Dearomatization of furans via [2,3]-Still-Wittig rearrangement

Furans and benzofurans of type 1 were dearomatized via the [2,3]-Still-Wittig rearrangement. Enol ethers 2 could be isolated or isomerized to the corresponding furans 3. The substitution pattern at the homofuranylic position had a strong influence on reaction behavior. Benzofurans rearranged with the greatest efficiency, and employment of a 3-substituted benzofuran (1; R′=CH₃) allowed the creation of a quaternary carbon center.     

Ring expansion of trans-divinyl ethylene oxide by oxonium ylide [2,3] sigmatropic rearrangement

Oxonium ylide [2,3] sigmatropic rearrangement of trans-divinyl ethylene oxide is observed upon its exposure to ethyl or t-butyl diazoacetate in the presence of either Rh(II) or Cu(II) catalysts to produce cis-6-vinyl-3,6-dihydropyran-2-carboxylates.     

Preparation and conformational study of CF3-containing enkephalin-derived oligopeptide

Incorporation of (2S,3S)-4,4,4-trifluorothreonine (F₃-Thr) instead of Thr in the enkephalin-derived hexapeptide led to the apparent conformational alteration due to the strong electron-withdrawing effect of the trifluoromethyl group by comparison with the original compound on the basis of their various NMR measurements.     

Lewis acid mediated [2,3]-sigmatropic rearrangement of allylic ammonium ylides

An investigation of the Lewis acid mediated [2,3]-sigmatropic rearrangement of allylic ammonium ylides has been conducted by employing various bases and boron Lewis acids. Using BBr₃ together with the phosphazene base 6, various allylic amines could be rearranged in good yields with low to excellent diastereoselectivity.     

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.     

Regioselective Synthesis ofPyrano[3,2-f]quinolin-2(7H)-onesand Furo[3,2-f]quinolin-2-ones

Summary.  A simple and efficient synthesis of the hitherto unreported ring systems pyrano[3,2-f]quinolin-2(7H)-one and furo[3,2-f]quinolin-2-one was accomplished via a thermal [3,3]-sigmatropic rearrangement. Received March 7, 2000. Accepted May 4, 2000     

Synthesis of functionalized cage propellanes and D3-Trishomocubanes via the ring-closing metathesis and acid-promoted rearrangement

Several functionalized cage propellanes and D₃-trishomocubanes containing spiro linkage have been reported starting with commercially available materials such as 1,4-hydroquinone and dicyclopentadiene. In this regard, Claisen rearrangement, Diels–Alder reaction (DA), ring-closing metathesis (RCM) and acid-promoted rearrangement have been used as key steps. The strategies described here, opens up new opportunities to assemble intricate cage systems that are difficult to construct by conventional methods. Carbocation intermediates generated during the rearrangement process play a prominent role in designing unusual cage systems. One of the rearranged cage compound's structure was unambiguously established by single crystal X-ray diffraction studies.     

Construction of 3-allylidene-4-vinyltetrahydrofurans and 3-allylidene-4-vinylpyrrolidines via sequential domino allylation/olefination of C-C triple bonds

An efficient method via sequential domino allylation/olefination of C-C triple bonds for the syntheses of five-membered heterocycles was developed by treatment 1,6-enynes with alkenes in the presence of a palladium catalyst. The configurations of the 1,3-dienes of the five-membered heterocycles are stereocontrolled.     

Diastereoselective synthesis of substituted prolines via 5-endo-trig cyclisations of aza-[2,3]-Wittig sigmatropic rearrangement products

The major diastereoisomers of aza-[2,3]-Wittig sigmatropic rearrangement products from α-amino acid derivatives are susceptible to a rare nucleophilic 5-endo-trig cyclisations of an amine onto a non-conjugated vinylsilane in high yield and complete diastereocontrol. Five examples are presented, with cyclisation yields between 35 and 87%. A rationale for the stereoselectivity of the cyclisation is forwarded based upon the steric control factors that have been documented for the aza-[2,3]-Wittig sigmatropic rearrangement. A discussion of the mechanism in the context of the reaction conditions is also presented. Oxidation of the silyl group to a hydroxyl group and complete removal were demonstrated for synthetic utility.     

Synthesis of 2,3-diaminodihydropyrroles via thioimidate cyclopropane rearrangement

The synthesis of novel 2,3-diaminodihydropyrroles is reported. The key step is a thiomethylimidate cyclopropane rearrangement to afford a pyrrolothiomethylimidate intermediate. Reaction of this intermediate with amines produced the title compounds in good yields. The scope and mechanism of the reaction is discussed.     

