IChO 2008 · Theory problem 3

Vinpocetine and its metabolites

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Vinpocetine (Cavinton®, Calan®) is one of the best selling original drugs developed in Hungary. Its preparation relies on a natural precursor, (+)-vincamine (\(\ce{C21H26N2O3}\)), which is isolated from the vine plant, vinca minor. The transformation of (+)-vincamine to vinpocetine is achieved in two steps depicted below.

Figure 1

All compounds (A to F) are enantiomerically pure compounds.

  • The elementary composition of A is: C 74.97%, H 7.19%, N 8.33%, O 9.55%.
  • B has 3 other stereoisomers.

3.1 Propose structures for the intermediate A and vinpocetine (B).

A study of the metabolism of any drug forms a substantial part of its documentation. There are four major metabolites each formed from vinpocetine (B): C and D are formed in hydrolysis or hydration reactions, while E and F are oxidation products.

Hints:

  • The acidity of the metabolites decreases in the order C >> E >> D. F does not contain an acidic hydrogen.
  • C and E each have 3 other stereoisomers, while D and F each have 7 other stereoisomers.
  • F is a pentacyclic zwitterion and it has the same elementary analysis as E:
    C 72.11 %, H 7.15 %, N 7.64 %, O 13.10 %.
  • The formation of E from B follows an electrophilic pattern.
  • The formation of D from B is both regio- and stereoselective.

3.2 Propose one possible structure for each of the metabolites C, D, E and F!

3.3 Draw a resonance structure for B that explains the regioselective formation of D and the absence of the alternate regioisomer in particular.

Official English text of the problem.

Answers

The following SMILES preserve the two surviving carbon stereocenters of the supplied vincamine structure.

  • 3.1

A — Methyl apovincaminate (apovincamine), \(\ce{C21H24N2O2}\):
COC(=O)C1=C[C@]2(CC)CCCN3CCc4c([C@@H]32)n1c1ccccc41

B — Vinpocetine (ethyl apovincaminate), \(\ce{C22H26N2O2}\):
CCOC(=O)C1=C[C@]2(CC)CCCN3CCc4c([C@@H]32)n1c1ccccc41

  • 3.2

C — Apovincaminic acid, \(\ce{C20H22N2O2}\):
O=C(O)C1=C[C@]2(CC)CCCN3CCc4c([C@@H]32)n1c1ccccc41

D — Ethyl vincaminate, \(\ce{C22H28N2O3}\), choosing the alcohol configuration shown for the starting vincamine:
CCOC(=O)[C@@]1(O)C[C@]2(CC)CCCN3CCc4c([C@@H]32)n1c1ccccc41

E — 10-Hydroxyvinpocetine, \(\ce{C22H26N2O3}\), one possible phenolic oxidation product:
CCOC(=O)C1=C[C@]2(CC)CCCN3CCc4c([C@@H]32)n1c1ccc(O)cc41

F — Vinpocetine N-oxide, \(\ce{C22H26N2O3}\), choosing one configuration at the oxidized tertiary-amine nitrogen:
CCOC(=O)C1=C[C@]2(CC)CCC[N@+]3([O-])CCc4c([C@@H]32)n1c1ccccc41

  • 3.3

Charge-separated enamine resonance contributor of vinpocetine:
CCOC(=O)C1[CH-][C@]2(CC)CCCN3CCC4=C([C@@H]32)[N+]=1c1ccccc14

In the drawing, make the bond between the indole nitrogen and the ester-bearing alkene carbon double, make the original alkene bond single, and place positive charge on that nitrogen and negative charge on the other alkene carbon. Preserve the remaining framework and carbon stereochemistry.

Solution

3.1 — Identification of A and B

Using standard atomic molar masses \(12.011,\ 1.008,\ 14.007,\ 15.999\ \mathrm{g\,mol^{-1}}\) for carbon, hydrogen, nitrogen and oxygen, respectively, a \(100.00\ \mathrm g\) sample of A gives the atomic ratio

\[ n_{\mathrm C}:n_{\mathrm H}:n_{\mathrm N}:n_{\mathrm O} = \frac{74.97}{12.011}: \frac{7.19}{1.008}: \frac{8.33}{14.007}: \frac{9.55}{15.999} \]

\[ =6.242:7.133:0.5947:0.5969 \approx21:24:2:2. \]

Thus A has molecular formula \(\ce{C21H24N2O2}\), with molar mass approximately \(336.44\ \mathrm{g\,mol^{-1}}\). Compared with vincamine, it has lost one molecule of water:

\[ \ce{C21H26N2O3 -> C21H24N2O2 + H2O}. \]

The starting (+)-vincamine is represented by
COC(=O)[C@@]1(O)C[C@]2(CC)CCCN3CCc4c([C@@H]32)n1c1ccccc41.

Acid-catalysed dehydration removes the hydroxyl group from the carbon bearing the methyl ester and a hydrogen from the adjacent methylene carbon. The resulting double bond lies between those two carbons. This gives A, methyl apovincaminate.

The hydroxyl-bearing stereocenter disappears upon formation of the alkene. The two other carbon stereocenters retain their configurations.

Sodium hydroxide then hydrolyses the methyl ester:

\[ \ce{C21H24N2O2 + NaOH -> C20H21N2NaO2 + CH3OH}. \]

The intermediate is sodium apovincaminate:
[Na+].O=C([O-])C1=C[C@]2(CC)CCCN3CCc4c([C@@H]32)n1c1ccccc41.

Its carboxylate oxygen alkylates bromoethane, producing the ethyl ester B:

\[ \ce{C20H21N2NaO2 + C2H5Br -> C22H26N2O2 + NaBr}. \]

Consequently, B is vinpocetine, ethyl apovincaminate. Its two carbon stereocenters give

\[ 2^2=4 \]

stereoisomers, hence three others besides the specified enantiomer. The endocyclic double bond in the six-membered ring does not introduce an independent pair of geometrical isomers.

