The idea of working backwards from final target molecule to starting materials (usually via one or more intermediates) when designing a synthesis.
The development of this thought process is widely attributed to E. Corey of Harvard University, who was awarded the 1990 Nobel Prize in Chemistry.
” Remembering E2 reaction mechanisms, any neighboring anti-periplanar proton and Br (leaving group) can be eliminate with a strong base to form a new pi bond.
In the case of the alkyne, we would need two leaving groups and two rounds of elimination to form an alkyne.
Let’s take a look at the retrosynthesis of 3,3-dimethylbutyne from 3,3 dmethylbutanol.
At first you may see the reagent has an alcohol group but maybe you wonder “how am I going to get to an alkyne, we haven’t even seen alkynes yet!Starting from an alkyne, remember that each double bond represents the presence of a pi bond that can be formed from an elimination reaction.So, think “what can we eliminate that would have resulted in pi bond formation?Thus, if we form two Br bonds to each of the carbons, we can then form the alkyne from 1,2-dibromo-3,3dimethylbutane.In the forward reaction, we can do this with a strong base such as sodium ethoxide (Na OEt) in ethanol to form the alkyne. To do this, we would have to carry out an addition reaction.The retrosynthetic analysis is not a synthesis form of organic chemistry, but an analytical approach based on the desired product.The target molecule is broken down into smaller and smaller fragments.” The first step is to see which bonds have changed between the products and reactants and we can see the bond colored in red.Since only one bond changed, this is relatively “easy” example.Practice is the only way to learn these tricks and the more reactions you know, the easier it will be to find shortcuts to faster synthesis schemes and solving more complicated examples which will surely be on your final exam.Sign up with Study today to master over 180 organic chemistry reactions and learn the mechanisms fast!