Tetrahydrofuran, isomerization

On the basis of fundamental experiments (see Section IV,A,2) some indenobenzazepine alkaloids have been efficiently synthesized from the corresponding protoberberines via 8,14-cycloberbines. For example, the cycloberbine 428 derived from the protoberberine 427 was heated with methanesulfonic acid in aqueous tetrahydrofuran to afford a 2 1 mixture of cis- and trans-indenobenzazepines 429 in 92% yield (Scheme 85). The mixture was methylated with methyl iodide to give the cts-N-methyl derivative 430 and the unchanged trans secondary amine (21%), which was very difficult to methylate and which gave the /V-methyl derivative only in 6% yield even on treatment with dimethyl sulfate for 43 hr. Contrary to the ordinary cases (Section IV,A,2), the trans derivative did not isomerize to the cis isomer 430 under various acidic conditions. Debenzylation of 430 by hydrogenolysis afforded fumarofine (417), which was converted to O-methylfumarofine (316) by methylation with diazomethane (215). 

The spontaneous isomerization of all-trans- carotenoids at room temperature is a slow process, and its rate depends on the solvent and the pigment structure. For example, the initial solutions of P-carotene in a mixture of tetrahydrofuran (THF), methanol, and acetonitrile containing ca. 95% of all -trans- and 5% of 9-cis- plus 13-co-isomers was transformed to 90% all-fra ns- p -ca rot ene and 9% of 9-cis- plus 13-cA-carotene after 24h of spontaneous isomerization at 25°C (Pesek et al. 

Isomeric tetrahydrofuran derivatives (47 and 48) are formed by copper(II)-catalyzed decomposition of methyl diazoacetate in 2-phenyloxetan (Scheme 62).99 Use of the chiral homogeneous catalyst, bis[N-(i )-a-phenylethylsali-cylaldiminato]copper(II) (49),100 causes asymmetric induction albeit with... 

During the study of the isomerization of diallyl ether with catalysts of this type, it was observed that the nature of the complex anion (Y) has a profound influence on the relative rate of isomerization with respect to cyclization. Diallyl ether can be isomerized to a mixture consisting mainly of allyl propenyl ethers (44) with a small amount of 2-methyl-3-methylene-tetrahydrofuran (45) and dipropenyl ether (46) the catalyst for this... 

All the rhodium catalysts gave exclusively -hexylsilanes from 1-hexene, but if the catalysts was used at about 10 4 molar ratio to reagents in benzene, no isomerization of the hexene was observed. A polar coordinating solvent such as tetrahydrofuran drastically reduced the rate of hydrosilation. The rhodium complexes were recoverable unchanged after completion of the reactions. 

In contrast to the high enantioselectivity achieved for the Z-isomeric substrates, hydrogenation of the S-isomeric substrates usually proceeds at a much lower rate and gives poor enantioselectivities [86]. With the Rh-BINAP system as the catalyst and tetrahydrofuran (THF) as solvent, hydrogenation of the Z-and S-isomeric substrates generates products with different configurations [2]. 

The isomeric propargylic stannylated aldehyde intermediate, on the other hand, could be prepared from the alcohol precursor without competing cyclization to an seven-membered enol ether product (Eq. 9.105). Treatment of this stannane with SnCl4 afforded the cis-disubstituted tetrahydrofuran stereoselectively. Presumably, this reaction proceeds through an allenyl trichlorostannane intermediate. 

Dibromoethane normally reacts with activated methylene groups to produce cyclopropyl derivatives [e.g. 25, 27], but not with 1,3-diphenylpropanone. Unlike the corresponding reaction of 1,3-dibromopropane with the ketone to form 2,6-diphenylcyclohexanone, 1,2-dibromoethane produces 2-benzylidene-3-phenyl-tetrahydrofuran and the isomeric 2-benzyl-3-phenyl-4,5-dihydrofuran via initial C-alkylation followed by ring closure onto the carbonyl oxygen atom (Scheme 6.2) [28],... 

Materials. A series of a,to-bis(hydroxyphenyl)PSU oligomers with different molecular weights were synthesized and characterized as was previously reported (12). Two samples of PPO (one from Aldrich and one from General Electric Co.) were both purified by precipitation from chloroform solution into methanol. A commercial sample (Dow Chemical) of an isomeric mixture of chloromethylstyrenes (C1MS, 40% para, 60% meta) was used as received. 1-Chloromethoxy-4-chlorobu-tane (CMCB) was prepared according to a procedure developed by Olah et al. (14) and modified by Daly et al. (15), i.e., from paraformaldehyde, tetrahydrofuran and anhydrous HC1. 

