Synthesis of cyclohexenones
The use of allyl-palladium catalysis enables a one-step α,β-dehydrogenation of ketones via their zinc enolates. The optimized protocol utilizes commercially available Zn(TMP)2 as base and diethyl allyl phosphate as oxidant, operates under salt-free conditions, and tolerates a diverse scope of cycloalkanones.
D. Huang, Y. Zhao, T. R. Newhouse, Org. Lett., 2018, 20, 684-687.
A cationic Rh(I)-catalyzed [5 + 1] cycloaddition of vinylcyclopropanes and CO affords either β,γ-cyclohexenones as major products or α,β-cyclohexenones exclusively, under different reaction conditions.
G.-J. Jiang, X.-F. Fu, Q. Li, Z.-X. Yu, Org. Lett., 2012, 14, 692-695.
Wittig reaction of aldehydes with (cyclopropylmethyl)triphenylphosphonium bromide delivered alkenyl cyclopropanes. UV irradiation in the presence of Fe(CO)5 converted these alkenyl cyclopropanes to 2-substituted cyclohexenones. This approach enabled a three-step synthesis of the tricyclic core of estrone methyl ether.
D. F. Taber, R. B. Sheth, J. Org. Chem., 2008, 73, 8030-8032.
The gold(I) complex MeAuPPh3 is a highly effective catalyst for the hydrative cyclization of 1,6-diynes to yield 3-methyl hex-2-enone derivatives with very good yield. A mechanism is proposed.
C. Zhang, D.-M. Cui, L.-Y. Yao, B.-S. Wang, Y.-Z. Hu, T. Hayashi, J. Org. Chem., 2008, 73, 7811-7813.
TFA-promoted exo carbocyclizations of nonterminal alkynals gave good to excellent yields of exo cycloalkenones. On the other hand, terminal 5-alkynals gave endo carbocyclizations to cyclohexenones. These carbocyclizations can be considered as tandem alkyne hydration/aldol condensation processes.
C. González-Rodríguez, L. Escalante, J. A. Varela, L. Castedo, C. Sáa, Org. Lett., 2009, 11, 1531-1533.
Pd(DMSO)2(TFA)2 as a catalyst enables a direct dehydrogenation of cyclohexanones and other cyclic ketones to the corresponding enones, using O2 as the oxidant. α,β-Unsaturated carbonyl compounds are versatile intermediates in the synthesis of pharmaceuticals and biologically active compounds. The substrate scope includes heterocyclic ketones and several natural-product precursors.
T. Diao, S. S. Stahl, J. Am. Chem. Soc., 2011, 133, 14566-14569.
Pd-catalyzed Suzuki-Miyaura and Sonogashira cross-coupling reactions of phosphonium salt-activated, cyclic 1,3-diones provide β-substituted cyclic enones in good isolated yield and high generality with respect to both substrates and coupling partners.
S.-M. Yang, G.-H. Kuo, M. D. Gaul, W. V. Murray, J. Org. Chem., 2016, 81, 3464-3469.
Olefin substrates can be converted to the corresponding enones or 1,4-enediones in very good yields in short reaction times using a Cu(II) 2-quinoxalinol salen complex as the catalyst and tert-butyl hydroperoxide (TBHP) as the oxidant via allylic activation. The reaction tolerates many additional functional groups.
Y. Li, T. B. Lee, T. Tang, A. V. Gamble, A. E. V. Gorden, J. Org. Chem., 2012, 77, 4628-4633.
Brřnsted acid-mediated cyclizations of siloxyalkynes with simple arenes and alkenes afforded substituted tetralone and cyclohexenone derivatives. A wide range of substrates can be employed in these carbocyclizations.
L. Zhang, S. A. Kozmin, J. Am. Chem. Soc., 2004, 126, 10204-10205.
A palladium-catalyzed oxidative cross-coupling of vinyl boronic acids and cyclic α-diazocarbonyl compounds enables an efficient synthesis of 1,3-diene compounds. Mechanistically, the reaction involves migratory insertion of palladium carbene as the key step.
