The Preparation of Cyclic Vinyl Ethers

Parham, William E.; Holmquist, H. E.

doi:10.1021/ja01147a008

Cited by 27 publications

(16 citation statements)

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“…cis- and trans- 2-Ethoxy-4-methyltetrahydrofurans (6a). (i) Following procedure A, a solution of 4a 41 (1.5 g, 7.2 mmol) in dry benzene (36 mL) was treated with tributylstannane (2.3 mL, 8.6 mmol) for 2 h. Short-path (Kugelrohr) distillation of the concentrated residue furnished a colorless oil (114 mg, 12%) which comprised the two tetrahydrofurans cis- and trans - 6a in a ratio of 4.7:1. An analogous experiment in benzene- d 6 gave a ratio between cis- and trans- 6a of 4.2:1.…”

Section: Methodsmentioning

confidence: 99%

Formation of Some Oxygen-Containing Heterocycles by Radical Cyclization: The Stereochemical Influence of Anomeric Effects

Beckwith

Page

1998

J. Org. Chem.

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The influence of the anomeric effect on radical cyclization has been examined by determining the stereochemical outcome of the ring closure of 10 suitably substituted radicals. The stereochemistry of the products formed from the acyclic precursors 4a-f indicates that for suitably constituted radicals anomeric interactions stabilize pseudoaxially substituted transition structures 14 thus affording products with stereochemistry the reverse of that normally observed.In recent times, the use of free radical reactions for chemical synthesis has grown steadily in popularity as the processes which govern and control their outcomes have become better understood. 1 Intramolecular addition reactions of suitably constituted alkenyl radicals and related species are particularly interesting since they readily afford a variety of ring systems commonly found in many natural products. 1 The advantages offered by such reactions generally include high regioselectivity, modest to good stereoselectivity, 1-5 and high chemoselectivity. [1][2][3][4][5] The highly regioselective exo-cyclization of the hex-5-enyl radical has been intensively studied. The accurate determination of its kinetics 6 has underpinned its widespread use as a mechanistic probe and as a radical clock. 7 Most monosubstituted hex-5-enyl radicals also undergo regioselective exo-cyclization while conforming to the general stereochemical guideline that 1-or 3-substituted radicals afford predominantly cis-disubstituted products, while 2-or 4-substituted radicals give mainly transdisubstituted products. 5 It is now generally accepted that the cyclization behavior of simple hex-5-enyl systems reflects the stereoelectronic demands of the transition structure. [2][3][4][5] The hypothesis that the strain energy engendered in attaining the optimum orbital overlap for 1,6-ring closure outweighs the steric and thermodynamic factors which would otherwise favor six-membered ring formation 2-5 has received support from theoretical analyses 8 and the modeling of transition structures by a combination of molecular orbital and molecular mechanics calculations. 8 These calculations confirm the original hypothesis that the transition structure for 1,5-exo-cyclization of hex-5-enyl radicals and related species resembles cyclohexane in its chair form. 2-5 Hence the stereoselectivity exhibited in the ring closure of monosubstituted hexenyl radicals and related species is seen to arise from the propensity of cyclizations to proceed through the more stable pseudoequatorially substituted cyclohexane-like conformation of the transition structure rather than through its less stable pseudoaxially substituted conformer. 8 Since calculations of the relative selectivities of the cyclized products tend to be overestimated when only axially substituted chairlike structures are considered, it appears that boatlike transition structures might also be involved in some reactions. 4,8 Cyclizations of hex-5-enyl type radicals containing heteroatoms have also been studied. 1,9,10 In general such spec...

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Section: Methodsmentioning

confidence: 99%

Formation of Some Oxygen-Containing Heterocycles by Radical Cyclization: The Stereochemical Influence of Anomeric Effects

Beckwith

Page

1998

J. Org. Chem.

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“…A cis-trans mixture of 2,4-dimethoxytetrahydropyran (1) (I) was heated at 180" with a catalytic amount of phosphorus pentoxide (4). Only extensive decomposition occurred.…”

Section: Resultsmentioning

confidence: 99%

Some reactions of 2,4-dimethoxytetrahydropyrans

Baldwin¹,

Brown²

1969

Can. J. Chem.

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Acid-catalyzed elimination of methanol from 2,4-dimethoxytetrahydropyran (1) produces 2-methoxy-5,6-dihydro-2H-pyran (3) rather than the expected olefin 4-methoxy-3,4-dihydro-2H-pyran (2).The reaction of 1,3-dibromo-5,5-dimethylhydantoin with 3 in ether -methanol gives a 2:l mixture of the isomers 3~-bromo-2cr,4cr-dimethoxytetrahydropyran (5a) and 3cr-bromo-2cr,4P-dimethoxytetrahydropyran (5b) respectively. A rationale is given to explain the preponderance of 5a over 5b and the highly selective attack of the bromine of the hydantoin and the methanol on C-3 and C-4 respectively of the double bond of 3. Reduction of 5ab with zinc in ethanol provides only compound 3.The room temperature reaction of 1 in a mixture of water and 1,2-dimethoxyethane containing Amberlite IR-120, produces 2-hydroxy-4-methoxytetrahydropyran (6) as an equilibrium mixture of cis and trans isomers in the ratio 1 :l. This gave a value of 0.9 kcal/mole for the anomeric effect in 6. Pyrolysis of the derivative, 2-acetoxy-4-methoxytetrahydropyran failed to produce the olefin 2 and resulted only in extensive decomposition.

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“…2,4-Dimethyl-5,6-dihydro-2H-pyran was synthesized from 2-methyl-1,3-pentadiene and powdered paraformaldehyde with hydroquinone in a stainless steel bomb at 190 °C as previously reported.24 To 14.7 g (0.131 mol) of this dimethylpyran in 100 ml of dry tetrahydrofuran was added 0.437 mol of 1 M borane-THF complex (Aldrich) under nitrogen at 0-5 °C. After the reaction mixture had stirred in an ice bath for 3 h, the temperature was raised to 25 °C and the mixture was stirred for an additional hour.…”

Section: Methodsmentioning

confidence: 99%

Trichothecene analogs. 1. 1,5-Dioxaspiro[2.5]octanes

Fullerton¹,

Chen²,

Hall³

1976

J. Med. Chem.

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Seven l,5-dioxaspiro[2.5]octanes were synthesized and tested in the mouse P388 lymphocytic leukemia screen and the mouse Ehrlich ascites screen. These compounds possess the "epoxypyran" structure which has been believed to be the active portion of the trichothecene class of sesquiterpene tumor inhibitors. Three of the compounds were found to have marginal to moderate activity in the Ehrlich ascites screen (inhibition 74.1-86.3%) and low activity in the P388 screen (T/C = 126-131). A carbocyclic analogue, l-oxaspiro[2.5]octane ( 9), was moderately active in both screens (inhibition 78.8%, T/C = 140). In the Ehrlich ascites screen, T-2 toxin (2) was about 25 times more potent than 9. None of the spirooctanes studied caused any skin irritation in 10-mg doses on the skin of rabbits, whereas 2 caused extensive necrosis at 0.1-mg doses.

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The Preparation of Cyclic Vinyl Ethers

Cited by 27 publications

References 0 publications

Formation of Some Oxygen-Containing Heterocycles by Radical Cyclization: The Stereochemical Influence of Anomeric Effects

Formation of Some Oxygen-Containing Heterocycles by Radical Cyclization: The Stereochemical Influence of Anomeric Effects

Some reactions of 2,4-dimethoxytetrahydropyrans

Trichothecene analogs. 1. 1,5-Dioxaspiro[2.5]octanes

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