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 were not hydrocarbons (C and H only) with 3 or less C or cyclopentane (shown on the question paper).
• Not counting carbons and therefore randomly adding or loosing C atoms and/ or changing the location of functional groups.
• Not controlling the location of functional groups (i.e. controlling regiochemistry)
• 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).
• Incorrect stereochemical consequences of reactions.
• Confusing NaNH₂ with Na / NH₃

A1

Common errors: poor control of regioselectivity or wrong reagents (e.g. attempting to use the radical version of HCl addition (there isn't one))

A2

Common errors: poor control of regioselectivity or wrong reagents (e.g. making pent-2-yne, using hydroboration, using NaBH4)

B1

Common errors: incorrect stereochemistry or wrong reagents (e.g. Na₂CO₃ does not epoxidise an alkene)

B2

Common errors: incorrect stereochemistry, wrong reagents to make the 1,2-diol

C1

Common errors: incorrect stereochemistry, wrong reagents (e.g. to add the required ethyl group)

C2

Common errors: incorrect stereochemistry (e.g. Br₂ addition to C=C gives trans-1,2-dibromides)