Science of Synthesis

Matsumoto, K.; Katsuki, T.; Arends, I. W. C. E., Science of Synthesis: Stereoselective Synthesis, (2011) 1, 105.

The most reliable ketone catalyst is Shiʼs ketone (‌74‌; ‌Scheme 51‌),[‌123‌,‌124‌] which can be prepared in two steps from d-fructose. Epoxidation with this catalyst is quite effective for E-disubstituted and trisubstituted alkenes, and high enantioselectivity (generally >90% ee) is obtained in the presence of typically 10–30 mol% catalyst (‌Scheme 51‌). Various functional groups, such as ethers, esters, and acetals, remain unaffected under the reaction conditions. Allylic and homoallylic alcohols are also good substrates for the epoxidation, as the hydroxy group is also compatible with the reaction conditions.[‌125‌] High enantioselectivities can be achieved with dienes, enynes, alkenylsilanes, or enol esters.[‌126‌–‌129‌] The major problem in ketone-catalyzed epoxidation is decomposition of the catalyst by Baeyer–Villiger oxidation, which diminishes the conversion of the substrate. The decomposition process is markedly affected by the pH of the reaction medium, but the enantioselectivity is unaffected.[‌130‌] Whereas rapid decomposition occurs when the reaction is carried out at pH 7–8, higher pH values suppress the decomposition and improve the substrate conversion. Thus, the epoxidation is typically performed at around pH 10.5, and the pH is controlled by the addition of a base such as potassium carbonate or potassium hydroxide. To obtain the desired epoxides in high yield, it is important to maintain a constant pH throughout the reaction, so the epoxidation is carried out under buffered conditions.

‌Meeeee 88‌ Meeeeeeeeeee Meeee e Meeeeeee-Meeeeee Meeeee Meeeee Meeeeeee[‌888‌]

‌Meeeeeeeeee 88‌

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‌Meeeee 88‌ Meeeeeeeeeeee-Meeeeeeeee Meeeee Meeeeeeee eee eee Meeeeeeeeee ee Meeeeeee eee M-Meeeeeeeeeeee Meeeeee[‌888‌–‌888‌]

‌Meeeeeeeeee 88‌

M8	M8	M8	Meeeeeee (eee%)	Meee (°M)	ee (%)	Meeee (%)	Mee
Me	M	M	88	−88	88	88	[‌888‌]
Me	Me	M	88	−88	88	88	[‌888‌]
		M	88	−88	88	88	[‌888‌]
		M	88	−88	88	88	[‌888‌]
M≡MMe	Me	M	88	−88	88	88	[‌888‌]
		M	88	8	88	88	[‌888‌]

Meeeeee Meeʼe eeeeee (‌88‌) ee eeeeeeeeeee ee e eeeeeeee eee eee eeeeeeeeeee ee eeeeeeee-eeeeeeeee eeeeeee, eee eeeeeeeee eeeeee ‌88‌ ee eeeeeeeee ee eee eeee ee α,β-eeeeeeeeeee eeeeee ‌88‌ (‌Meeeee 88‌).[‌888‌] Mee eeeeeeee ee M-eeeeeeeeeeeee eee eeeeeeeeeeeeee eeeeeeeeee eeeeeeee ee e eeeeee eeeeeeeeeeeeeeee eeeeee, eeeeeeee eee eeeeeeeeeee ee M-eeeeeeeeeeeee eeeeeeeeee eeeeee eeee eeeeeeeeee eeeeeeeeeeeeeeeeee. Meeeee ‌88‌ eeee eeeeeee eeee eeeeeeeeeeeeeeeeee ee eee eeeeeeeeeee ee eeeeeee eeeeeee eeeeeee eeeeeeee-eeeeeeeee eeee.[‌888‌]

‌Meeeee 88‌ Meeeeeeeee Meeeeeeeeee ee α,β-Meeeeeeeeee Meeeee Meeeeeeee ee e Meeeeeeee Meeeee[‌888‌]

‌Meeeeeeeeee 88‌

Meeeeeeeeeee Meeeeeeee

8-Meeeee-8-eeeeeeeeeeeee (‌88‌, M8 = Me; M8 = M; M8 = Me); Meeeeee Meeeeeeee:[‌888‌]

(M)-Meee-8-eeeeeeeeeee (8.888 e, 8 eeee) eee eeeeeeeee ee MeMM/MM8(MMe)8 (8:8; 88 eM). Meeeee [e 8.88 M eeee ee Me8M8M8•88M8M ee 8.8 eM ee Me8(eeee) (88 eM)], Me8MMMM8 (88 ee, 8.88 eeee), eee Meeʼe eeeeee ‌88‌ (8.8888 e, 8.8 eeee) eeee eeeee eeee eeeeeeee. Mee eeeeeee eee eeeeee ee eeeee −88 °M (eeeeeeee eeeeeeeeeee; eeeeeeee eeeeeeeeeee −88 ee −88 °M) eeee e MeMe/eee eeee. M eeee ee Meeee (8.88 e, 8.88 eeee) ee 8.8 eM ee Me8(eeee) (8.8 eM) eee e eeee ee M8MM8 (8.8 e, 8.8 eeee) ee M8M (8.8 eM) eeee eeeeeeeeee eeeee eeeeeeee eeee 8 e eeee eeeeeee eeeee ee eeee eeeeeeee eeeeeee. Mee eeeeeeee eee eeee eeeeeeeeeee eeeeeeee ee eeeeeeee ee eeeeeee eee M8M. Mee eeeeeee eee eeeeeeeee eeee eeeeeee (8 × 88 eM), eeeeee eeee eeeee, eee eeeee (Me8MM8). Mee eeeeeee eee eeeeeeee ee eeeee eeeeeeeeeeeeee (eeeeee eee eeeeeeee eeee 8% Me8M ee eeeeeee, eeeeeee/Me8M 8:8 ee 88:8) ee eeee e eeeeeeeee eee; eeeee: 8.888 e (88%); 88.8% ee (eeeeee MM).

References