Evans aldol reaction template

Bu₂BOTf and Hünig’s base turn (S)-4-benzyl-3-propionyloxazolidin-2-one into its Z-boron enolate; benzaldehyde adds through a chair transition state to give the (2S,3S) syn aldol with high diastereoselectivity.

Mechanism step by step

A Z-boron enolate of an oxazolidinone imide reacts through a Zimmerman–Traxler chair: the “Evans syn” aldol

1. Boron triflate binds – The imide C=O oxygen displaces triflate from Bu₂BOTf
2. Z-enolate – Hünig’s base removes the α-H: the Z-boron enolate (OBBu₂ and CH₃ cis)
3. Aldol through the chair – Six atoms (B, O, C, C, C, O) in a chair: the C–C bond forms as the aldehyde O binds boron (see the chair below)

Key points

• Zimmerman–Traxler: a chair-like six-membered transition state; the Z-enolate puts its CH₃ and the aldehyde R both pseudo-equatorial, so Z → syn (an E-enolate would give anti).
• Bu₂BOTf with a hindered amine gives the Z-boron enolate; short B–O bonds make the chair tight, so boron aldols are more selective than lithium aldols.
• Face selection: the oxazolidinone C=O points away from the enolate C–O (dipoles opposed) and the benzyl group blocks one face, giving the “Evans syn” isomer (dr typically > 99 : 1).
• The auxiliary is removed afterwards (LiOH/H₂O₂ → acid, LiBH₄ → alcohol) and recovered.

Drawn with 155 atoms, 12 arrows, 38 shapes – every one editable in MolDraw.

Sources

• Clayden, Greeves & Warren, Organic Chemistry, 2nd ed., OUP 2012, ch. 33 Textbook
• Clayden, Greeves & Warren, Organic Chemistry, 2nd ed., OUP 2012, ch. 41 Textbook
• Kürti & Czakó, Strategic Applications of Named Reactions in Organic Synthesis, Elsevier 2005 Textbook