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78 Organic Chemistry Solutions Manual Problem 12 Precocene is a compound that causes insect larvae to pupate and can also be found in some plants (Ageratum spp.) where it may act as an insecticide. It was isolated in minute amounts and has the following spectroscopic details. Propose a structure for precocene. Spectra of precocene: Mass spectrum: m/z (high resolution gives C₁₃H₁₆O₃), M-15 (100%) and M-30 (weak) Infrared: CH and fingerprint only. 1H NMR: δH (p.p.m.) 1.34 (6H, s), 3.80 (3H, s), 3.82 (3H, s), 5.54 (1H, d, J 10 Hz), 6.37 (1H, d, J 10 Hz), 6.42 (1H, s), and 6.58 (1H, s). Purpose of the problem Your first attempt at determining the structure of a natural product without any hints - not even a wrong structure. Suggested solution The mass spectrum gives us the formula (C₁₃H₁₆O₃), which looks like an aromatic compound as there are so few hydrogens, and the base peak at M 15 suggests that a methyl group is lost rather easily. The infrared suggests that all three oxygen atoms are present as ethers. The NMR shows us these details: 1.34 (6H, s): two identical methyl groups, probably CMe₂. Rather large shift. 3.80 (3H, s) and 3.82 (3H, s): two different OMe groups. 5.54 (1H, d, J 10 Hz) and 6.37 (1H, d, J 10 Hz): two cis protons on an alkene. 6.42 (1H, s) and 6.58 (1H, S ): two isolated protons on an aromatic ring, probably 1,4-related as there is no coupling. Rather small shifts - electron-rich ring. If all this is true, we have these fragments. ? H H ? H Me Me C₁₃H₁₆O₂ ? ? ? ? H ? ? This adds up to C₁₃H₁₆O₂, so we have one more oxygen atom to fit in somewhere. It is very important that you now start to join up these fragments and see what you get. You quickly find that no OMe chains of atoms are possible as there are not then enough substituents to go on the benzene ring (see the structure in the margin). We must find four substituents for the ring and there are only four other pieces OMe to be joined up. The only chain-terminating units are the two OMe groups. The obvious solution is to put the cis double bond in another ring fused on to the benzene ring and hence making two of the required four substituents. We can then combine the other groups in various ways such as these. MeO MeO MeO MeO 0 MeO 0 MeO 0 The five-membered ring is no good because the coupling constant between the alkene protons would be much smaller than 10. The two six-membered rings are good because they make the benzene ring electron-rich (three OR substituents). The middle structure is not right because the