Abstract
A range of achiral bispidines have been synthesized and evaluated as the stoichiometric ligand in the two-ligand catalytic asymmetric deprotonation of N-Boc pyrrolidine.
Highlights
Organolithium/chiral diamine complexes are established as versatile and useful reagents for the asymmetric synthesis of chiral molecules.[1,2] In this context, (–)-sparteine is the most widelyinvestigated diamine[3] following the pioneering work by Hoppe[4] and Beak.[5]
Reagents comprising organolithium reagents (e.g. s-BuLi or n-BuLi) and (–)-sparteine have been used for the asymmetric synthesis of a diverse range of compounds including amines,[6] alcohols,[7] phosphines,[8] ferrocenes[9] and paracyclophanes.[10]
We have optimised the two-step synthesis of bis-i-Pr-bispidine 6 and carried out the synthesis of other bispidines for use in the two-ligand catalytic asymmetric deprotonation of N-Boc pyrrolidine 4
Summary
Organolithium/chiral diamine complexes are established as versatile and useful reagents for the asymmetric synthesis of chiral molecules.[1,2] In this context, (–)-sparteine is the most widelyinvestigated diamine[3] following the pioneering work by Hoppe[4] and Beak.[5]. Use of 1.3 eq s-BuLi, 0.2 eq (+)-sparteine surrogate 7 and 1.2 eq bis-i-Pr-bispidine 6 gave silyl adduct (R)-5 in 66% yield and 94:6 er It is known[18] that sBuLi/(+)-sparteine surrogate 7 lithiates N-Boc pyrrolidine 4 faster than s-BuLi/(–)-sparteine 3 and we believe that this facilitates more efficient catalytic turn-over. In both of the examples shown in Scheme 2, success was achieved using a designed recycling diamine, bis-i-Pr-bispidine 6. In this paper, we present our efforts at identifying an alternative recycling bispidine ligand to bispidine 6
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