Ascaridol (A) is a natural organic compound that has an exotic structure. It can be found in the vol — Organic Chemistry Chemistry Question
Ascaridol
Ascaridol (A) is a natural organic compound that has an exotic structure. It can be found in the volatile oil of the goosefoot (Chenopodium album) and many other plants.
The following information is available:
a) Pure A can only be distilled in high vacuum because at elevated temperatures it explodes.
b) The 13C NMR spectrum shows the presence of only one C=C double bond in A.
c) A solution of A (in diethyl ether) does not react with sodium. Reduction with LiAlH4 leads to B.
d) If B is reacted with NaBH4 in the presence of acetic acid, then reacted with H2O2 in basic solution, the product is a mixture of two structural isomers.
e) Reaction of B with one equivalent of hydrogen gas in the presence of a metal catalyst leads to C. A reacts with twice as much hydrogen as B in the same reaction, and also yields C. C does not react with chromic acid in acetone.
f) The dehydration of C leads to the elimination of two equivalents of H2O and two organic compounds D and E are formed. Treatment of D with ozone followed by a reductive workup (Zn/H2O) leads to one equivalent of glyoxal (ethanedial) and one equivalent of 6-methyl-heptane-2,5-dione. The same reaction with E leads to one equivalent of 3-oxo-butanal and the same amount of 4-methyl-3-oxo-pentanal.
g) It is assumed that under natural conditions A forms via the reaction of D and F catalyzed by chlorophyll in the presence of light.
Determine the structure of A-F.
Model Answer
As glyoxal is symmetric the structure of D can be determined:
Based on its ozonolysis products, E has two possible structures:
As D and E are formed on dehydration of the same alcohol, both have the same carbon skeleton. Therefore, E1 is the correct structure.
Possible structures of C are:
Dehydration of C4 and C5 only produces E and D, respectively, thus neither of these structures are correct. As C cannot be oxidized by chromic acid, it does not contain any secondary hydroxyl groups. C2 is therefore the correct structure.
B contains one double bond. It can be either in the ring or in the isopropyl group:
Hydroboration of B1 leads to two structural isomers; that of B2 to a single product, so the former is the right alternative.
The composition of B is C10H18O2, while that of A is C10H16O2. The NMR spectrum shows that A contains only one double bond. As A does not react with sodium, it does not have any hydroxyl groups. The elemental composition precludes the possibility that A contains an ether and a hydroxyl group. Thus, in A, neither of the oxygen atoms are bonded to a hydrogen atom, despite this A contains only two hydrogen atoms less than B. This is only possible if the two oxygen atoms are bonded to each other, i.e., A contains a peroxide bond. This finding explains its reduction by LiAlH4 and hydrogen to a diol, as well as its explosive property.
Formally A can be formed via an addition reaction from D and oxygen (F). This reaction indeed happens on irradiation in the presence of a photosensitiser (e.g., chlorophyll). It can be shown that reaction occurs with the excited (singlet) state of oxygen, not the triplet ground state. The reaction is a Diels–Alder type addition with singlet oxygen as a dienophile.