Studies on retro-[1,4] Brook rearrangement of 3-silyl allyloxysilanes. Observation of the formation of unusual 3,3-bissilyl enols

Detailed investigations of the retro-[1,4] Brook rearrangement of 3-silyl allyloxysilanes are described. Based on control experiments and NMR studies, rationalizations are proposed for the formation of 3,3-bissilyl enols, unusual compounds that are stable to acidic hydrolysis but that can be transformed into the corresponding aldehydes under basic hydrolysis conditions. These studies further show that the 3,3-bissilyl enolates can be O-alkylated by alkyl halides with complete chemoselectivity. This reaction provides a practical entry to various 3,3-bissilyl aldehydes and enol derivatives. As a demonstration of the synthetic utility of this approach, 3,3-bissilyl aldehyde was converted into bissilyl divinyl ketone, which can undergo an SiO₂-promoted Nazarov reaction to give cyclic β-silyl enone smoothly.     

Creation of quarternary stereocentres via [3,3]-sigmatropic rearrangement of allylic thiocyanates. A synthetic approach to (+)-myriocin

A stereoselective approach to the advanced precursor of (+)-myriocin, 3-deoxy-1,2:5,6-di-O-isopropylidene-3-methoxycarbonylamino-α-d-glucofuranose 3-C-carboxylic acid, via the [3,3]-sigmatropic rearrangement of allylic thiocyanates prepared from d-glucose is presented. From the observed results, supported by DFT calculations, we can conclude that the [3,3]-sigmatropic rearrangement of the thiocyanato group in allylic hexofuranosides is strongly influenced by the steric interaction of the 1,2-O-isopropylidene group in the transition states.     

Base-promoted aromatic [3,3] sigmatropic rearrangement of N-acyl-O-arylhydroxylamine derivatives

The base-promoted aromatic [3,3] sigmatropic rearrangement of N-acyl-O-arylhydroxylamines giving α-(2-hydroxyphenyl)amides was successfully demonstrated. The substrates were prepared from N-substituted hydroxylamines by N-acylation followed by copper(I)-mediated O-arylation with boronic acids. Treatment of the substrates with lithium hexamethyldisilazide (LiHMDS) in THF at 0?°C to room temperature generated the corresponding amide enolates. The aromatic [3,3] rearrangement of the enolates provided the desired products in moderate to good yields. A crossover experiment produced only intramolecular products and clarified that the reaction proceeds via the aromatic [3,3] sigmatropic rearrangement, not a bond-cleavage–recombination process. Our method is a formal α-arylation of amides.     

Synthesis of benzoxepins via rearrangement of dihydrofurans derived from carbonyl ylide [3+2] cycloaddition

Highly substituted benzoxepins can be prepared via BBr₃-induced rearrangement of functionalized dihydrofurans derived from carbonyl ylide [3+2] cycloaddition.     

A new approach to the synthesis of trifluoromethylated products of the [3,3]-sigmatropic rearrangement

CH-acids of general formula CF₃CH(X)CF₃ (X = CF₃, CO₂R) react with 2,3-unsaturated alcohols in the presence of bases to give trifluoromethylated products of the [3,3]-sigmatropic rearrangement. These reactions provide a convenient method for the synthesis of 2-alkenyl-2-trifluoromethylmalonates.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 7, pp. 1273–1277, July, 1994.     

Efficient synthesis of indoles using [3,3]-sigmatropic rearrangement of N-trifluoroacetyl enehydrazines

[3,3]-Sigmatropic rearrangement of N-trifluoroacetyl enehydrazines provides a novel method for the construction of indoles. N-Trifluoroacetyl enehydrazine having a cyclopentene ring smoothly underwent [3,3]-sigmatropic rearrangement followed by cyclization to give indolines in excellent yield. On the other hand, both cyclohexenyl N-trifluoroacetyl enehydrazine and acyclic N-trifluoroacetyl enehydrazine gave indoles in good yield. Additionally, the substituent effect on the benzene ring was also studied. The rearrangement of N-trifluoroacetyl enehydrazines proceeded smoothly even under either aqueous or solvent-free conditions.     

Domino "[3+3]-cyclization-homo-Michael" reactions of 1,3-bissilyl enol ethers with 1,1-diacylcyclopropanes

The Lewis acid mediated domino "[3+3]-cyclization-homo-Michael" reaction of 1,3-bissilyl enol ethers with 1,1-diacylcyclopropanes allows an efficient one-pot synthesis of functionalized salicylates containing a halogenated side chain. A great variety of substitution patterns could be realized by variation of the starting materials and of the Lewis acid. The mechanism of the domino process was studied.     

alpha-Substituted acylsilanes via a highly selective [1,4]-Wittig rearrangement of alpha-benzyloxyallylsilane

alpha-Benzyloxyallylsilane undergoes efficient [1,4]-Wittig rearrangement to generate an enolate intermediate that can be trapped with various electrophiles, thereby providing a new synthetic approach to substituted acylsilanes.     

