A rhodium-catalyzed asymmetric synthesis of silicon-stereogenic dibenzooxasilines has been developed. High enantioselectivities have been achieved by employing (S,S)-Me-Duphos as the ligand through "enantioselective transmetalation".

Pericyclic reactions are a class of transformations that comprise sigmatropic rearrangements, group transfer reactions, cycloadditions and electrocyclic reactions. Since Woodward and Hoffmann delineated a rationale for the mechanism and stereochemistry of these reactions, they have become powerful synthetic tools. [1][2][3] Whilst sigmatropic rearrangements and cycloadditions are cornerstones of contemporary synthetic methodology, many electrocyclic reactions are not fully exploited. The high temperatures often required to initiate these transformations, and difficulties associated with assembling polyene precursors, often preclude their use in complex-molecule synthesis. There are no general methods for the asymmetric catalysis of electrocyclic reactions, and as a consequence opportunities for exerting stereocontrol in these manifolds are limited. [4,5] The archetypal 6p electrocyclic reaction-the hexatriene cyclization-proceeds suprafacially, and isoelectronic charged systems such as pentadienyl anion derivatives electrocyclize through the same mode (Scheme 1 A). [6,7] We reasoned that exploring these anionic manifolds could provide an opportunity for exerting stereocontrol in an electrocyclic reaction. Here we describe this approach, and outline a catalytic asymmetric process for the generation of functionalized indolines. [8] It has been demonstrated that the rates of electrocyclization of both 6p and 8p systems are dependant on the nature of the substituents in and around the extended p-system. Synergistic combinations of electron-withdrawing and donating groups have been demonstrated to significantly lower the barrier to cyclization.[9] Similarly, the presence and positioning of heteroatoms within the p-system is known to have profound effects on the propensity for cyclization.[10] In a series of pioneering investigations by Speckamp et al.[11] and others, [12] the [1,5]-electrocyclic reaction [13] of 2-aza-pentadienyl anions has been demonstrated to be a powerful and efficient process. [14] We rationalized that in order to control the absolute direction of rotation of the orbitals in the ringclosing process (the torquoselectivity), [15] we needed to block one p-face of the delocalized anionic component of the reaction and rely on the stereospecificity of the electrocyclic process to direct the stereochemical outcome (Scheme 1 B). One way to accomplish this would be to exploit tight ionpairing in an organic solvent, using a chiral counterion to select one of the two faces of a pentadienyl anion or equivalent, and hence influence the enantioselectivity of the cyclization reaction. Asymmetric phase-transfer catalysis [16] relies on the tight-ion pairing principle, and has been demonstrated to be a powerful and practical approach to the generation of enantioenriched materials. As a consequence, we examined a series of cinchona alkaloid-derived quaternary ammonium salts 1-6 as chiral counterions for our approach to asymmetric electrocyclization (Scheme 2). [17] We prepared a series of model benzaldimines 7-9 (a...

The asymmetric synthesis of β-substituted lactones by catalytic asymmetric conjugate addition of alkyl groups to α,β-unsaturated lactones is reported. The method uses alkylzirconium nucleophiles prepared in situ from alkenes and the Schwartz reagent. Enantioselective additions to 6- and 7-membered lactones proceed at rt, tolerate a wide variety of functional groups, and are readily scalable. The method was used in a formal asymmetric synthesis of mitsugashiwalactone.

A catalytic enantioselective electrocyclic cascade leads to the construction of topologically complex systems comprising multiple rings with up to three stereocentres. This phase-transfer catalysed process offers a new strategy for the rapid and enantioselective generation of complex products bearing allcarbon quaternary stereogenic centres.

Scheme 1. Strategic approach to asymmetric electrocyclization.Scheme 2. Catalysts examined in this study.