3B Chemistry Lecture 19 Tuesday, April 7th, 2009
More on carbohydrates: How we can use carbohydrates.
If you take both of these cis and trans forms and react
them with acetone and an acid catalyst, you know it is a
1,2 diol and reactions with ketones or aldehydes will
give acetals or ketals. We also know we can use this as a
method for protecting diols. We will protect the OH
groups. Weve only reacted one of them and not both;
the cis will form much faster than the trans. The trans
version is obviously more strained. That observation is
important with using sugars in synthesis. Nature
provides us with carbohydrates with lots of stereocenters
which are fixed and potentially an aldehyde group. We
also have a bunch of hydroxyl group. In this next
example we have our squiggly line again which is a
mixture of alpha and beta enamers. This is galactose and
if thrown into aetone with an acid catalyst, what we will
end up with is this molecule. We have protected all but
one hydroxyl group with the acetone function. We have
2 ketals. We started with a mix of alpha and beta in
equilibrium. The hydroxyl group doesnt do anything ; it
can make a hemiketal but it will jus fall apart due to
ketone favorability. Now we can do chemistry at the C6
hydroxyl group. We can oxidize it with PCC and no
water so it can stop at the aldehyde. We can add a Wittig
reagent and then H2, with Pd/C, and lastly we can add
H3O+ which will remove the ketal protecting groups.
Aldehyde, alkene, alkane = we have added a methyl
group to our sugar. The H3O+ will hydrolyze it. We
have 2 enameric products that we get from this reaction;
but isnt there some aldehyde and furanose form
present? Yes, but they arent the major products. The
most important part: weve removed the hydroxyl group
at C6 and made it into a methyl group. We have changed
the sugar quite a bit by doing this.
Anything done to that C6 hydroxy group: you cant use
any source of protons in water. That will kcick off the
protecting groups in the course of the synthesis, so add
H3O+ to the end of the synthesis. Glucose in a mixture
of alpha and beta enamers. Reacting it with excess
acetone will end up getting : furanose. We started off
with a pyranose. What is going on here: the hydroxyl
groups are trans which dont react very quickly. They
can potentially react, but something else is going on
here: we know that it is in equilibrium with a small
amount of the furanose form. The 2 hydroxy groups on
the fischer arent part of the ring. We can do a lot of
things to it like use PCC; however you cant do this with
CrO3, H2SO4 because thats acid. We can then add
LiAlH4 and H3O+. PCC will give us the ketone, and
LiAlH4 will reduce the ketone to an alkoxide. Where is
it going to come from? If it comes from the bottrom
well go to exactly where we started and coming from
the top will give us a new compound. It will come
mainly from the top. It is a bicyclic system= 2 rings.
That big thing is on a single bond and can rotate and
move out of the way of LiAlH4 which is the source of
H-. It approaches the carbonyl as that ion. It is not the
little H- you think of. In reality, it is the big thing that
approaches the molecule. First it comes from the top;
then H3O+ can get us to the last molecule. It is a 6
membered ring now because 6 predominates in solution.
We can do several things with the remaining hydroxyl
group; This was protecting group chemistry in sugars.
Lets take a sugar (glucose) and add an alcohol like
methanol with an acid catalyst. It is a cyclic hemiacetal
and adding an alcohol to it will make an acetal. We get
alpha and beta acetals and their carbohydrate names are
glycosides. We added methanol but lost water. you can
pick whatever alcohol you want. Nature takes sugars and
makes bigger ones using this method. This next
molecule is one sugar: a cyclic hemiacetal reacting with
an alcohol. It just has 5 alcohol groups on it. In this
case, we have this alchol function react with it to create
another glycoside: it is a glycosidic linkage and it is still
an acetal. It is called maltose. This is called an alpha 1,4
linkage. Alpha refers to the enameric oxygen which
forms the glycosidic linkage and then it is 1, (carbon #1)
and 4 is where the hydroxyl group is on the next sugar. It
is also called a disaccharide of glucose. We are only
missing one piece of information: the alpha beta on the
other end of the sugar isnt defined though. This linkage
wont fall apart unless we throw in an acid and then
well get 2 molecules of glucose. It is a disaccharide. A
monosaccharide is any sugar thrown into aqeous acid
and if you break it into constituents that cant do
anything further, we have a monosaccharide. A
disaccharide can be broken up into 2 monosaccharides if
thrown into aqeous acid. We can even have a
trisaccharide; this one in particular is called cellibiose. It
is a glucose molecule that can be rotated 180 degrees. It
is a beta 1,4 linkage. We also have lactose. This is
sucrose. Fructose doesnt prefer furanose form; but an
enzyme can grab the furanose form and link it together
with the pyranose form which is favored. a
bunch Sometimes of sugars can be strung together; how many
monosaccharide units can be in the entire bunch?
