Enantioenriched 1-chloro-2-(1,3-dioxolan-2-yl)ethyllithium was generated by PhLi initiated sulfoxide-ligand exchange and deployed in situ for sequential double stereospecific reagent-controlled homologation (StReCH) of B-(2-chloro-pyrid-5-yl) pinacol boronate. This process afforded highly functionalized contiguous stereodiad motifs (typically, % ee ≥ 98%, dr ≥ 85:15) amenable to subsequent annulative transformations as demonstrated by the concise synthesis (5-7 steps) of cyclic adducts related to the analgesic alkaloid epibatidine.
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Heteroarenes are ubiquitous motifs in bioactive molecules, conferring favourable physical properties when compared to their arene counterparts. In particular, semisaturated heteroarenes possess attractive solubility properties and a higher fraction of sp carbons, which can improve binding affinity and specificity. However, these desirable structures remain rare owing to limitations in current synthetic methods.
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School of Chemistry, University of Bristol, Cantock's Close, BS8 1TS, United Kingdom.
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Department of Chemistry, The City College of New York (CCNY), 160 Convent Avenue, New York, NY, 10031, USA.
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Chemistry
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Department of Chemistry, Oregon State University, Corvallis, OR 97331 (USA).
Four putative functionalized α-chloroakyllithiums RCH2CHLiCl, where R=CHCH2(18 a), CCH (18 b), CH2OBn (18 c), and CH[O(CH2)2O] (18 d), were generated in situ by sulfoxide-lithium exchange from α-chlorosulfoxides, and investigated for the stereospecific reagent-controlled homologation (StReCH) of phenethyl and 2-chloropyrid-5-yl (17) pinacol boronic esters. Deuterium labeling experiments revealed that α-chloroalkyllithiums are quenched by proton transfer from their α-chlorosulfoxide precursors and it was established that this effect compromises the yield of StReCH reactions. Use of α-deuterated α-chlorosulfoxides was discovered to ameliorate the problem by retarding the rate of acid-base chemistry between the carbenoid and its precursor.
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