Abstract
The protocol of lithiation by means of lithium and a catalytic (5% molar) amount of DTBB (4,4'-di-tert-butylbiphenyl), applied to omega-chloro ortho ester 6 under Barbier-type conditions gives, after final acid-catalyzed methanolysis, the corresponding functionalized esters 8 or 9 (for chlorotrimethylsilane as electrophile) or, after ortho ester deprotection and acid catalyzed treatment, the delta-lactones 11. The procedure is also practical for bicyclic ortho esters 14: the beta-chloro OBO ester derivate generates the gamma- lactones 15 and the gamma-chloro OBO ester gives corresponding esters 8.
Highlights
Among organometallic reagents, organolithium compounds are the most reactive ones due to the high ionic nature of the carbon-lithium bond [1]
Our laboratory has approached the preparation of this organolithium reagent starting from α-chloroacetic acid by chlorine-lithium exchange using an excess of lithium in the presence of catalytic amount of DTBB (4,4’-di-tert-butylbiphenyl, 5% molar) [6]
We report the preparation of masked β, γ, and δ-lithium ester enolates starting from the corresponding ω-chloro ortho esters, by a chlorinelithium exchange using the arene-catalyze lithiation process, and their reaction with electrophilic compounds [16]
Summary
Organolithium compounds are the most reactive ones due to the high ionic nature of the carbon-lithium bond [1]. Carboxylate homoenolates can be prepared by deprotonation only when a stabilizing group is present at the β-position [7] Another procedure to prepare these organolithium intermediates is via halogen-lithium exchange, either bromo-lithium [8] or chloro-lithium, the later being performed in our group using the same methodology as for the corresponding α-enolate (arenecatalyzed lithiation) [6]. Our laboratory has been studying, during more than a decade, the preparation of functionalized organolithium intermediates [10] starting from different substrates (chlorinated, non-halogenated, or heterocyclic precursors [11]) by using an arene-catalytic lithiation under very mild reaction conditions [12,13,14] Employing this methodology organolithium compounds bearing a masked carbonyl function (masked lithium ω-enolates) have been prepared [12,15]. We report the preparation of masked β-, γ-, and δ-lithium ester enolates starting from the corresponding ω-chloro ortho esters, by a chlorinelithium exchange using the arene-catalyze lithiation process, and their reaction with electrophilic compounds [16]
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