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 Incoming links: HSQC

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

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Figure 1.
Proton letters and carbon numbers assigned to the two side chains
. **Details**.

On the ester-containing side chain, read outward from the cyclohexene:

ring - 11; F, F C H_{2} - O - 1 C (= O) - 9; D CH = 2; A CH - 3 (ipso) Ph .

(1)

A is the downfield beta-vinylic proton and D the alpha-vinylic proton next to the ester.

The phenyl ortho pair is B on carbon 7; the meta pair is C on carbon 6; its para proton is the remaining C on carbon 5. Thus the C envelope integrates to three protons, while B integrates to two. Carbon 3 is the unprotonated ipso site.

In the other substituent, 10 is the quaternary C-OH carbon. The methyl assignments are N on 13 and O on 16, reading the very closely spaced HSQC cross peaks together with the positive methyl responses in DEPT-135. Their mutual long-range correlations and correlations to 10 support the gem-dimethyl group. The two methyl carbons are distinguishable in this chiral molecular environment; their exact stereotopic names are not determined here.

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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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Tertiary alcohol 2026-10-20

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An alcohol whose hydroxyl-bearing carbon is bonded to three carbon substituents. Its oxygenated carbon carries no directly attached proton, so it lacks an ordinary carbon-proton HSQC cross peak.

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