Spiroketals via oxidative rearrangement of enol ethers

Oxidative rearrangement of cyclic enol ethers leads to alpha-alkoxyesters. In the presence of a neighboring spiroether, this approach provides a stereoselective access to spiroketals. A modified proposal for the biosynthesis of acutumine is presented.     

Formation of yohimbanones via a novel rearrangement

An investigation of the reaction of tryptophan and its methyl ester with ninhydrin has been conducted. In the reaction of tryptophan with ninhydrin only one product, yohimbanone (1), is isolated. In contrast, an intermediate, Pictet-Spengler product 3, is isolated from the reaction of tryptophan methyl ester with ninhydrin. The isolation of this intermediate provides support for a proposed mechanism of this novel rearrangement, which is presented herein.     

Indirect hydroquinone succinoylation via a photo-fries rearrangement. Application to the synthesis of enol lactones.

A photo-Fries rearrangement of hydroquinone succinates is the key step that allows the preparation of some γ-aryl enol lactones by dehydration of the corresponding aroyl propionic acids.     

Optimum geometries and relative energies for cytosine,thymine, uracil,the imino tautomer of cytosine,the enol tautomer of thymine,and the enol tautomer of uracil by the MINDO/2 SCF MO method

A MINDO /2 SCF MO geometry optimization of cytosine (C), thymine (T), uracil (U), the imino tautomer of cytosine (C*), the enol tautomer of thymine (T*), and the enol tautomer of uracil (U*)was made. The optimized geometries for cytosine, thymine, and uracil agree well with crystallographic data. The optimized geometries for the tautomers show the correct trends in bond lengthening and bond angle except for the C4—O4 length and C4—O4—H angle of T* and U*. The energies of tautomerization were found to be 10.3, ?9.0, and ?14.2 kcal/mol for C?C*, T?T*, and U?U*, respectively, when optimized geometries are used. The overestimation of the C4—O4—H angle is speculated to arise because of an inadequacy in the parametrization of the one-center integrals in MINDO /2.     

Synthesis of cyclobutanones and four-membered enol ethers by using a rearrangement reaction of enol triflates

A new synthetic method of cyclobutanone derivatives and four-membered enol ethers via an intramolecular cyclization of a ketone enolate was developed. The cyclization precursors, enol triflates having a silyloxy group at the β′-position, were synthesized from the corresponding β-hydroxy ketones, which were prepared via an aldol reaction of a cycloalkanone and an aldehyde. Under the influence of TBAF, the enol triflates afforded a cyclobutanone or a four-membered enol ether through rearrangement of the trifluoromethanesulfonyl group followed by an intramolecular C- or O-alkylation reaction.     

A novel rearrangement of eudesmanolide enol ester epoxide: facile 1-step construction of the salvialane sesquiterpene skeleton

A novel carbon–carbon rearrangement for the eudesmanolide sesquiterpene enol ester epoxide in the presence of a catalytic amount of Lewis acids (3–10 mol%) or an excess of NaOMe is disclosed. This reaction was developed to give a facile, one-step procedure for the construction of naturally occurring and bioactive salvialane sesquiterpenes. The reaction conditions were investigated extensively and a possible reaction mechanism is also discussed.     

Efficient synthesis of polysubstituted isochromanones via a novel photochemical rearrangement

A novel and convenient approach to the synthesis of polysubstituted isochromanones is described. Irradiation of 2-formyl phenylalkeno-derivatives with UV light in benzene solution afforded the corresponding products in up to 98% yield. The possible reaction mechanism is proposed and further supported by the isotopic experiments.     

Regioselective synthesis of alpha-methyl 2-methyleneglutarate via a novel lactonization-elimination rearrangement

[reaction: see text] A facile route to the alpha-methyl ester of 2-methyleneglutarate via a three-step sequence from 3-hydroxymethylcyclopentene is described. Regioselective formation of the monoacid from a diester precursor proceeds via a novel fluoride-mediated, tandem deprotection/rearrangement of O-silyl 2-(hydroxymethyl)dimethylglutarate.     

Synthesis of functionalized polycyclic compounds via a novel aromatic oxy-Cope rearrangement

Polycyclic compounds have been prepared by aromatic dianionic oxy-Cope rearrangements involving the π bonds of two phenyl rings in a [3,3] sigmatropic rearrangement.     