Another question: how many different sugars make up
the bunch? DIFFERENT SUGARS. For this molecule,
we would think it has 5 monosaccharide units but we
dont know what each unit is. We have a beta 1,4
linkage and an alpha 1,6 linkage. And then we want to
ignore the alpha or beta because they are up and down in
solution anyway. But all the other hydroxyl groups are
fixed. You need to recognize which sugars are the same.
Two are the same and one is different. So we have 2
different monosaccharides making up the trisaccharide.
You just have to look at something and recognize a
hidden carbonyl group; and what will happen with
H3O+. Well end up with an OH in various locations.
For lactose, if we throw H3O+, well end up with
galactose and glucose. We just took 2 molecules of
glucose, threw in acid, and maltose appeared. We cannot
make maltose in the laboratory though because look at
the situation: we have 5 hydroxylg rouops on the glucose
on the right so we can use any of them to make a
glycositic linkage. And besides that, we have the choice
of making alpha or beta. We know its in solution (a
mixture of both alpha and beta). We have 5 possible
linkages there and then one on the end. It can be alpha or
beta for each glycosidic system. There are over 40
possibilites. Then if you add 2 glucoses together and you
want to stop at 2 glucoses, it will not happen: it will
contiuosly link. We cant do this just by taking 2 sugars;
nature does it all the time though with enzymes. We can
make things like this if we use lots of protecting groups.
Polymers: A bunch of glucoses which are beta 1,4
linked. This is a linear polymer. We have a short hand
abbreviation: its made up of monomers that are
repeating in this case. This is called a biopolymer or a
polysaccharide. It is the most abundanct biopolymer on
our planet. This is cellulose. It makes up 90% of a
cottonball and 50% of a tree. It is a structural polymer.
Cellulose does not dissolve in water. What about all the
hydroxyl groups? Most of them spend their time
hydrogen bonding with other hydroxyl groups of other
cellulose strands that are lined up right next to them.
Strands of cellulose are sometimes braided in between
each other so were left with a rope of fiber that has all
kinds of hydrogen bonding going on in the middle ; on
the outside there are still hydroxyl groups but not
enough to get dissolved in water. Cotton absorbs water
though. Cellulose is a great source of energy too. It has a
lot of glucose in it though. Our stomach is aqeous acidic
so we can release these molecules. Animals make use of
cellulose like cows, sheep, pigs: any hoofed animals that
eat grass. Grass is made of cellulose and it provides
glucose to the animals. In their guts, they have Beta
gluocosidases and it cleaves beta linkages into smaller
pieces and all the animals mentioned have multiple
stomachs so that we can digest and make smaller pieces.
This is why these animals can get energy from cellulose
but we cant. Cellulose has been used for a long time but
it doesnt dissolve in anything which has made it harder.
When you try to melt it, it just decomposes. One thing
you can do is take pyridine and combine it with another
reagent: it will turn each hydroxyl group into an esther.
We took something that wasnt soluble in anything and
turned it into something where no H bonding is possible.
This is starting to look more like an organic molecule
that can dissolve in typical organic solvents. It is called
cellulose triacetate. It dissolves and melts without
decomposing.
If we put cellulose in sodium hydroxide and carbon
disulfide, then we end up with a zanthate salt. RO- +
CS2 = nucleophile adds to electrophile to get the
zanthate. (This is carboxylic acid chemistry). If you add
an acid to a zanthate like sulfuric acid it will get
protonated and is not stable so it will decompose to the
starting alcohol and CS2. We make a mixture of zanthate
salts. It is ionic and soluble in water. Cellulose was not
soluble in water but zanthate salts are. We are doing this
to get it to dissolve in water. if you pass this molecule in
a plate with holes in it, so that you are pressurizing a
system with little holes in the plate, then out comes a
stream of liquid. You will get a shower of sulfuric acid.
Zanthate in the presence of sulfuric acid will turn back
into the alcohol. As this stream comes out, its hit with
sulfuric acid and turned back into cellulose. When you
get cellulose naturally, you are stuck with what you got.
We are shooting out through holes and cellulose is
coming out as a spray; we grab hold of a strand of
cellulose and pull it; we are taking the cellulose fibers
and aligning them as long linear fibers and this feels like
silk (a protein). This cellulose polymer now feels like
rayon. Rayon is a 100% natural fiber just manipulated a
little bit. Chemically its nothing different from
cellulose; we used a biopolymer. We can do the same
thing with a plate using a slit in the plate and it will
create cellophane.