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Terada reported a ring expansion reaction of 1,3‐dithianes in the presence of pyrrole using CPA 6u, forming 1,4‐dithiepane derivatives with high enantioselectivity. Indole also participated in the reaction successfully [168]. DFT calculation revealed that an unprecedented enantioselective 1,2‐sulfur rearrangement/stereospecific nucleophilic addition sequence is the concrete mechanism, which is controlled by the chiral counteranion (Scheme 2.81).

Scheme 2.81. Ring expansion reaction of 1,3‐dithianes.

Terada subsequently reported a vinylogous Wagner‐Meerwein shift through the activation of trichloroimidate as the leaving group to yield α‐vinyl cyclopentanones with high enantioselectivity using chiral phosphoramide 14c as the chiral Brønsted acid catalyst. Quaternary carbon centers were constructed with 57–95% ee (Scheme 2.82) [169].

Scheme 2.82. Vinylogous Wagner‐Meerwein shift.

Source: Based on [169].

Sun reported a House‐Meinwald rearrangement of racemic tetrasubstituted oxirane using CPA 6v or 6w to afford 2,2‐diarylcyclohexanone [170]. Chiral ketones bearing all‐carbon quaternary center at the α‐position were obtained efficiently with 86–97% ee (Scheme 2.83).

Scheme 2.83. House‐Meinwald rearrangement.

Zhu and co‐workers reported pinacol rearrangement of 1,2‐tertiary diols to furnish 2‐alkynyl‐2‐arylcyclohexanones catalyzed by N‐triflyl chiral phosphoramide 14c (Scheme 2.84a) [171]. They also reported Meinwald rearrangement of spirooxiranes to give 2,2‐diarylcyclohexanones (Scheme 2.84b).Both reactions resulted in the formation of quaternary all‐carbon stereocenters with 81–94% ee and 72–91% ee, respectively.

Scheme 2.84. Construction of quaternary stereocenters by Pinacol rearrangement (a) (

Source: Based on [171]

) and Meinwald rearrangement (b).