Highly Stereoselective α-Hydroxyalkylation/Chlorination of α,β-Acetylenic Ketones—An Efficient Approach to β-Halogeno Baylis–Hillman Adducts

Halogeno Aldol Reaction of Ethyl Vinyl Ketone and Aldehydes Mediated by Titanium Tetrachloride

“…This is similar to ,,--unsaturated aldehyde-based system where the dynamically controlled syn stereoselection was proven to be dominant at -78 °C in the same solvent in which TiCl 4 /(n-Bu) 4 NI combination was employed as the halogen source (I -) [8c]. The working hypothesis of this reaction is proposed as shown in Scheme 4 [7,10]. The initial step involves the addition of TiCl 4 to the ethyl vinyl ketone to generate the titanium enolate.…”

Chalcogenide–Lewis Acid Mediated Tandem Michael Aldol Reaction — an Alternative to the Morita–Baylis–Hillman Reaction and a New Development

“…On the other hand, reactions of methyl propiolate ( 30 , R 2 = OMe) provided 3‐chloro‐2‐(α‐hydroxybenzyl)acrylates ( 31 , R 2 = OMe) in 36–75 % yields as a mixture of geometrical isomers ( E / Z = 1:1.6–1:7). When a chalcogenide catalyst was not used, the isomer ratios were changed in the case of 3‐butyn‐2‐one, and no product was formed in the reactions of methyl propiolate 8c,16. These findings indicate that a chalcogenide plays an important role in the reaction of the electron‐deficient alkynes.…”

“…Li et al8c reported the reactions of alkynyl ketones with aldehydes in the presence of only TiCl 4 . At almost the same time, we reported the reactions mediated by Me 2 S–TiCl 4 16.…”

Titanium(IV) Bromide and Boron(III) Tribromide Promoted Baylis-Hillman Reactions of Arylaldehydes with But-3-yn-2-one

“…From the point of view of synthetic chemistry, the a-bromomethylene aldols 1 are more useful than the corresponding a-chloromethylene aldols because they can be more easily subjected to the transition-metal-catalyzed reactions such as allylic substitution and Suzuki coupling reactions. However, only one example in which TiBr 4 was used as the Lewis acid for this reaction has been examined at room temperature in these reports [5] [6]. Thus, we more carefully examined the reaction of arylaldehydes with but-3-yn-2-one in the presence of the Lewis acids TiBr 4 or BBr 3 (1.4 equiv.)…”

“…However, other arylaldehydes needed higher temperature and longer time to complete the reaction ( Table 1, Entries 5 ± 8). The geometry of the major isomer was determined by 1 H-NMR NOESY spectroscopic data and comparison of the spectral data with those of the corresponding a-chloromethylene aldols [5] [6].…”