Article pubs.acs.org/joc
Cu-Mediated Rearrangements of Allenylcyclopropanols to Cyclopentenones: Two Divergent Pathways Naresh Tumma, Eppa Gyanchander, and Jin Kun Cha* Department of Chemistry, Wayne State University, 5101 Cass Ave, Detroit, Michigan 48202, United States S Supporting Information *
ABSTRACT: Two CuCN-mediated rearrangement reactions of allenylcyclopropanols to cyclopentenones have been achieved by means of Et2Zn/CuCN·2LiCl or CuCN·2LiCl to afford 5-alkyl or 4-alkyl cyclopentenone regioisomers: the former conditions afford 5-alkyl substituted cyclopentenones via β-carbon elimination, whereas the latter result in the 4-alkyl substituted regioisomers with concomitant oxidation at the γ-position, via a free radical mechanism.
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INTRODUCTION Cyclopentenones serve as versatile intermediates for the synthesis of a broad range of natural products and bioactive compounds. The development of efficient methods for preparing cyclopentenones has been an active area of research.1 In addition to the well-known Pauson−Khand and Nazarov reactions, transition metal-catalyzed cycloisomerization has provided new approaches to cyclopentenones. By following up on Trost’s Ru(II)-catalyzed ring expansion of alkynylcyclopropanols to cyclopentenones,2 we recently reported comparative mechanistic studies of the rearrangement reactions of alkynyl- and allenylcyclopropanols, which afforded the identical cyclopentenone products, yet via different reaction pathways (Scheme 1).3 Their mechanisms share β-carbon elimination (A → B) as a key common feature.4,5 Since inexpensive CuCN was effectively utilized in SN2′ alkylation of cyclopropanols with allylic and propargylic derivatives via β-carbon elimination,6 the action of Cu(I) on allenylcyclopropanols was anticipated to provide cyclopentenones. We report herein a divergent behavior in the CuCN-catalyzed rearrangement of allenylcyclopropanols, depending on the use of Et2Zn as an additive.
Scheme 1. Ring Expansion Reactions
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RESULTS AND DISCUSSION Sequential treatment of cyclopropanol 7a in THF with diethylzinc and THF-soluble CuCN·2LiCl7 indeed afforded cyclopentenone 8 (Table 1), which had been obtained previously under Ru(II)-catalyzed conditions (in THF at reflux).3 The reaction was completed at rt in 45 min. The yield was slightly improved to 68% in a 0.05 M concentration solution (entries 1 and 2). Comparable results were also obtained with the cis-dialkyl isomer 7b. A substoichiometric amount of the Cu(I) catalyst gave similar results, albeit the reaction proceeded slowly (16−18 h for entries 4 and 5 vs 2 d).
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DOI: 10.1021/acs.joc.7b00432 J. Org. Chem. 2017, 82, 4379−4385
