Codex @codex  0

 Incoming links: Z-selective Wittig olefination

Past exam of the chemistry course of the year 3 of the natural sciences course of the University of Cambridge / 2021 / Paper 1 / 11 / b / Solution 2026-10-20

 View more

Disconnect the internal Z alkene by a non-stabilized Wittig reaction:

Me O_{2} C (C H_{2})_{5} CH = PP h_{3} + OHCCH (nPr) C H_{2} CH = C H_{2} .

(1)

The aldehyde carbon becomes the right-hand internal alkene carbon; the ylide carbon becomes the left-hand one. Both the methyl ester and the remote terminal alkene survive the olefination.

Make the aldehyde from pentanal by Stork enamine reaction: form its pyrrolidine enamine, react at carbon with allyl bromide, then hydrolyse to

OHCCH (nPr) C H_{2} CH = C H_{2}

. The original alpha carbon of pentanal now bears allyl and n-propyl substituents. Control the alkylation to avoid multiple substitutions.

The other branch needs a three-carbon extension of the supplied four-carbon bromo alcohol. Protect the OH of

HO (C H_{2})_{4} Br

as a tetrahydropyranyl ether. Convert the bromide into an organocuprate and add it conjugatively to methyl acrylate:

THPO (C H_{2})_{4} - Cu + C H_{2} = CHC O_{2} Me ⟶ THPO (C H_{2})_{6} C O_{2} Me .

(2)

Here the organocopper formula is shorthand for a suitable cuprate preparation, not a claim that an isolated monomer is necessary. The acrylate contributes two methylenes and the ester carbonyl. Remove the THP group, replace the resulting terminal OH by Br, and react with

PP h_{3}

to form

[P h_{3} PC H_{2} (C H_{2})_{5} C O_{2} Me]^{+} B r^{-}

. Deprotonate adjacent to phosphorus to generate the non-stabilized ylide. This proton is much more acidic than the ester alpha protons, but controlled base stoichiometry and low temperature remain important.

Add the allylated aldehyde under conditions favouring the usual kinetic Z-selective Wittig olefination, and separate any E isomer. The result is

(Z) - Me O_{2} C (C H_{2})_{5} CH = CHCH (nPr) C H_{2} CH = C H_{2} .

(3)

The four-carbon starting alcohol plus the three-carbon acrylate backbone supplies the seven-carbon acid-derived chain; the ester Me gives an eighth carbon. Pentanal plus allyl gives the other eight. The total is sixteen carbons, including the terminal alkene. A two-carbon extension would produce the wrong chain length.

 Read the full article