B. (2S, 3R)-2,4-Dimethyl-1,3-pentanediol3. To a stirred solution of (+)-2 (2.75 g, 5 mmol, 96 4 isomeric purity) in tetrahydrofuran (THF) (50 mL) is added lithium aluminum hydride (0.19 g, 5 mmol) at 0°C. The reaction mixture is stirred at room temperature for 1 hr and quenched by the careful addition of sodium sulfate decahydrate (5 g). The mixture is stirred vigorously for 30 min and filtered. The filtrate is concentrated, dissolved in 75 mL of a 1 1 mixture of hexane and dichloromethane. This solution is dried over sodium sulfate, filtered and concentrated under reduced pressure. Trituration of the resulting oil with hexane (50 ml) results in the precipitation of auxiliary alcohol 4 (1.6-1.8 g) which is recovered by filtration (Note 11). The residue is separated by chromatography over silica gel (40 g) (Note 2) with hexane and ethyl acetate (3 1-1 1) to afford additional 4 (0.2-0.4 g. Note 12) and 3 (0.60 g, 92%) (Notes 13, 14). 

R = CH2CH—CH2/, dimethyl formamide with the bromide to prepare 29 (R = CH2C=CH), and tetrahydrofuran to prepare the 9-(3-dimethyl-aminopropyl) derivatives of the four isomeric nitrocarbazoles using the tosylates as alkylating agents. ... 

Epoxy-l-butene (1) is a versatile intermediate for the production of commodity, specialty and fine chemicals (2). An important derivative of 1 is 2,5-dihydrofuran (2,5-DHF). This heterocycle is useful in the production of tetrahydrofuran (3), 2,3-dihydrofuran (4), 1,4-butanediol (5), and many fine chemicals (e.g., 3-formyltetrahydrofuran (6) and cyclopropanes (7)). The homogeneous, Lewis acid and iodide salt-catalyzed rearrangement (isomerization) of 1 to 2,5-DHF has been known since 1976 (8) and is the only practical method for 2,5-DHF synthesis. 

Ojima has reported a rhodium-catalyzed protocol for the disilylative cyclization of diynes with hydrosilanes to form alkylidene cyclopentanes and/or cyclopentenes. As an example, reaction of dipropargylhexylamine with triethyl-silane catalyzed by Rh(acac)(GO)2 under an atmosphere of CO at 65 °G for 10 h gave an 83 17 mixture of the disilylated alkylidene pyrrolidine derivative 92b (X = N-//-hexyl) and the disilylated dihydro-1/ -pyrrole 92c (X = N-//-hexyl) in 76% combined yield (Equation (60)). Compounds 92b and 92c were presumably formed via hydrosilyla-tion and hydrosilylation/isomerization, respectively, of the initially formed silylated dialkylidene cyclopentane 92a (Equation (60)). The 92b 92c ratio was substrate dependent. Rhodium-catalyzed disilylative cyclization of dipro-pargyl ether formed the disilylated alkylidene tetrahydrofuran 92b (X = O) as the exclusive product in low yield, whereas the reaction of dimethyl dipropargylmalonate formed cyclopentene 92c [X = C(C02Et)2] as the exclusive product in 74% isolated yield (Equation (60)). 

Tetrahydrofurans 53 should be of synthetic interest because of the many natural products containing a bicyclic tetrahydrofuranoid skeleton or structural element [30]. The double bond would allow further synthetic transformations. While the chances of a regioselective isomerization of 46 to the allyl aminosulfoxonium salts 52 seemed to be good, the prospects of a cyclization of the latter with formation of 53 were considered to be less promising because of the poor nucleophilicity of the silyloxy group. 

The acyclic sulfonimidoyl-substituted amino acids 24 were selected as starting material for the synthesis of the unsaturated prolines of type 63. Because of the facile synthesis of the unsaturated bicyclic tetrahydrofurans 53 from the vinyl aminosulfoxonium salts 46 (cf. Scheme 1.3.20), it was speculated that upon treatment with a base the vinyl aminosulfoxonium salts 67 would experience a similar isomerization with formation of the allyl aminosulfoxonium salts 69, which in turn could suffer an intramolecular substitution of the allylic aminosulfoxonium group (Scheme 1.3.24). The methylation of sulfoximines 24 with Me30Bp4 gave... 

In agreement with this mechanism, it was found that the epoxide (4RS)-4,4-(epoxy-methano)tricyclo[5.1.0.02,5]octane-e c/o-8-carbaldehyde 2,2-dimethylpropaneT,3-diyl acetal (1) gave 4-oxotricyclo[6.1.0.02,6]nonane- ,/ttreatment with lithium iodide in tetrahydrofuran.71 Several examples employing this oxaspirohexane to cyclopentanone isomerization method are shown (see Table 7).69-80 Lithium bromide in the presence of hexamethylphosphoric triamide was also effective in these transformations.70,74,76 79,80... 

Fluorination of bicyclic derivatives, which has been used in prostaglandin synthesis from nor-bornadicne, gives a 1 1 mixture of two isomeric ot-fluoro esters, e.g. 3.18 The reaction is performed in tetrahydrofuran at — 78 °C, while a violent explosion occurs if the reaction is performed at — 40 C (upon quenching the reaction with water). 

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