Past exam of the chemistry course of the year 3 of the natural sciences course of the University of Cambridge / 2022 / Paper 3 / 39

The two substituents are $-\mathrm{C}{\mathrm{H}}_2-\mathrm{O}-\mathrm{C}(\mathrm{O})-\mathrm{CH}=\mathrm{CH}-\mathrm{Ph}$, with the alkene E, and $-\mathrm{C}(\mathrm{OH})(\mathrm{C}{\mathrm{H}}_3{)}_2$. The free bond at the left of each drawing attaches to the cyclohexene.

The A-D COSY correlation and $16.2\mathrm{Hz}$ 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.

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

Use a cyclohexene half-chair, not a fully puckered saturated-cyclohexane chair. The alkene carbons and their directly attached bonds form the approximately planar end; the saturated part puckers. Put the $\mathrm{C}(\mathrm{OH})(\mathrm{C}{\mathrm{H}}_3{)}_2$ substituent pseudoequatorial at the opposite saturated methine to avoid a crowded pseudoaxial arrangement. Its methine proton is correspondingly pseudoaxial. This is a preferred conformation; the achiral spectra do not assign an absolute configuration.

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.

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.

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 $\mathrm{C}(\mathrm{OH})(\mathrm{C}{\mathrm{H}}_3{)}_2$ 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.