Diastereoselectivity in the Allylation of N-Trialkylsilylimines of O-Protected (2S)-Lactal − Some Unexpected Results

Abstract

A comparison of the diastereoselective addition of allylMgCl to differently O-protected N-trialkylsilyllactaldimine and to N-benzyllactaldimines is reported. The stereoselectivity strongly depends on the O-and N-protecting groups, on the metal cation, and on the reaction temperature, and varies from predominance of the anti diastereoisomer to predominance of the syn one. O-TBS-and O-TIPS-silyl imines displayed better anti diastereoselectivity than their O-alkyl counterparts. The N-protection produced unexpected results:

Acyclic Stereocontrol in the Additions of Nucleophilic Alkenes to α‐Chiral N‐Sulfonyl Imines

“…Instead, substitution with a more reactive nucleophile (methallylTMS, 1 b ) gave the anti products at low temperature, and better anti selectivity was observed at higher temperatures. A similar effect has been reported in chiral N ‐silyl imines and was attributed to entropic factors . Presumably, this temperature‐ and reactivity‐dependent selectivity change could be due to a change in the dominant reaction mechanism.…”

Solvent effects on stereoselectivity: more than just an environment

“…Moreover, in the temperature range À40 to À46 1C, we have an abrupt jump in diastereoselectivity. 25 This interruption marks two linear trends, one at a higher T and one at a lower T, with quite similar slopes but different intercepts, meaning that the jump is derived exclusively from a change in DDS a . This break in the Eyring plot can be readily explained in terms of the interconversion of solute-solvent clusters, such as at the inversion temperature, and, in our opinion, it can be effectively considered a proper T inv .…”

Stereoselective Allylation Reactions of Imines and Related Compounds

“…However, the selectivities are not very high and only with an additional chiral auxiliary on the nitrogen atom (in a typical double asymmetric induction experiment) a ds=98% is achieved. In fact, the influence of the nitrogen substituent regarding the diastereofacial induction had been pointed out by Cainelli and Galletti [27]. These authors demonstrate that the diastereoselectivity of the allylation of α-alkoxy imines using allylmagnesium chloride is strongly dependent on the O-and N-protecting groups, and on the reaction temperature.…”