Preparation of 8-Amido-2-dimethylamino-1,2,3,4-tetrahydro-2-dibenzofurans and several fluorinated derivatives via [3,3]-sigmatropic rearrangement of O-aryloximes

Methodology to prepare 8-amido-2-amino-1,2,3,4-tetrahydro-2-dibenzofurans, analogues with a fluorine substituent incorporated in the 6-, 7-, and 9-positions, and a difluorinated analogue with fluorines in the 6- and 9-positions is described. The tetrahydrodibenzofuran ring systems are prepared by acid-catalyzed [3,3]-sigmatropic rearrangement of O-aryloximes. Regioselective reactions to prepare the requisite O-aryloxime intermediates from commercially available fluorobenzene derivatives are discussed.     

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.     

Efficient synthesis of 2- and 3-substituted-2,3-dihydro [1,4]dioxino[2,3-b]pyridine derivatives

A versatile new approach for the synthesis in three steps of 2-substituted-2,3-dihydro[1,4]dioxino[2,3-b]pyridines B via a Smiles rearrangement using easily available reagents is described. A study illustrating the influence of experimental conditions on the progress of the reaction is reported.     

Synthesis of 1,3-diaminated stereotriads via rearrangement of 1,4-diazaspiro[2.2]pentanes

The synthesis of 1,3-diaminated stereotriads via the bis-aziridination of allenes is reported. The reactive 1,4-diazaspiro[2.2]pentane intermediates undergo a mild Br?nsted acid-promoted rearrangement to yield 1,3-diaminated ketones in good yields with excellent stereocontrol. Directed reduction of the ketone can be achieved to yield a C-N/C-O/C-N stereotriad in high dr. The ability to transfer the axial chirality of the substrates to the products allows for the facile preparation of enantioenriched stereotriads from allenes in two simple steps.     

Synthesis and regioselectivity of the [3,3]-sigmatropic rearrangement of substituted 2-allylthio- and 2-allylseleno-1,4-dihydropyridines

The reaction of 3-cyano-1,4-dihydropyridine-2-thiolates and the corresponding selenolates with allyl bromide gave 2-allylthio- and 2-allylseleno-3-cyano-1,4-dihydropyridines, which, upon heating in various solvents or in the solid state, undergo [3,3]-sigmatropic rearrangement to give 3-cyano-3-allyl-1,2,3,4-tetrahydropyridine-2-thiones and the corresponding selenones. The resultant pyridinethiones are alkylated by alkyl halides at the sulfur atom and are oxidized by iodine to give disulfides.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 8, pp. 1888–1895, August, 1991.     

[3,3]-sigmatropic rearrangement of allyl 2-benzothienyl sulfide

Allyl 2-benzothienyl sulfide at 20–120 °C undergoes a [3,3]-sigmatropic rearrangement to give 3-allylbenzothiophene-2-thiol. The kinetic parameters of the reaction were studied. Under the experimental conditions the thiol undergoes cyclization to give 2-methyl-2,3-dihydrobenzothieno[2,3-b]thiophene, 2-methyl-benzothieno[2,3-b]thiophene, and benzothieno[2,3-b]dihydrothiopyran. Allyl 3-methyl-2-benzothienyl sulfide does not form a thiol even at 150–190 °C but rather forms only bis(3-methyl-2-benzothienyl) disulfide.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 5, pp. 615–618, May, 1981.     

Efficient synthesis of [1,3]oxazino[2,3-a]quinoline derivatives by a novel 1,4-dipolar cycloaddition involving a quinoline-DMAD zwitterion and carbonyl compounds

An efficient synthesis of [1,3]oxazino[2,3-a]quinoline derivatives via a three-component reaction of quinoline, DMAD and carbonyl compounds is described.     

Synthesis of optically active β-alkyl aspartate via [3,3] sigmatropic rearrangement of α-acyloxytrialkylsilane

The synthesis of four types of optically active β-carbon-substituted analogs of threo-β-hydroxy aspartate (THA) and a β-carbon-substituted analog of threo-β-benzyloxy aspartate (TBOA), which are potent blockers of excitatory amino acid transporters in the mammalian central nervous system, via the chirality-transferring ester-enolate Claisen rearrangement of α-acyloxytrialkylsilane is described.     

Efficient and green telescoped process to 2-methoxy-3-methyl-[1,4]benzoquinone

A telescoped process for the preparation of 2-methoxy-3-methyl-[1,4]benzoquinone is disclosed. When this novel process is compared to the prevailing method that utilizes Na2Cr2O7 as the oxidant, the novel process represents a high yielding (95%), green, and environmentally benign alternative with H2O2 and HNO3 as the oxidants and CH3COOH as the reaction medium.     

Highly diastereoselective synthesis of dispiro[1,4-dithiane/dithiepane]bisoxindoles via Stevens rearrangement

Synthesis of dispiro[1,4-dithianes/dithiepanes]bisoxindoles from the reaction of 3-diazo-2-oxindoles and spiro-1,3-dithiolaneoxindole or -1,3-dithianeoxindole derivatives in the presence of rhodium(II) acetate dimer as a catalyst was demonstrated under mild conditions in a highly diastereoselective manner. These dispiro-compounds were obtained via intermolecular sulfonium ylides and followed by Stevens rearrangement.