Science of Synthesis

Terada, M., Science of Synthesis: Stereoselective Synthesis, (2011) 3, 338.

Although the enantioselective intermolecular carbonyl-ene reaction has been extensively investigated, the enantioselective ene reaction with electron-deficient alkenes as the enophile has received limited attention even taking into account the intramolecular variant.[‌105‌,‌106‌] The first catalytic enantioselective intermolecular reaction[‌107‌] emerged during the development of the enantioselective Diels–Alder reaction, using α,β-unsaturated aldehydes ‌64‌ with cyclopentadiene as the ene component. In this intriguing process diphenylprolinol silyl ether ‌65‌[‌108‌,‌109‌] proved to be an outstanding organocatalyst in the presence of 4-nitrophenol, affording the ene adducts in an efficient and highly enantioselective manner, albeit as a mixture of diene regioisomers ‌66‌ and ‌67‌ (‌Scheme 25‌). Interestingly, the 4-nitrophenol is critical for the success of the reaction, since other strong acids, such as trifluoroacetic acid and hydrogen chloride are unsuccessful.

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M8	Meeeeeee ‌88‌ (eee%)	Meee (e)	Meeee (‌88‌/‌88‌)	eee (%)	Meeee (%)	Mee
Me	88	88	88:88	88	88	[‌888‌]
8-eeeeeeee	88	8	88:88	88	88	[‌888‌]
8-M8MM8M8	88	8	88:88	88	88e	[‌888‌]
8-MeM8M8	88	8	88:88	88	88	[‌888‌]
8-MeMM8M8	88	8	88:88	88	88	[‌888‌]
	88	8	88:88	88	88	[‌888‌]
8-eeeee	88	8	88:88	88	88	[‌888‌]
8-eeeeeee	88	8	88:88	88	88	[‌888‌]
8-MeMM8M8	88	8	88:88	88	88	[‌888‌]

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References