Synthetic methods. IV. Halogenation of carbonyl compounds via silyl enol ethers

Stereoselective Synthesis and Physicochemical Properties of Liquid‐Crystal Compounds Possessing a trans‐2,5‐Disubstituted Tetrahydropyran Ring with Negative Dielectric Anisotropy

“…According to the second synthetic plan (strategy II, Scheme ), another method to synthesize 1 began with the hetero‐Diels–Alder reaction of cyclohexanecarboxaldehyde 8 and siloxydiene 13 11. As shown in Scheme , a mixture of 8 and 13 in toluene was treated with Me 2 AlCl at −78 °C to produce dihydropyran 12 , which was then treated in situ with NBS to provide bromide 11 in 80 % yield almost entirely as the cis isomer ( cis / trans =15:1 by 1 H NMR spectroscopy), the relative stereochemistry of which was determined by 1 H NMR spectroscopy 16. 17 The stereoselectivity of bromination was explained by transition‐state III , in which the bromonium cation was accessed from the β face of the silylenol ether, leading to the formation of the favorable chair‐like conformation.…”

“…[11] As shown in Scheme 6, a mixture of 8 and 13 in toluene was treated with Me 2 AlCl at À78 8C to produce dihydropyran 12, which was then treated in situ with NBS to provide bromide 11 in 80 % yield almost entirely as the cis isomer (cis/trans = 15:1 by 1 H NMR spectroscopy), the relative stereochemistry of which was determined by 1 H NMR spectroscopy. [16,17] The stereoselectivity of bromination was explained by transition-state III, in which the bromonium cation was accessed from the b face of the silylenol ether, leading to the formation of the favorable chair-like conformation. Because cycloadduct 11 was prone to isomerize into the trans form during the chromatography on silica gel, cycloadduct 11 was immediately treated with aryl Grignard reagent 15 to produce bromohydrin 23 in 60 % yield.…”

Synthesis of ( – )-cis-Neocnidilide

“…The corresponding acetate was obtained by treating with Ac20 in pyridine and submitted to the 1H-NMR analysis employing Eu(hfch. (8). A mixture of acrylic acid (2.07 g), pivaloyl chloride (3.47 g) and Et3N (2.91 g) in dry benzene (35ml) was stirred at room temperature for 30min, and the resulting precipitate was filtered off.…”

Stereospecific Synthesis of (−)-β-Turmerone and (−)-Bisacurol

“…amide (LDA)/Me 3 SiCl in THF at À 788, leaving the vicinal center of chirality at C(2) of 7a,b untouched. Subsequent bromination could be performed by treatment with N-bromosuccinimide (NBS ) in THF at 08 [12]. Evaluation of the NMR data of the crude bromo ketones demonstrates that in both cases, one diastereoisomer was formed predominantly (probably the axial bromo compounds).…”