Alkylation Reactions Electrophilic Addition

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

isopentenyl diphosphate

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)].

Was this article helpful?

0 0
Reducing Blood Pressure Naturally

Reducing Blood Pressure Naturally

Do You Suffer From High Blood Pressure? Do You Feel Like This Silent Killer Might Be Stalking You? Have you been diagnosed or pre-hypertension and hypertension? Then JOIN THE CROWD Nearly 1 in 3 adults in the United States suffer from High Blood Pressure and only 1 in 3 adults are actually aware that they have it.

Get My Free Ebook


  • lotta
    How to alkylate double bond?
    8 years ago

Post a comment