Part 6: SYNTHESIS

Answers to the synthesis problems are given below.  These answers have been selected as they are short and efficient, but there are probably other reasonable solutions.  Note some targets have specific stereochemistry that needs to be considered.

When planning syntheses, one should try to work backwards by functional group analysis. The diagrams show the "retrosynthesis" - the design or plan and then below that the reaction scheme step-by-step (as required in the question). Red arrows try to show carbon-carbon bond forming reactions, blue arrows are functional group manipulations. The blue text rationalise the retrosynthetic analysis and information about important reactions are given in green.

Common errors:

• Lots of students made more use of Chem 351 reactions over 353 reactions. It's important to note that the 353 materials are often more selective and therefore give better control than the 351 reactions students elected to use.
• Not reading the instructions and using starting materials that are not hydrocarbons (C and H only) with 3 or less C (similar criteria as long exams!)
• Not counting carbons and therefore randomly adding or loosing C atoms.
• pKas ! i.e. not paying attention to the presence of acidic and basic groups.
• Reaction knowldege ? Need a LG to do SN or E reactions, wrong reagents for the desired task (e.g. alkyl halides eliminate with base not acid, alcohols with acid not base).
• Stereochemical consequences of addition reactions and SN reactions.

A1

Common errors: (1) trying to use NaNH₂ to make a vinyl anion (check the pKas), (2) expecting radical bromination of pentane to give 1-bromopentane as the major product (3) trying to use pent-2-ene rather than pent-1-ene

A2

Common errors: (1) need LG to be able to eliminate to make an alkene (2) stereochemistry of Br2 addition is anti

B1

Common errors: (1) using the right starting material and reagents to make 1-bromopropane at the start of the synthesis

B2

Common errors: (1) could not draw meso compound (2) confusing NaNH2 with Na / NH3

C1

Common errors: (1) not recognsing the epoxide route to control the stereo and regiochemistry

C2

Common errors: (1) did not recognise the SN2 to make the ester (2) confusing NaNH2 with Na / NH3

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