Y. Xia, Y. Xia, Z. Liu, Y. Zhang, J. Wang, J. Org. Chem., 2014, 79, 7711-7717.
Chiral BINOL-derived Brřnsted acids catalyze a highly enantioselective asymmetric Morita-Baylis-Hillman (MBH) reaction of cyclohexenone with aldehydes. The reaction requires 2-20 mol% of the chiral Brřnsted acid and triethylphosphine as the nucleophilic promoter.
N. T. McDougal, S. E. Schaus, J. Am. Chem. Soc., 2003, 125, 12094-12095.
A mild, palladium-catalyzed intramolecular oxidative alkylation of various ζ-alkenyl β-diketones and ζ-alkenyl β-keto esters in the presence of CuCl2 at room temperature formed the corresponding 2-cyclohexenones in high yield.
T. Pei, X. Wang, R. A. Widenhoefer, J. Am. Chem. Soc., 2003, 125, 648-649.
A solvent-free ytterbium(III) triflate promoted, zinc(II) chloride catalyzed Conia-ene reaction allows the construction of cyclic enones. In the presence of zinc(II) chloride and ytterbium(III) triflate, a variety of linear β-alkynic β-keto esters and β-diketones were cyclized under neat conditions in good yields. The selectivity toward five- or six-membered-ring carbocycles depends on substituents at the terminal alkynes.
Y. Liu, R.-J. Song, J.-H. Li, Synthesis, 2010, 3663-3669.
A highly stereoselective tandem Michael addition-Wittig reaction of (3-carboxy-2-oxopropylidene)triphenylphosphorane and α,β-unsaturated aldehydes gives multifunctional 6-carboxycyclohex-2-en-1-ones in excellent diastereo- and enantioselectivities by employing the combined catalysis of a bulky chiral secondary amine, LiClO4, and DABCO.
Y.-k. Liu, C. Ma, K. Jian, T.-Y. Liu, Y.-C. Chen, Org. Lett., 2009, 11, 2848-2851.
An efficient and regioselective Yb(OTf)3-promoted palladium-catalyzed oxidative cyclization of γ-heteroalkenyl β-keto amides has been developed. Under simple aerobic condition, various six-, seven-, and eight-membered-ring N- and O-heterocycles were obtained in excellent yield.
K.-T. Yip, J.-H. Li, O.-Y. Lee, D. Yang, Org. Lett., 2005, 7, 5717-5719.
A simple chiral primary amine catalyses a highly efficient reaction for the synthesis of both Wieland-Miescher ketone and Hajos-Parrish ketone as well as their analogues in high enantioselectivity and excellent yields. This procedure represents one of the most efficient methods for the synthesis of these versatile chiral building blocks even in gram scale with 1 mol% catalyst loading.
P. Zhou, L. Zhang, S. Luo, J.-P. Cheng, J. Org. Chem., 2012, 77, 2526-2530.
[Cp*RhCl2]2 is a very effective catalyst for C-C coupling of quinones with aryl and alkyl boronic acids under mild conditions. This reaction provides a practical, efficient method for the synthesis of aryl- and alkylquinones in good yields at room temperature and under base-free conditions.
D. Wang, B. Ge, L. Du, H. Miao, Y. Ding, Synlett, 2014, 25, 2895-2898.
A scalable, direct functionalization of various quinones with several boronic acids proceeds readily at room temperature in an open flask using catalytic silver(I) nitrate in the presence of a persulfate co-oxidant. The scope with respect to quinones is broad, with a variety of alkyl- and arylboronic acids undergoing efficient cross-coupling.
Y. Fujiwara, V. Domingo, I. B. Seiple, R. Gianatassio, M. Del Bel, P. P. S. Baran, J. Am. Chem. Soc., 2011, 133, 3292-3295.