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The pinacol reaction is extremely well-studied and tolerates many different reductants, including electrochemical syntheses.Variants are known for homo- and cross-coupling, intra- and inter-molecular reactions with appropriate diastereo- or enantioselectivity; [2] as of 2006, the only unsettled frontier was enantioselective cross-coupling of aliphatic aldehydes. [3]
Pinacol is a branched alcohol which finds use in organic syntheses. It is a diol that has hydroxyl groups on vicinal carbon atoms. A white solid that melts just above room temperature, pinacol is notable for undergoing the pinacol rearrangement in the presence of acid and for being the namesake of the pinacol coupling reaction .
The reaction product he obtained instead he called paraceton which he believed to be an acetone dimer. In his second publication in 1860 he reacted paraceton with sulfuric acid (the actual pinacol rearrangement). Again Fittig was unable to assign a molecular structure to the reaction product which he assumed to be another isomer or a polymer.
It has the formula [(CH 3) 4 C 2 O 2 B] 2; the pinacol groups are sometimes abbreviated as "pin", so the structure is sometimes represented as B 2 pin 2. It is a colourless solid that is soluble in organic solvents. It is a commercially available reagent for making pinacol boronic esters for organic synthesis.
Pinacol type rearrangements are often used for this type of contraction. [20] Like the expansion reaction this proceeds with an electron donating group aiding in the migration. Contraction reactions of one ring can be coupled with an expansion of another to give an unequal bicycle from equally sized fused ring.
In the Baeyer-Villiger reaction, the more substituted group, in general, migrates. [3] In the pinacol rearrangement, the order of migratory aptitude has not been determined unambiguously, [2] but some trends have been determined. For example, relative migratory aptitudes for alkyl substituents is Hydride >C(CH 3) 3 > C 2 H 5 > phenyl > CH 3. [2]
Spiro compounds are often prepared by diverse rearrangement reactions. For example, the pinacol-pinacolone rearrangement is illustrated below. [3]: 985 is employed in the preparation of aspiro[4.5]decane. [12]]. The synthesis of a spiro-keto compound form a symmetrical diol
Other key reactions encountered in this synthesis are a pinacol coupling and a Reformatskii reaction. In terms of raw materials the C20 framework is built up from L-serine (C3), isobutyric acid (C4), glycolic acid (C2), methyl bromide (C1), methyl iodide (C1), 2,3-dibromopropene (C3), acetic acid (C2) and homoallyl bromide (C4).