we fragment here we have a two carbon fragment and then a six
carbon fragment, but we said that we need alcohols of five or fewer carbons so this
is gonna make it a lot easier on ourselves if we do this, so that's where
we're going to break this molecule apart and if we go retrosynthetically
remember that this kind of back arrow is a retrosynthetic arrow it does not mean
a reaction, it means back in time, and so what can have come from this is that, and
and that, so we have these two compounds and this is a valid retro
synthetic step because if we combine these compounds we will get that.
and so now we still have more to do because we said we need to be starting
from alcohols, well where did these in turn come from?
well this can have come from an alcohol, this retrosynthetic step is
valid and remember that this does not imply a reduction, what this means is a
reverse oxidation, this is back in time this is valid because we know that if we
have this propanol if we oxidize it with PCC we can get that aldehyde. and then from
here we know that this can have come from the alkyl bromide, which can in turn
have come from the alcohol. these again are valid retrosynthetic steps because
there is some reaction that we know that would induce the transformation in the
forward direction. so this so now, we have our our starting material
this is a five carbon alcohol, this is a three
carbon alcohol, these meet the criteria that we stated about valid starting materials
so now that we have the valid starting materials let's just discuss in the
forward direction what needs to happen so that we can prove to ourselves that
this is a valid synthetic pathway. so let's first take our our propanol there
here's our propanol, we know that if we use PCC, PCC is the way to get an
aldehyde from an alcohol, from a primary alcohol, so we just did the forward
direction of this retrosynthetic step, now let's also build up this fragment so
let's take our five carbon alcohol, we know that if we use PBr3 we can
turn an alcohol into an alkyl bromide, and then we also know that if we use
magnesium in ether we can turn this into a Grignard reagent, and then now we
know if we take this guy and this guy and combine them and then do some
acidic work-up, we will get our product because this carbon would attack that
carbon, which would make this bond that we initially broke apart in the first
step of the retrosynthetic analysis. so once again to summarize the key is to
look at what kind of molecule we are making, in this case an alcohol, and then
think: what kind of reactions do I know that make alcohols? in this case Grignard
was our choice. pull the parent molecule, pull the target molecule
apart in a way that makes sense because we know the reaction that will put it
back together, and then go backwards until we have starting material that
meets the criteria of the problem.
thanks for watching guys, subscribe to my channel for more tutorials and as always
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