Sharpless asymmetric epoxidation of endiynes; epoxides

Zac Etheridge ([email protected]),
A contribution from the Caddick Group Group, Sussex

Chemicals Used
tert-butylhydroperoxide (5.5M soln in decane, fluka, 0.9 mmol, 0.164 mL) titanium tetra-isopropoxide (distilled, 0.045 mL, 0.15 mmol) (-)-diethyl tartrate (distilled, 0.032 mL, 0.19 mmol) dichloromethane (distilled from calcium hydride) powdered 3A molecular sieves (activated by bunsen burner) enediyne (55 mg, 0.15 mmol)

Procedure
The enediyne is dissolved in DCM (2 mL), freshly activated molecular sieves added and the mixture stirred under nitrogen for 1 hour. Meanwhile to further freshly activated molecular sieves stirring in DCM (1.5 mL) at -18C is added the titanium tetraisopropoxide and the diethyl tartrate. This mixture is stirred for 1 hour then the TBHP added, folowed by the endiyne via canula. The mixture is allowed to warm to 0C and stirred for 15 hours then quenched with trimethylphosphite (1 mL) then 10% aqueous tartaric acid (2 mL) added. The organics are extracted 3 times with ether then washed with sodium bicarbonate solution and brine then dried over magnesium sulphate, filtered and concentrated. Column chromatography eluting with 5% ether/PE affords the product as a yellow oil (0.038g, 69%)

Author's Comments
It is absolutley imperative this reaction is kept completely anhydrous. All reagents except TBHP must be freshly distilled, no matter what purity they are claimed to be. Molecular sieves should be freshly activated for several minutes by heating with a bunsen, then allowed to cool under nitrogen before use in drying the enediyne and in activating the DET/Ti(OiPr)₄ mixture. A successful epoxidation will result in a lower spot by TLC, although it is possible to visualise both the DET and TBHP, so care should be taken that these are not being confused with the product.Note that in order for this reaction to be successful, the primary hydroxy group must be protected.

Data
1H NMR 3.82 (2H, d, J = 3.5, OCH₂), 3.76 (1H, app. q, J = 3.5, OCH), 3.61 (1H, s, CCH), 2.40 (1H, s, CCCH), 2.29 (1H, d, J = 4.1, OH), 0.90 (9H, t, J = 7.9, (SiCH₂CH₃)₃), 0.81 (9H, s, SiC(CH₃)₃), 0.51 (6H, q, J = 7.9, (SiCH₂CH₃)₃), 0.00 (6H, s, Si(CH₃)₂).

Lead Reference
Hirama M., Iguchi S., Wang G.X., J. Org. Chem. 2001, 66, 2146

Other References
Sharpless K.B. et al J. Am. Chem. Soc. 1987, 109, 5765.

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