4d Flashcards
okay lets think of aldol reactions,, whats the 2 diff ways u can do it
deprotonate and form an enolate,, use the O- and double bond to attack the other molecule
protonate the carbonyl and then deporonate the alpha hydrogens to form an enol,, then use this to attack the other moelcule.
under what conditions is the carbonyl first protonated in the aldol reaction
when u have acidic conditions
when we have 2 stereogenic centres,, how many total stereoisomers do we have
2^n
where n is the total amount of stereocentres we have
if n = 2 we have 4 as our answer
2 enantiomers
2 diastereoisomers
when we have an enolate aka an O- ,, what do we rlly have
we never just have the O-
it will always be associated with smt
aka like a Li!!! if we used LDA to deprotonate!!!
when we perform the aldol reaction we have two O- in total tho,, do each have to be associated with the Li?? and if so how do we show this
yes they both have to be associated with the Li,, bc u cant have O- just by itself!!!!!
meaning they both have to be associated with the Li,, aka at the same time aswell,, so u get like a 6 membered cyclohexane type ring!!!! with Li at the top connecting the O’s together!!!!
whats important about the t6 membered cyclohexane type Li transition state,,
, it helps us form the correct configuration of our product!!!
what type of enolates can we have
we can have a Z enolate and an E enolate
what does a Z enolate give us
a Z enolate will give us a SYN product!!!!
an E enolate will give us what
an E enolate will give us an ANTI product!!!
when we draw these things and decide whether the things are syn // anti,, what are we acc referring to
the R group and the OH!!!
R2 and OH
aka the substituents on the enolate that told us if the 2 things are E or Z enolates
those are the 2 things we care about
okay so how do we acc draw the cyclohexane chair
we draw the cyclohexane chair normally
but then we label all the carbons on the reactants //SM and then add these to the cyclohexane,,,
so we add the O of th carbonyl and the Li // the associated thing from the base.
if R2 and R3 are pointing in opp directions,, we need to make sure we dont put them both equirorial,, they will have to be both axial but pointing in opp directions.
we then draw the curly arrows of the aldol reaction as normal
then cut off any bits we dont leave.
once youve drawn the cyclohexane ring and all the substituents and even drew the aldol reaction the way it occurs,, what do we then need to do to make sure we have syn // anti
we have to look at the moelcule in a specific direction,, the direction u made a video on,, then look to see if the substituents are poiting up or down
if theyre pointing up they get a wedge and if they point down they get a dash.
obvs if they have the same orientation theyre syn and if theyre different theyre dashed
sometimes the product will have dashed for both substituents telling us its a syn,, but we also get syn but we get them both wedged,, whats the diff
the diff is how u arrange the cyclohexane chair,, soemtimes u can put the aldehyde at the front and sometimes at the back!!!
theyre enantiomers of eahcother as its just their mirror image
whats the diff between syn and anti molecules
they have different biological properties.
whats an important thing we need to keep in mind when drawing the cycohexane ring
keep looking at the relationship of each substituent to the others in the SM.
if the O and R2 were on diff sides,, make sure u draw it with R2 pointing away from it!!!
aka if the framework and therefore o is going down,, make sure u draw the R2 equitorial,, aka not facing down.
okay so so far this has been for reagents thast didnt have a chiral centre. what if they did tho
if they have a chiral centre then its long
u need to do the keflin anh procedure on the aldehyde to see which face u attack it using the nucleophile!!!
aka look to see whihc group is the biggesta nd put this on a side,, then move anticlockwise to see where to put the other substituents
then attack the drawing and see which face that corresponds to,, and then obvs look to see if u have syn or anti!!!
