Part I, copper(I) iodide dimethyl sulfide catalyzed 1,4- addition of alkenyl groups from alkenylzirconium and alkenylzinc reagents and their application toward the total synthesis of azaspirene : Part II, aqueous Wittig chemistry employing stabilized ylides and aldehydes
- Author(s): El-Batta, Amer Adnan
- et al.
Hydrozirconation of alkynes, utilizing Schwartz's reagent, }Cp2Zr(H)Cl}, is a superb protocol for making regioselective alkenylzirconocene reagents. We recently reported the direct conjugate addition of the alkenyl group in high yield from an alkenylzirconocene in the presence of a catalytic amount of the CuI0.75SMe2 complex. Specifically, this Cu(I) additive is the most efficient when compared to several other copper(I) and copper(II) sources. While organozirconocenes have been used in many applications for organic synthesis, the transmetallation from zirconium to zinc has been demonstrated to be a more efficient subsequent procedure in the synthesis of carbon- carbon bonds. The Zr->Zn transmetalation protocol illustrates that the 1,4-addition of mixed alkenyl- alkylzincate reagents to enones and enals is faster than the corresponding alkenylzirconocene reagents using catalytic amount of the CuI0.75DMS complex. This protocol was also applied to N-enoyl derived oxazolidinones and the stereoselectivity was measured. Quite remarkably, employing TMSOTf as a Lewis acid achieved the 1,4-products in high yields, with only 10 mol% of the CuI0.75DMS catalyst was utilized. Encouraged by the unique action of the mixed alkenyl-alkylzincate/CuI0.75SMe2 components, it is proposed that this protocol be used for the insertion of the hexadiene unit in the azaspirene structure. Azaspirene, a novel angiogenesis inhibitor, has biological activity that treats angiogenesis-related diseases such as cancer and rheumatoid arthritis. An efficient investigation for total synthesis of azaspirene was studied, aiming to achieve a concise stereocontrolled route to this unique anti-cancer agent as well as a small library thereof. Simple synthetic routes to achieve two key fragments in the target molecule were accomplished. The overall synthetic strategy proposed herein is of great economical use compared to few other reported synthetic studies toward the target molecule. In addition, water is demonstrated to be an excellent medium for the synthesis of carbon-carbon double bonds. Excellent yields and E/Z- ratios were reported using a broad range of aldehydes and phosphoranes as well as in situ formed ylides. The rate of Wittig reactions in water is unexpectedly accelerated compared to conventional Wittig reactions in organic solvents, and water is the medium of choice for this type of chemistry. The role of water in Wittig reactions and its mechanism are discussed