Terpenes hydrogen eliminations

The 1,2-hydrogen elimination reaction (p-elimination) is, as we have seen, a ubiquitous termination step in terpene biogenetic schemes. There is accordingly no need to cite specific examples nor to present precedent from carbonium ion chemistry. 

Mevalonic acid is indeed the true precursor of the terpenes but it is a C6 compound and so it must lose a carbon atom to give the C5 precursor. The spare carbon atom becomes C02 by an elimination reaction. First, the primary alcohol is pyrophosphorylated with ATP (Chapter 49) then the CO2H group and the tertiary alcohol are lost in a concerted elimination. We know it is concerted because labelling the diastereotopic hydrogen atoms on the CH2CO2H group reveals that the elimination is stereospecific. 

Both isopentenyl pyrophosphate and dimethyiaiiyi pyrophosphate are needed for the biosynthesis of terpenes. Therefore, some isopentenyl pyrophosphate is converted to dimethyiaiiyi pyrophosphate by an enzyme-catalyzed isomerization reaction. The isomerization involves addition of a proton to the sp carbon of isopentenyl pyrophosphate that is bonded to the greater number of hydrogens (Section 4.4) and elimination of a proton from the carbocation intermediate in accordance with Zaitsev s rule (Section 11.2). 

Some reaction systems, which have been described in the patent literature for the production of meat aromas, regard thiamine as precursor. 3-Methyl-2-butene-l-thiol is one of the roast odorants of coffee (cf. 21.1.3.3.7) and can cause on off-flavor in beer (cf. Table 5.5). In general, only very small amounts are formed which are still aroma active on account of the very low odor threshold (Table 5.21). The formation of the thiol is explained by the fact that the 3-methyl-2-butene radical is formed from terpenes by photolysis (beer) or under the drastic conditions of the roasting process (coffee). This radical then meets a SH -radical formed from cysteine under these conditions. In the case of beer, humulons (cf. 20.1.2.3.2) are under discussion as the source of the alkyl radical. In coffee 3-methyl-2-butene-l-ol (prenyl alcohol) is also a possible precursor, which yields the thiol after water elimination and hydrogen sulfide addition. 

---