As indicated above, the C5 isoprene unit in the form of dimethylallyl diphosphate (DMAPP)
can be used to alkylate a nucleophile. In the elaboration of terpenoids and steroids, two or more C5 units are joined together, and the reactions are rationalized in terms of carbocation chemistry, including electrophilic addition of carbo-cations on to alkenes. DMAPP may ionize to generate a resonance-stabilized allylic carboca-tion as shown in Figure 2.4(d), and this can then react with an alkene [e.g. isopentenyl diphosphate (IPP)] as depicted in Figure 2.5(a). The resultant carbocation may then lose a proton to give the uncharged product geranyl diphos-phate (GPP). Where the alkene and carbocation functions reside in the same molecule, this type of mechanism can be responsible for cyclization reactions [Figure 2.5(a)].
The initial carbocation may be generated by a number of mechanisms, important examples being loss of a leaving group, especially diphos-phate (i.e. SN1 type ionization), protonation of an alkene, and protonation/ring opening of epox-ides [Figure 2.5(b)]. S -Adenosylmethionine may also alkylate alkenes by an electrophilic addition mechanism, adding a C1 unit, and generating an intermediate carbocation.
Alkylation reactions: electrophilic addition
(a) inter- and intra-molecular additions electrophilic addition of cation on to alkene
geranyl diphosphate (GPP)
intramolecular addition: cyclization
(b) generation of carbocation loss of leaving group
(c) discharge of carbocation
loss of proton protonation of alkene
methylation of alkene via SAM H
cyclization / loss of proton
protonation and ring opening of epoxide
quenching with nucleophile (water)
The final carbocation may be discharged by loss of a proton (giving an alkene or sometimes a cyclopropane ring) or by quenching with a suitable nucleophile, especially water [Figure 2.5(c)].
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