Five-membered 2,3-dioxo heterocycles. 2. Synthesis and [1,5]-sigmatropic rearrangement of 4-acyl-3-diphenylmethoxy-1,5-diphenyl-2,5-dihydropyrrol-2-ones

4-Acyl-1,5-diphenyltetrahydropyrrole-2,3-diones react with diphenyldiazomethane to give O-alkylation products, which upon heating undergo [1,5]-sigmatropic suprasurface rearrangement to form 4-[methyl- and 4-[-aryl(diphenylmethoxy)-methylene]-1,5-diphenyltetrahydropyrrole-2,3-diones.See [1] for Communication 1.Translated from Khimiya Geterotsiklicheskikh Soedinenii. No. 5, pp. 625–628, May, 1987.     

Synthesis of functionalized tetrahydroisoquinolines via palladium-catalyzed 6-exo-dig carbocyclization of 2-bromo-N-propargylbenzylamines

An efficient twostep synthetic strategy for tetrahydroisoquinolines has been described. The first step involves CuI catalyzed three-component coupling reaction of terminal alkyne, aldehyde and amine that provides the requisite propargyl amine. Regio- and stereoselective palladium-catalyzed 6-exo-dig carbocyclization of propargyl amine, which provides a concise access to functionalized tetrahydroisoquinolines in good yields has been developed.     

Stereoselective synthesis of spirocyclic oxindoles based on a one-pot Ullmann coupling/Claisen rearrangement and its application to the synthesis of a hexahydropyrrolo[2,3-b]indole alkaloid

An efficient and convenient approach to the synthesis of spirocyclic oxindoles from iodoindoles has been developed. The most striking feature of this approach is that the sequential intramolecular Ullmann coupling and Claisen rearrangement proceeds in a one-pot manner to afford 3-spiro-2-oxindoles in good yield with excellent diastereoselectivity. Application of this one-pot reaction to chiral non-racemic tertiary alcohol substrates resulted in complete chirality transfer to the spirocyclic quaternary carbon. Using this method, asymmetric total synthesis of (−)-debromoflustramine B was accomplished.     

Improved Ritter reaction with CF3-containing oxirane for an access to central units of protease inhibitors

The influence of fluorinated alcohols on the BF₃·Et₂O-promoted Ritter type reaction has been investigated. Trifluoroethanol/BF₃·Et₂O system allowed access to the central unit of protease inhibitors by the ring opening of fluorinated epoxide 2 with various nitriles.     

Synthesis of functionalized triarylmethanes by combination of FeCl3-catalyzed benzylations of acetylacetone with [3+3] cyclocondensations

Functionalized triarylmethanes are prepared in two steps by FeCl₃-catalyzed benzylation of acetylacetone to give 3-(diarylmethyl)pentane-2,4-diones and subsequent formal [3+3] cyclization of the latter with 1,3-bis(trimethylsilyloxy)-1,3-dienes.     

Palladium-catalyzed synthesis and fluorescence study of 2,3-diaryl-5-ethynylbenzo[e]indoles

A series of new ethynylated benzoindoles were synthesized starting with the nucleophilic attack of naphthoquinone by lithium acetylides. The resulting 1,4-diethynylnaphthalenes were applied in palladium catalyzed CN coupling - hydroamination domino reactions. The obtained compounds fluoresce in the near ultraviolet and blue spectral region with high quantum yields (65–78%).     

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.     

Stereoselective synthesis of CF3-substituted trans- and cis-enediynes via carbocupration or hydrostannation reactions of CF3-containing diyne

CF₃-containing diyne, readily prepared from (Z)-2,3,3,3-tetrafluoro-1-iodoprop-1-ene in two steps, was smoothly subjected to carbocupration reaction with various organocuprates, followed by treatment of the resultant carbocupration adducts with iodine, the corresponding vinyl iodides being formed in good yields in a cis-addition manner. Thus obtained iodide could be successfully converted into CF₃-substituted trans-enediynes in high yields. Also, hydrostannation reaction of the diyne proceeded well in a trans-addition manner to give the corresponding vinylstannane in high yield. The vinylstannane was also found to be effectively convertible to CF₃-substituted cis-enediynes in high yields.     

Synthesis of 2′,3′-dideoxy-6′-fluorocarbocyclic nucleosides via Reformatskii-Claisen rearrangement

2′,3′-Dideoxy-6′-fluorocarbocyclic nucleosides, analogues of highly bioactive carbovir and abacavir were synthesized. The notable steps were the incorporation of fluoromethylene group by way of silicon-induced Reformatskii-Claisen rearrangement of allyl bromofluoroacetate, the construction of the carbocyclic ring via ring-closing metathesis (RCM) and the introduction of base by Mitsunobu reaction.