text: Many retrosynthetic disconnections important for organic synthesis planning use carbocationic synthons. Carbon-carbon bonds, for example, exist ubiquitously in organic molecules, and are usually disconnected during a retrosynthetic analysis to yield carbocationic and carbanionic synthons. Carbon-heteroatom bonds, such as those found in alkyl halides, alcohols, and amides, can also be traced backwards retrosynthetically to polar C-X bond disconnections yielding a carbocation on carbon. oxonium and acylium ions are carbocationic synthons for carbonyl compounds such as ketones, aldehydes and carboxylic acid derivatives. An oxonium-type synthon was used in a disconnection en route to the hops ether, a key component of beer (see fig.1). In the forward direction, the researchers used an intramolecular aldol reaction catalyzed by titanium tetrachloride to form the tetrahydrofuran ring of hops ether.
  Another common disconnection that features carbocationic synthons is the Pictet-Spengler reaction. The mechanism of the reaction involves C-C pi-bond attack onto an iminium ion, usually formed in situ from the condensation of an amine and an aldehyde. The Pictet-Spengler reaction has been used extensively for the synthesis of numerous indole and isoquinoline alkaloids.
  Carbanion alkylation is a common strategy used to create carbon-carbon bonds. The alkylating agent is usually an alkyl halide or an equivalent compound with a good leaving group on carbon. Allyl halides are particularly attractive for S2-type reactions due to the increased reactivity added by the allyl system. Celestolide (4-acetyl-6-t-butyl-1,1-dimethylindane, a component of musk perfume) can be synthesized using a benzyl anion alkylation with 3-chloro-2-methyl-1-propene as an intermediate step. The synthesis is fairly straightforward, and has been adapted for teaching purposes in an undergraduate laboratory.