Reaction of Dianions Derived from γ-Substituted β-Keto Esters with Epoxides

Isomerisation of (E)‐2‐Tetrahydrofurylidenealkanecarboxylic Esters and Amides into Their (Z) Isomers by Chelation Control with Metallated Bases or Lewis Acids

“…Such stereocontrol has already been mentioned in the literature for the preparation of (E)-2-(2-tetrahydrofurylidene)propionates bearing a methyl group at C-3 of the ring. [8] From the (E) substrates 1, the two vinylogous enolates B and D could be generated by use of LDA, but only one Scheme 4 geometry (B) would allow the formation of the chelate A, resulting in the (Z) isomers 2 after ethanolysis. As 1,3-allylic strains became more important, the equilibrium between the two intermediate species would be in favour of the desired isomer and the (Z) selectivity would increase.…”

“…[1] During the course of our studies on the total synthesis of Pamamycin-607, [2] which contains three tetrahydrofuran rings, we used a diastereodivergent strategy to prepare two fragments (C 8 ϪC 18 and C 1Ј ϪC 11Ј ) differing in the syn and anti configuration of one chiral centre α to the tetrahydrofuran ring (Scheme 1). The key step of this strategy was the (E)/(Z) isomerisation of the substituted vinyltetrahydrofuran intermediate, which was obtained only in the more stable (E) configuration.…”

Total Synthesis of the Spliceosome Modulator Pladienolide B via Asymmetric Alcohol‐Mediated syn‐ and anti‐Diastereoselective Carbonyl Crotylation

“… [22] Exclusive epoxidation of the more substituted alkene was observed, providing glycidol 2 in highly enantiomerically enriched form. The unprotected epoxide was exposed to the dienolate derived from tert ‐butyl acetoacetate followed by acetic anhydride to form the product of epoxide ring opening 3 in 86 % yield [23] . Compound exists as a dynamic mixture of hydroxy ketone 3 and, predominantly, the 5‐membered lactol.…”

“…The unprotected epoxide was exposed to the dienolate derived from tert-butyl acetoacetate followed by acetic anhydride to form the product of epoxide ring opening 3 in 86 % yield. [23] Compound exists as a dynamic mixture of hydroxy ketone 3 and, predominantly, the 5-membered lactol.…”

Furanylidene systems from a tosylic acid-mediated, tandem desilylation-cyclization reaction of silyl ethers of monoalkynylated β-keto carbonyls

“…Compounds 5a -c were synthesized according to a known procedure with one modifi cation (Lygo and O ' Connor, 1992 ). Modifi cation: the fi nal reaction mixture was cooled in an ice-water bath (0 ° C) and quenched with an aqueous solution of saturated ammonium chloride.…”