Diastereomeric Enols and Enol Derivatives of 8‐Fluoro‐5,11‐dihydro‐10H‐dibenzo[a,d]cyclohepten‐10‐one with a Chiral 11‐Substituent

Abstract

Aldol condensation of 8‐fluoro‐5,11‐dihydro‐10H‐dibenzo[a,d]cyclohepten‐10‐one and (4S)‐2,2‐dimethyl‐1,3‐dioxolane‐4‐carbaldehyde provides access to the corresponding (E,Z)‐α,β‐unsaturated ketones with a (4R)‐chiral 11‐substituent. Subsequent hydrogenation affords a mixture of (11R)/(11S)‐epimeric ketones, which undergoes base‐catalysed equilibration resulting in selective crystallisation of the (11R) epimer. The enol intermediate and acyclic enol derivatives are shown to exist as two slowly interconverting conformers with a high inversion barrier of the 10,11‐disubstituted ring. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2006)

Mechanisms and Beyond: Elucidation of Fluxional Dynamics by Exchange NMR Spectroscopy

“…Application of 1 H‐ 1 H EXSY is critical for identifying rotation pathways in slowly interconverting tricyclic pyridones 43 (Scheme ), where a high ring flip barrier is a norm. This can be compared with certain bulky derivatives of dibenzosuberone . Mannuronic acid derivatives stand out because of their high reactivity and uncommon conformational behaviour.…”

“…This can be compared with certain bulky derivatives of dibenzosuberone. [73] Mannuronic acid derivatives stand out because of their high reactivity and uncommon conformational behaviour.T he 19 F- 19 FE XSY resultss how that a-mannuronic acid ester 44 flips easily and preferentially populates the unusual 1 C 4 -chair conformation (Scheme14). [74] Helicity ring flip ( Figure 6-II) occurs in bridged ansa-titanocenes slowly even in the crystal undergoing a1 808 reorientation about the pseudo-C 2 axis bisecting the X-Ti-X inter bond angle (21 kcal mol À1 energy barrier).…”

Determination of the geometry of acetoxyendiynes and acetoxyenynes by NMR heteronuclear 13C1H scalar couplings and 13C NMR chemical shifts. Structural assignment of the oxylipin natural products peyssonenynes A and B

“…[1] The series of acetoxyenynes 3a-d (Scheme 1) used as model systems was derived from propargyl ketones 6a-d by enol acetate formation using acetic anhydride and triethylamine in CH 2 Cl 2 . [19] In turn, ketones 6a-d were obtained from the addition of the anion of TIPS-acetylene (nBuLi, THF, −78…”

Further improvements of the dibutyl tin oxide-catalyzed regioselective diol tosylation

“…10 Initially, we wanted to avoid the use of tin derivatives because of the potential toxicity associated with organotins: unfortunately, the uncatalyzed reaction gave only 76% of desired monotosyl derivative 6a, accompanied by 8% of the starting material 2a and 16% ditosylated product 7a.…”

“…Calcd for C 10 To a solution of 2a (34.4 g, 0.1 mol) in toluene (1.2 L/mol) was added 1 (25 mg, 0.1 mol %) at 25°C. The mixture was stirred for 1 h. Diisopropyl ethylamine (21 ml, 1.2 equiv) was added and the reaction mixture was stirred for 5 min.…”