Diastereocontrol via the phenol- and palladium(II)-catalyzed Claisen rearrangement with cyclic enol ethers

Taking the judicious choice of either 2,6-dimethylphenol or PdCl₂(RCN)₂ as the catalyst, the Claisen rearrangements with the enol ethers of cyclic ketones are shown to proceed with a high level of either anti or syn diastereoselection, respectively.     

Lactone enol cation-radicals: gas-phase generation,structure, energetics,and reactivity of the ionized enol of butane-4-lactone

The cation-radical of 2-hydroxyoxol-2-ene (1(+*)) represents the first lactone enol ion whose structure and gas-phase ion chemistry have been studied by experiment and theory. Ion 1(+*) was generated by the McLafferty rearrangement in ionized 2-acetylbutane-4-lactone and characterized by accurate mass measurements, isotope labeling, metastable ion and collisionally activated dissociation (CAD) spectra. Metastable 1(+*) undergoes competitive losses of H-4 and CO that show interesting deuterium and (13)C isotope effects. The elimination of CO from metastable 1(+*) shows a bimodal distribution of kinetic energy release and produces (*)CH(2)CH(2)CHdbond;OH(+) (14(+*)) and CH(3)CHdbond;CHOH(+*) (15(+*)) in ratios which are subject to deuterium isotope effects. Ab initio calculations at the G2(MP2) level of theory show that 1(+*) is 105 kJ mol(-1) more stable than its oxo form, [butane-4-lactone](+*)(2(+*)). The elimination of CO from 1(+*) involves multiple isomerizations by hydrogen migrations and proceeds through ion-molecule complexes of CO with 14(+*) and 15(+*). In addition, CO is calculated to catalyze an exothermic isomerization 14(+*) --> 15(+*) in the ion-molecule complexes. Multiple consecutive hydrogen migrations in metastable 1(+*), as modeled by RRKM calculations on the G2(MP2) potential energy surface, explain the unusual deuterium kinetic isotope effects on the CO elimination.     

Decarboxylative elimination of enol triflates as a general synthesis of acetylenes

The enol trifluoromethanesulfonates 4, 8, 12, 17 and 20 of tert-butyl beta-ketodiesters and beta-ketoesters can be hydrolysed to the corresponding carboxylic acids by dissolution in trifluoroacetic acid. The dicarboxylic acids undergo mild decarboxylative elimination to give the acetylenic acids 4 and 9 in aqueous sodium bicarbonate solution at room temperature. Similarly, the monocarboxylic acids give the terminal and mid-chain acetylenes 13, 18, 21, and 24 by refluxing in acetone with potassium carbonate. One of the substituents on the acetylenes can be methyl, primary alkyl, secondary alkyl or ethynyl, and the other can be a carboxylic acid, hydrogen or primary alkyl, but the enol trifluoromethanesulfonates could not be prepared when one of the substituents was tert-butyl, nor when both substituents on the precursor to the acetylene were secondary alkyl.     

On the fragmentation pathway of the ionized enol of glycine in the gas phase

Density functional and second-order many body perturbation approaches were used to compute the potential energy surface for the fragmentation of the ionized enol of glycine [H2NCH = C(OH)2]+* into water and aminoketene radical cation [H2N-HC = CO]+*. Two possible pathways were considered. The potential energy surfaces obtained are very similar and both predict the existence of a molecular complex in which the water is coordinated to the aminoketene moiety in two different fashions with a noticeable binding energy. The fragmentation is kinetically controlled by the step in which the molecular complex is formed from the most stable cation enol of glycine. Our quantum-mechanical data confirm the hypothesis that the ylide ion [H3NCHCOOH]+* is an intermediate in the water loss.     

Arylation of [6,6]-spiroacetal enol ethers: reactivity and rearrangement

Attempts to selectively arylate [6,6]-spiroacetal enol ethers at the 2-position delivered unexpected results. Palladium-mediated arylation conditions afforded the double-Heck product, whereas reaction with benzenesulfinic acid resulted in a facile rearrangement into the corresponding 5-phenylsulfonyl-3,4,5,6-tetrahydrochromans, providing access to 5-aryl-3,4,5,6-tetrahydrochroman and hexahydrochroman derivatives.