Deprotonation lithium carbenoid generation

Configurational stability has also been confirmed for various metalated carbamates by Hoppe and coworkers. Remarkably, carbamate-protected alcohols such as 20 are deprotonated enantioselectively, when treated with i-butyllithium in the presence of (—)-sparteine. The lithium carbenoids like 21 (R = alkyl) thus generated turn out to retain their configuration (equation 11). Similar results have been obtained for a-lithiated amines and carbamate protected amines " . As a rule, dipole stabilization of the organolithium compounds in general also enhances the configurational stability of a-oxygen-substituted lithium carbenoids. 

Due to the acidifying effect of a second halogen atom, dihalo-snbstituted lithium carbenoids 30 can be obtained by deprotonation of geminal dihaloalkanes, the most frequently applied method for the generation of lithium carbenoids with this snbstimtion... 

Reactions of halogen-stabilized carbenoids with imines have been carried out using prefoimed lithium species (e.g. equation 36), or via a carbenoid generated from diiodomethane utilizing zinc-copper couple (Simmons-Smith conditions). A stereospecific ring closure is observed after the addition of lithiodichloromethane to a benzaldimine (equation 37).The addition of lithiochloro(phenylsulfon-yl)methane to aromatic imines affords 2-phenylsulfonyl-substituted aziridines, which can be deproton-ated and alkylated in excellent yield (Scheme 20). 

Note added in proof. ktotA synthesis of (-)-xialenon A using bicyclic alcohol 77 [Eq. (26)] represents the first application of enantioselective a-hthiation transannular C-H insertion of epoxides in natural product synthesis [78]. The first enantioselective generation - intermo-lecular nucleophile trapping of a lithium carbenoid has been described, via enantioselective a-deprotonation of the epoxides of oxa- and aza-bicyclic alkenes [e.g. 93 Eq. (30)] using alkyl-lithiums in the presence of (-)-sparteine or bisoxazoline 5 [Eq. (3)] [79]. Insertion of a second... 

Most lithium halocarbenoids decompose at temperatures above — 80 to — 90 °C. An essential condition for a successful synthesis via a (preformed) lithium halocarbenoid is that its generation proceeds at a high rate at temperatures below this range. For this reason, the more polar THF is preferred to Et20 as a main solvent for the deprotonation or lithium-halogen exchange reactions by which the carbenoids are formed. 

Oxiranyllithiums are generated from activated oxiranes by deprotonation or by tin/lithium permutation. Frequently oxiranyllithium/2-lithiooxyethylidene equilibria are invoked for the ensuing transformations. However, evaluated in the light of recent investigations, they are are mostly carbenoid- rather than carbene-mediated. 

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