4-Ipomeanol (4-IPO) is a pulmonary pre-toxin isolated from sweet potatoes infected with the fungus Fusarium solani. One of the 4-IPO metabolites is toxic to the lungs, liver and kidney in humans and animals. This metabolite can covalently bind to proteins, thereby interfering with normal cell processes.
The toxic metabolite, an enedial, is mostly formed in the bronchiolar exocrine cells (club cells) in the lungs of rodents. Necrosis of bronchiolar cells is therefore the primary damaging effect of the toxin, due to this location of metabolism. The secondary pathological effects are edema, congestion and hemorrhage caused by the destruction of the bronchiolar exocrine cells. In humans the metabolite is mostly formed in the liver, and causes liver toxicity.
Structure and reactivity
4-Ipomeanol is a chemical compound belonging to the family of furans. It consists of a furan ring, which is substituted at the third carbon of the furan ring with a pentanone containing a hydroxyl group. A furan ring is a five-membered aromatic ring consisting of one oxygen atom and four carbon atoms; a pentanone is a ketone consisting of five carbon atoms. Other 3-substituted furans are ipomeanine (IPN), 1-ipomeanol (1-IPO) and 1,4-ipomeanol (DIOL), differing in the locations of the hydroxyl groups. 4-IPO has three functional groups which determine the molecule's reactivity. These are the furan ring, the ketone group and the alcohol group.
Reactivity of furan ring
The furan ring is aromatic according to Hückel’s rule, so the furan makes the compound relatively stable. Therefore, this ring will not react easily with other compounds. Furan is heterocyclic, which means that it is cyclic, but one or more of the ring atoms is not a carbon atom. In the case of furan this heteroatom is an oxygen atom. This atom is sp2 hybridized and has one lone pair in an sp2 orbital and a second lone pair in a p orbital, overlapping with the p orbitals of adjacent carbons. This results in a pi bond formation.
Reactivity of ketone
A ketone group is polar, as oxygen is more electronegative than carbon. Therefore, the carbonyl atom of the ketone group is electron deficient, thus electrophilic, and can easily react with nucleophiles. However, it does not undergo substitution reactions, as the attached molecule is a too strong base to be eliminated. As a result of this, irreversible nucleophilic addition reactions are possible: a nucleophile can add to the carbonyl carbon, but due to the lack of a good leaving group, no base is eliminated. This intermediate molecule takes up a proton, so a hydroxyl group is formed. When there is sufficient acid present, the hydroxyl group can be protonated further, making it a good leaving group. This functional group will then react as an alcohol group.
As described above is the basic reaction mechanism of ketones. In this way ketones can undergo several organic reactions, reacting with compounds like grignard reagents, acetylide, cyanide, and hydride ions, amines, water, alcohols and peroxyacids.
Reactivity of alcohol
The alcohol group is a strongly basic leaving group that cannot undergo nucleophilic substitution reactions. However, it becomes a better leaving group after protonation, which converts the leaving group from OH− to H2O. H2O is a weaker base than OH− and can thus undergo substitution reactions with weakly basic nucleophiles. This reaction occurs following a SN2 mechanism, as 4-IPO is a secondary alcohol.
Dehydration of the at the pentanone substituted alcohol group is also possible, following an E1 reaction mechanism. This reaction is acid-catalyzed and results in the loss of a water molecule. Like with the SN1 reaction, protonation of the leaving group is required first. Water then is eliminated, leaving behind a carbocation, which leads eventually to alkene formation. Furthermore, secondary alcohols can undergo oxidation reactions. This results in the formation of a ketone.
Synthesis
4-IPO can be isolated from sweet potatoes infected with the fungus Fusarium solani. However, it can also be synthesized from the commercially available chemical diethyl 3,4-furandicarboxylate.
Partial hydrolysis of diethyl 3,4-furandicarboxylate (I) with an equimolar quantity of NaOH results in the monoester 4-(ethoxycarbonyl)furan-3-carboxylic acid (II) occurs. This is followed by decarboxylation by heating with a copper powder yielded ethyl 3-furoate (III). The Claisen condensation is used to form ethyl 3-furoylacetate (IV). By reacting with propylene oxide, lactone (V) is formed. Decarboxylation is achieved by gently heating lactone in the presence of diluted acid (VI).