For a drawing, retain the original dashed bonds to the bridgehead hydrogen and ethyl group, replace the alcohol-containing segment by the alkene, and change the ester methyl group to ethyl for B.

The small species used here are water O, sodium hydroxide [Na+].[OH-], methanol CO, bromoethane CCBr, and sodium bromide [Na+].[Br-]. The stated catalyst and solvents are sulfuric acid OS(=O)(=O)O, dichloromethane ClCCl, and ethanol CCO.

3.2 — Identification of C, D, E and F

The common analysis of E and F gives

\[ \frac{72.11}{12.011}: \frac{7.15}{1.008}: \frac{7.64}{14.007}: \frac{13.10}{15.999} \approx22:26:2:3. \]

Therefore,

\[ \boxed{\textbf{E},\textbf{F}:\ \ce{C22H26N2O3}}, \qquad M\approx366.46\ \mathrm{g\,mol^{-1}}. \]

Each is formed by adding one oxygen atom to B, without changing its total hydrogen count.

C: ester hydrolysis. Hydrolysis of the ethyl ester gives apovincaminic acid:

\[ \ce{C22H26N2O2 + H2O -> C20H22N2O2 + C2H5OH}. \]

The carboxylic acid group explains why C is the most acidic metabolite. Hydrolysis leaves the two carbon stereocenters unchanged, giving four stereoisomers in total.

D: hydration of the alkene. Addition of water gives

\[ \ce{C22H26N2O2 + H2O -> C22H28N2O3}. \]

The hydroxyl group must attach to the alkene carbon already bonded to the indole nitrogen and ester group. The adjacent alkene carbon becomes a methylene group. This creates one new carbon stereocenter, giving three in total and therefore eight stereoisomers.

The selected D is ethyl vincaminate. Draw it like the original vincamine, with an ethyl ester replacing the methyl ester: the hydroxyl bond projects toward the viewer, and the ester bond projects behind the plane.

Putting hydroxyl on the other alkene carbon would make both former alkene carbons stereogenic. That alternative would have four carbon stereocenters, inconsistent with the stated eight stereoisomers.

E: aromatic hydroxylation. Its intermediate acidity suggests a phenol. Electrophilic aromatic hydroxylation introduces one oxygen atom while replacing an aromatic hydrogen by hydroxyl; the total hydrogen count is unchanged.

One possible structure is 10-hydroxyvinpocetine. In the supplied drawing, put hydroxyl on the upper-left benzene carbon, para to the benzene fusion carbon bonded directly to the indole nitrogen. This introduces no new stereocenter, so E has four stereoisomers.

F: tertiary-amine oxidation. Oxidation of the saturated tertiary-amine nitrogen gives the amine oxide, containing the formally charged linkage \(\ce{N^{+}-O^{-}}\).

This assignment explains all its properties:

  • It adds one oxygen atom without changing the hydrogen count.
  • It retains all five rings.
  • It is a zwitterion, with net charge zero.
  • It introduces no hydroxyl or nitrogen-bound hydrogen.
  • The tetrahedral amine-oxide nitrogen has four distinguishable substituents and becomes a configurationally stable stereocenter.

Thus F has two carbon stereocenters and one nitrogen stereocenter, giving eight stereoisomers. The SMILES supplied above chooses the \(S\) configuration at this nitrogen.

For either oxidation, an atom-balanced formal equation is

\[ \ce{2C22H26N2O2 + O2 -> 2C22H26N2O3}, \]

with the product being E or F, respectively. This expresses the net elemental change; the problem does not specify the biological oxidant. Dioxygen is O=O.

The assignments satisfy the stereochemical counts:

Compound Characteristic group Configurationally stable stereocenters Total stereoisomers
C Carboxylic acid 2 carbon 4
D Tertiary alcohol 3 carbon 8
E Phenol 2 carbon 4
F Tertiary-amine N-oxide 2 carbon + 1 nitrogen 8

The acidity order follows from stabilization of the conjugate bases: carboxylate is strongly stabilized by delocalization over two oxygens, phenoxide is stabilized by aromatic delocalization, and the alcohol’s alkoxide lacks comparable resonance stabilization. Hence

\[ \mathbf C\gg\mathbf E\gg\mathbf D. \]

3.3 — Resonance and hydration regioselectivity

Call the ester-bearing alkene carbon \(C_\alpha\), and the adjacent alkene carbon \(C_\beta\). The relevant local resonance relationship is

\[ \ce{N-C(CO2Et)=CH <-> N^{+}=C(CO2Et)-CH^{-}}, \]

where the remaining ring bonds are understood.

Starting from a Kekulé representation, move the indole nitrogen’s lone pair into the bond to \(C_\alpha\), and move the alkene \(\pi\)-electron pair onto \(C_\beta\). The full charge-separated contributor is the structure given in Answers 3.3.

This contributor identifies \(C_\beta\) as the electron-rich end of the alkene. Protonation there gives an iminium ion whose positive charge is stabilized by nitrogen donation. This vinpocetine-derived iminium ion is
CCOC(=O)C1C[C@]2(CC)CCCN3CCC4=C([C@@H]32)[N+]=1c1ccccc14.

Water attacks its electrophilic \(C_\alpha\); subsequent deprotonation produces D, with hydroxyl on the ester-bearing carbon.

The opposite protonation would place positive charge at \(C_\beta\), while the newly saturated \(C_\alpha\) would interrupt conjugation with nitrogen. That pathway lacks the iminium stabilization, explaining the observed hydration regioselectivity and the absence of the alternate regioisomer.

The solution was checked against the official solution.