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Cinnamate ester 2026-10-20

An ester containing the conjugated fragment Ph-CH=CH-C(=O)-O-R. Its trans alkene commonly has a large vicinal proton coupling, while HMBC can connect an alkoxy methylene to the ester carbonyl.

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Past exam of the chemistry course of the year 3 of the natural sciences course of the University of Cambridge / 2022 / Paper 3 / 39 / a / Solution 2026-10-20

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Figure 1.
The cinnamoyloxymethyl and tertiary-alcohol side chains, with their spectral assignments
. **Details**.

The two substituents are $- C H_{2} - O - C (O) - CH = CH - Ph$ , with the alkene E, and $- C (OH) (C H_{3})_{2}$ . The free bond at the left of each drawing attaches to the cyclohexene.

The A-D COSY correlation and

16.2 Hz

coupling identify the trans cinnamate double bond. Five aromatic protons and four phenyl-carbon environments identify its monosubstituted phenyl ring. F is a two-proton OCH

_{2}

signal; its HMBC correlation to carbonyl 1 joins that methylene through oxygen to the ester. The two three-proton singlets N/O, their correlations to oxygenated quaternary carbon 10, and the missing OH resonance give the tertiary-alcohol substituent.

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Past exam of the chemistry course of the year 3 of the natural sciences course of the University of Cambridge / 2022 / Paper 3 / 39 / d / Solution 2026-10-20

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Figure 1.
Cyclohexene assignments with explicit ambiguity for the overlapping allylic methylenes
. **Details**.

Number ring positions

r_{1}

through

r_{6}

around the ring, with

r_{1} = r_{2}

the double bond. These auxiliary ring-position labels are distinct from the printed carbon-signal numbers.

$r_{1}$ : quaternary alkene carbon 4, attached to the OCH $_{2}$ side chain.
$r_{2}$ : alkene CH 8, proton E.
$r_{3}$ : allylic CH $_{2}$ , one of carbon signals 14/15; its protons are K and one of G/H/I.
$r_{4}$ : methine 12, proton L, attached to the tertiary-alcohol carbon 10.
$r_{5}$ : CH $_{2}$ 17, protons J and M.
$r_{6}$ : the other allylic CH $_{2}$ 15/14, carrying the remaining two of G/H/I.

The HSQC pairs J and M with 17 and places L on 12. It also places K and one unresolved member of G/H/I on one of the closely spaced allylic carbons, with the other two G/H/I protons on the other. COSY connects the vinylic E to the adjacent allylic part and connects the methine L to its neighboring methylenes; HMBC links F to the substituted alkene end and the gem-dimethyl group to the methine end.

The printed overlap does not justify a unique assignment of every G/H/I letter or a forced 14-versus-15 assignment to those two ring positions. The drawing retains those alternatives explicitly. In the half-chair, L is pseudoaxial; the resolved large/small couplings are consistent with pseudoaxial/pseudoequatorial methylene partners, but exact axial letter labels within the overlapped group remain uncertain.

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Past exam of the chemistry course of the year 3 of the natural sciences course of the University of Cambridge / 2022 / Paper 3 / 39 / e / Solution 2026-10-20

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Figure 1.
Complete constitution: cinnamoyloxymethyl at cyclohexene position 1 and a tertiary-alcohol substituent at position 4
. **Details**.

The constitution is [4-(2-hydroxypropan-2-yl)cyclohex-1-en-1-yl]methyl (E)-3-phenylprop-2-enoate, with absolute configuration at the saturated substituted ring carbon unspecified.

The OCH

_{2}

side chain is attached to the unprotonated alkene carbon 4: F correlates through the ring junction to the alkene carbons and an allylic methylene, while its correlation to 1 fixes the ester linkage. The

C (OH) (C H_{3})_{2}

side chain is attached to methine carbon 12, as shown by the methyl-to-12 HMBC correlations and the contiguous ring COSY network.

The formula check gives 19 carbons and 24 hydrogens including OH. Its degree of unsaturation is eight: four from phenyl, one ester carbonyl, one cinnamate C=C, one cyclohexene C=C and one ring. No additional double bond or hidden second carbonyl is needed.

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