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Carbonyl allylation has been employed in the synthesis of polyketide natural products and other oxygenated molecules with a contiguous array of stereocenters. For example, allylstannanation of a threose-derived aldehyde affords the macrolide antascomicin B, which structurally resembles FK506 and rapamycin, and is a potent binder of FKBP12. [12]
In organic chemistry, an allyl group is a substituent with the structural formula −CH 2 −HC=CH 2. It consists of a methylene bridge ( −CH 2 − ) attached to a vinyl group ( −CH=CH 2 ). [ 1 ] [ 2 ] The name is derived from the scientific name for garlic , Allium sativum .
In this type of substitution reaction, one group of the substrate participates initially in the reaction and thereby affects the reaction. A classic example of NGP is the reaction of a sulfur or nitrogen mustard with a nucleophile, the rate of reaction is much higher for the sulfur mustard and a nucleophile than it would be for a primary or secondary alkyl chloride without a heteroatom.
According to the general principle, the Lewis acid first activates the electrophilic carbon in presence of allyltrimethylsilane which then undergoes nucleophilic attack from electrons on the allylic silane. [5] The silicon plays the key role in stabilizing the carbocation of carbon at the β-position.
The semipinacol rearrangement is a rearrangement reaction in organic chemistry involving a heterosubstituted alcohol of the type R 1 R 2 (HO)C–C(X)R 3 R 4. The hetero substituent can be a halogen (Cl, Br, I), a tosylate, a mesylate or a thiol group. This reaction proceeds by removal of the leaving group X forming a carbocation as
In chemistry, a carbonium ion is a cation that has a pentacoordinated carbon atom. [2] They are a type of carbocation. In older literature, the name "carbonium ion" was used for what is today called carbenium. Carbonium ions charge is delocalized in three-center, two-electron bonds. The more stable members are often bi- or polycyclic. [3]
An allylic rearrangement or allylic shift is an organic chemical reaction in which reaction at a center vicinal to a double bond causes the double bond to shift to an adjacent pair of atoms: It is encountered in both nucleophilic and electrophilic substitution , although it is usually suppressed relative to non-allylic substitution.
A widely accepted mechanism involves a carbocation intermediate. The Fries rearrangement. In the first reaction step a Lewis acid for instance aluminium chloride AlCl 3 co-ordinates to the carbonyl oxygen atom of the acyl group. This oxygen atom is more electron rich than the phenolic oxygen atom and is the preferred Lewis base.