94989-83-0Relevant articles and documents
Acyclic Stereoselection. 36. Simple Diastereoselection in the Lewis Acid Mediated Reactions of Enol Silanes with Aldehydes
Heathcock, Clayton H.,Davidsen, Steven K.,Hug, Kathleen T.,Flippin, Lee A.
, p. 3027 - 3037 (2007/10/02)
The Lewis acid mediated aldol reactions of enol silanes with aldehydes have been investigated.The effects of enol silane structure, both nature of the ligand at the silyloxy carbon and the geometry of the double bond, the aldehyde structure, and the nature of the Lewis acid have been studied.In general, the reactions of prochiral enol silanes with prochiral aldehydes show little simple diastereoselection (Table I).An exception is Z enol silane 7, derived from ethyl tert-butyl ketone, which shows synthetically useful anti selectivity.Enol silane 36 may therefore be used as an anti-selective propionate equivalent.The chiral α-alkoxy aldehyde 43 shows a high diastereofacial preference in its reactions with enol silanes 42c and 42d provided a Lewis acid capable of expanding its coordination beyond four is used (TiCl4 or SnCl4) (Table II).However, with the related ketene acetal 41b, only modest diastereofacial selectivity is seen (Table II).Aldehyde 43 also shows a high diastereofacial preference, in the chelation-controlled sense, in its reactions with prochiral enol silanes 5-9.However, the simple diastereoselection observed in the latter reactions (Table III) is quite different from that observed in the reactions of prochiral aldehydes with the same enol silanes.For example, enol silane 7, which shows good anti selectivity in its reactions with prochiral aldehydes, gives a 15:1 mixture of the two syn aldols in its reaction with 43; while the reverse is true with the propiophenone-derived enol silanes 8 and 9.Finally, the results obtained in this study, along with those reported by other investigators, have been formulated into a coherent mechanistic rationale involving open transition states of the sort depicted in Figures 1 and 3.
ALDOL ADDITIONS to α- AND β-ALKOXY ALDEHYDES : THE EFFECT OF CHELATION ON SIMPLE DIASTEREOSELECTIVITY
Reetz, M. T.,Kesseler, K.,Jung, A.
, p. 4327 - 4336 (2007/10/02)
The TiCl4 or SnCl4 mediated reaction of enol silanes with chiral α- or β-alkoxy aldehydes constitutes the only presently known, general way to perform aldol additions with chelation-control (asymmetric induction >90percent).If the enol silane is prochiral
