Development of new optical active carbene reagents and its application to the synthesis of unnatural sugars

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It has been known for a long time that the treatment of azolium salts, represented in thiazolium salts and triazolium salts, with weak base generate an anion or a resonance-stabilized carbene which is able to act as catalyst to promote the Stetter reaction and benzoin condensation. During this research these reaction showed us new aspects: 1) formation of quaternary carbon with Stetter reaction and 2) selective cross-benzoin condensation.The construction of quaternary carbon with Stetter reaction was accomplished with the intramolecular reaction using salicylaldehyde derivatives as substrates. This reaction was carried out with good yield under mild conditions. And also this reaction is the first example of the construction of quaternary carbon with Stetter reaction. In the cross-benzoin reaction, we found that the combination of aromatic aldehyde and aliphatic aldehyde is very important for the selective acyloin synthesis, and especially, thiazolium salts is better than other azolium salts in this cross-benzoin reaction.These successes led us the development of new chiral azolium salts. Two types of chiral azolium salts were synthesized with general methods. One was thiazolium salts derived from binaphthol, the other was triazolium salt derived from norephedrine. And these salts were used for the Stetter reaction and benzoin reaction. Unfortunately, the chiral thiazolium salts derived from binaphthol had no catalytic activity. On the other hand, triazolium salts derived from norephedrine acted as catalyst to give 82%ee of an acyloin product.