Ynolates have a triple bond instead of the double bond in enolates. Compared to enolates, ynolates have attracted much less attention. Due to the lack of general and convenient methods for ynolate synthesis, there are only a few scattered reports in the literature. Because ynolates are not only precursors of alkynyl ethers like silyl ynol ethers, but also ketene anion equivalents that act as ketene precursors, their chemistry should be as interesting as that of enolates. For example, an ynolate reacts with an electrophile to give a ketene, which can then react with a nucleophile to afford an enolate, which is a versatile nucleophile. Thus, ynolates enable a “Negative-Positive Switching Multi-Reaction Process”.
See our review: Top. Curr. Chem. 327, 1-32 (2012),Synlett, 2231 (2008),Tetrahedron, 63, 10 (2007), Synthesis 2275 (2003); Chem. Soc Rev 367 (1998).
Secondary Orbital Interactions
(4) Short-Step Synthesis of Heterocycles
Ynolates react with nitrones, aldimines, and isocyanates to provide isoxazolidinones, b-lactams, and azetidinediones, respectively. Recently, we have found a new synthetic method for multisubstituted furans and pyrroles. These products provide access to bioactive natural products.
Heterocycles, 49, 113-116 (1998); Tetrahedron Lett., 41, 5943-5946 (2000); Org. Lett. 4, 3119-3121 (2002); Synthesis, 1441-1445 (2003); Tetrahedron: Asymmetry, 16, 2821-2831 (2005); Heterocycles, 66, 39-43 (2005).
Since our development of a new synthetic method for ynolates, we have found new synthetic reactions with ynolates. These results should be only part of the immense ynolate chemistry. We are also investigating the synthetic applications of these unique reactions.
(1) Novel Synthesis of Ynolates
We have developed a new synthetic methodology for ynolates via thermal cleavage of ester dianions, prepared from easily available a,a-dibromo esters.
Tetrahedron Lett., 38, 4433-4436 (1997); Tetrahedron, 54, 2411-2422 (1998); Tetrahedron Lett., 42, 8357-8360 (2001); Chem. Pharm. Bull., 51, 477-478 (2003).
(2) Successive Reactions Initiated by Ynolates
We have developed formal [n + 1]-type cycloadditions initiated by ynolates, as the “first negative-positive switching process”. Ynolates add to carbonyl groups at low temperature to give highly nucleophilic b-lactone enolates, which are not readily available by other methods. The b-lactone enolates undergo Dieckmann condensation and Michael addition to furnish the corresponding bicyclic compounds, which are easily decarboxylated to provide multi-substituted carbocycles in good yields and in one-pot. We have used this method to synthesize natural products in short steps.J. Am. Chem. Soc., 121, 6507-6508 (1999); Org. Lett., 3, 2029-2031 (2001); J. Org. Chem. 66, 7818-7824 (2001); Tetrahedron Lett. 43, 5039-5041 (2002).
(3) Torquoselective Olefination
b-Lactone enolates, which are prepared by reactions of ynolates with carbonyls, are ring-opened at room temperature and give a,b-unsaturated carboxylates, that is, polysubstituted olefins. We were surprised by the high geometrical selectivities and noticed that this is an olefination reaction! We have found excellent Z-selectivities in the reactions of acylsilanes and a-oxyketones, which yield tetrasubstituted olefins. This is the first successful example of highly stereoselective olefinations of ketones. Although the olefination of esters with high E-selectivity has been presumed to be very difficult, we have successfully achieved this type of reaction. The torquoselectivity in the ring-opening determines the stereoselectivity, which can be elucidated by the orbital interactions between the breaking C-O bond and several orbitals of the substituents. We call the reaction “Torquoselective Olefination”, which is a new methodology for olefination. This olefination is especially effective for sterically hindered substrates and for preparing tetrasubstituted olefins. The selectivity could be assumed by theoretical calculations.Tetrahedron Lett., 39, 4857-4860 (1998); Heterocycles, 52, 545-548 (2000); Tetrahedron Lett., 41, 5947-5950 (2000); J. Org. Chem., 65, 5443-5445 (2000); J. Am. Chem. Soc. 124, 6840-6841 (2002); J. Org. Chem. 69, 3912-3916 (2004); Org. Lett. 22, 3945-3948 (2004); Chem. Commun. 2477-2479 (2005); Chem. Eur. J. 12, 524-536 (2006); J. Am. Chem. Soc., 128, 1062-1063 (2006); J. Am. Chem. Soc., 131, 2092–2093 (2009).