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Shiina macrolactonization
Shiina macrolactonization (or Shiina lactonization) is an organic chemical reaction that synthesizes cyclic compounds by using aromatic carboxylic acid anhydrides as dehydration condensation agents. In 1994, Prof. Isamu Shiina (Tokyo University of Science, Japan) reported an acidic cyclization method using Lewis acid catalyst, and, in 2002, a basic cyclization using nucleophilic catalyst.
==Mechanism==
The slow addition of hydroxycarboxylic acids (seco acids) into a system containing aromatic carboxylic acid anhydride and catalyst produces corresponding lactones (cyclic esters) through the process shown in the following figure. In acidic Shiina macrolactonization, Lewis acid catalysts are used, while nucleophilic catalysts are used for Shiina macrolactonization under basic conditions.
In the acidic reaction, 4-trifluoromethylbenzoic anhydride (TFBA) is mainly used as a dehydration condensation agent. First, the Lewis acid catalyst activates the TFBA, and then a carboxyl group in seco acid reacts with the activated TFBA to produce mixed anhydride (MA) once. Then, a carbonyl group derived from the seco acid in MA is selectively activated and is attacked by a hydroxyl group in the seco acid through intramolecular nucleophilic substitution. Simultaneously, residual aromatic carboxylic acid salt, which is derived from the MA, acts as a deprotonation agent, causing the cyclization to progress and produce the desired lactone. To balance the reaction, each TFBA accepts the atoms of one water molecule from its starting material, i.e., the hydroxycarboxylic acid, and then changes itself into two molecules of 4-trifluoromethylbenzoic acid at the end of the reaction. Since the Lewis acid catalyst is reproduced at the end of the reaction, only a small proportion of catalyst is needed relative to the starting material to drive the reaction forward.
In the basic reaction, 2-methyl-6-nitrobenzoic anhydride (MNBA) is primarily used as a dehydration condensation agent. First, the nucleophilic catalyst acts on the MNBA to produce activated acyl carboxylate. The reaction of carboxyl group in the seco acid with the activated acyl carboxylate produces the corresponding MA, in the same manner as in the acidic reaction. Then, the nucleophilic catalyst acts selectively on a carbonyl group derived from the seco acid in MA to again produce activated acyl carboxylate. The hydroxyl group in the seco acid attacks its host molecule through intramolecular nucleophilic substitution, and at the same time, carboxylate anion, derived from 2-methyl-6-nitrobenzoic acid, acts as a deprotonation agent, promoting the progression of the cyclization and producing the desired lactone. To balance the reaction, each MNBA accepts the atoms of one water molecule from its starting material, changing itself into two molecules of the amine salt of 2-methyl-6-nitrobenzoic acid, and thus, terminating the reaction. Because the nucleophilic catalyst is reproduced at the end of the reaction, only small stoichiometric quantities are required.
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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