Stereoselective [4 + 1] Annulation Reactions with Silyl Vinylketenes Derived from Fischer Carbene Complexes

Abstract

Stable silyl vinylketenes were prepared via the thermal reaction of Fischer carbene complexes with triisopropylsilyl- or tert-butyldimethylsilyl-substituted alkynes. The ability of these silyl vinylketenes to participate with carbenoid reagents in [4 + 1] annulation reactions was investigated. The best results were obtained with diazomethane and substituted diazomethane reagents, which provided cyclopentenone products in excellent yields and essentially complete stereoselectivity.

Total synthesis of (±)-naphthacemycin A9, possessing both antibacterial activity against methicillin-resistant Staphylococcus aureus and circumventing effect of β-lactam resistance

“…Therefore, to access 9, 2,4-dimethoxy-6-methylphenyl boronic acid, prepared from a corresponding bromobenzene (40), was subjected to Suzuki-Miyaura coupling with 2-bromo-5-methoxyphenyl iodide (41) 39 to furnish the biphenyl product (42) in 76% for 2 steps (Scheme 4). Chromination of 42 gave the corresponding chromium-carbene complex (43) in 77% yield.…”

“…40 In addition, in the approach for synthesizing annulated hydroquinones, the 4-aryl groups of respective cyclobutenones can be converted to quinone products via selective electrocyclic ring opening of the cyclobutenone to the corresponding conjugated ketene, followed by ring closure to provide the hydroquinone, as reported by Moore and Perri (Scheme 4). 41 Encouraged by these reports, we carried out thermal rearrangement of the cyclobutenone product (44), and after optimization of the reaction condition, this approach was turned out to be successful, in that 44 could be converted under microwave irradiation at 160-180°C in toluene to yield the half methyl-ether quinone, which was immediately methylated to furnish 45 in 42% (69% based on recovered 44) (Scheme 4).…”

P(NMe2)3-mediated reductive [1+4] annulation of isatins with enones: a facile synthesis of spirooxindole-dihydrofurans

“…10 In this context, although the carbenes have been rarely applied to [1 + 4] annulation reactions due to their intrinsic reactivity preference for cyclopropanations with alkenes and conjugated dienes, 11 several pioneering [1 + 4] annulation reactions of nucleophilic carbenes with vinyl ketenes, vinyl isocyanates, and electron-deficient conjugated dienes have been successfully achieved by Danheiser, 12 Rigby,13 Spino,14 and others. 15 Intrigued by these precedented studies and the attractive reactivity of Kukhtin-Ramirez adducts as carbene equivalents or 1,1dipoles, we envisioned that a [1 + 4] annulation of Kukhtin-Ramirez adducts with electron-deficient conjugated enones would provide a new entry to five-membered oxaheterocycles like dihydrofurans (Scheme 2, bottom, eq. 3).…”

“…Under the standard conditions, readily available isatins 1d-h bearing electron-donating or -withdrawing group R 2 on the benzene ring uneventfully afforded their corresponding [1 + 4] annulation products in moderate to excellent yields with high diastereoselectivity (entries [15][16][17][18][19].…”

Recent Progress in the Synthetic Assembly of 2-Cyclopentenones

“…A selection of stable silyl vinyl ketenes bearing tricarbonylchromium(0) arene substituents 57 were prepared from Fischer carbene complexes and alkynes, and reacted with diazomethane, or a derivative thereof, to give the (4+1)annulation products 58 in excellent yields and in a completely stereoselective fashion (Scheme 18). 59 Removal of the chromium moiety was subsequently achieved using cerium(IV) ammonium nitrate. This process was applied in an elegant intramolecular mode, to provide an efficient synthesis of the rocaglamide skeleton, whereby the Fischer carbene alkyne 59 was transformed in a three-step process into adduct 60 in good yield and complete stereoselectivity (Scheme 18).…”