3- Bromo-4//-pyran

Pyran-2-one, 3,5-bis(methoxycarbonyl)-6-methyl-synthesis, 3, 789-790 Pyran-2-one, 3-bromo-synthesis, 3, 679 Pyran-2-one, 4,6-dialkoxy-reactions... 

B. 2H-Pyran-2-one. A mixture of 9.81 g. (0.100 mole) of 5,6-dihydro-2/I-pyran-2-one, 200 mg. of benzoyl peroxide, 18.6 g. (0.105 mole) of A-bromosuccinimide [2,5-Pyrrolidinedione, 1-bromo-] (Note 7), and 800 ml. of carbon tetrachloride is prepared in a 2-1., three-necked, round-bottomed flask equipped with a reflux condenser and a mechanical stirrer. The resulting suspension is stirred and heated to reflux. After 1.5 hours at reflux, most of the solid is dissolved, and the solution gives a negative test with starch-iodide paper. The reaction mixture is then allowed to cool, dining which time succinimide [2,5-Pyrrolidine-dione] crystallizes out. The precipitate is removed by filtration, and the filtrate is concentrated under reduced pressure to leave crude 5-bromo-5,6 -dihy dro 211 -pyran-2 one [ 211 Py ran - 2-one, 5-bromo-5,6-dihydro] as an oil. 

Propionic acid, 2-iodo-3-nitro-, ethyl ester [Propanoic acid, 2-iodo-3-mtro-, ethyl ester], 65 2//-Pyran, 3,4-dihydro-, 51 2//-PYR AN-2-ONE, 49 2H Pyran 2-one, 5 bromo 5,6-dihydro, 50 27/-PYRAN-2-ONE, 5,6 DIHYDRO-, 49 PYRIDINE, 2,3,4,5 TETRAHYDRO, 118 Pyridines, -substituted, 34 a Pyrone-6-carboxyhc acid [2H Pyran-6-Larboxyhc acid 2-oxo ], 51 Pyrroles, 34... 

This is manifest in the reactivity of 180/180-Z1 which was generated from 3-bromo-41-f-pyran (283) by /3-elimination of hydrogen bromide with KOtBu (Scheme 6.61). Whether or not this reaction was conducted in the presence of styrene or furan, the only product identified was tert-butyl 4H-pyran-4-yl ether (284). This is in line with the relationship of the intermediate to a pyrylium ion. Thus, the addition of the tert-butoxide ion to 180/180-Zj has to be expected at the 4-position with formation of the vinyl anion 285, which is then protonated to give 284. Likewise, the attack of the nucleophile is predicted at C2and C6 leading to the vinyl anions 286, which... 

Scheme 6.76 Generation of l-oxa-2,3-cyclohexadiene (351) from 5-bromo-3,4-dihydro-2H-pyran (376) and trapping products of351 obtained from furan, 2,3-dimethyl-1,3-butadiene, 1,1-diphenyl-ethylene, ( )-l-phenylpropene, ( )-2-butene, (Z)-2-butene and tert-butyl alcohol , according to Schlosserand co-workers.

Scheme 6.80 Reactions of 5-bromo-3,4-dihydro-2H-pyran (376) with complex bases composed of NaNH2 or NaNH2-NaOtBu and enolates of ketones, accordingto Caub re and co-workers.

I)idchydro-3,4-dihydro-2//-pyran, easily generated from 5-bromo-3,4-dihydro-2//-pyran, was trapped with enolates to give mixtures of cyclobutanes20. 

An Improved Preparation of 3-Bromo-2(H)-pyran-2-one An Ambiphilic Diene for Diels-Alder Cycloadditions. 

Lozano and Barba [82] electrolyzed 2-chloro-2-phenylacetyl chloride at mercury in methylene chloride and obtained derivatives of pyran-2-one and pyran-4-one, where R is H and C6H5CH2CO, respectively, at high and low current density (Scheme 6). In later work [83], the same workers reduced 2-bromo-2,2-diphenylacetyl bromide in methylene dichloride saturated with hydrogen sulfide gas three sulfur-containing... 

A mixture of 2,4,6-triphenyl-4-(4-bromo-butyl)-4//-pyran (28%) and l,4-bis-(2,4,6-triphenyl-4//-pyranyl)-butane (6%) is obtained when 1,4-dibromobutane is used as the electrophile. 

B. 1,3-Dimethyl-6H-benzo[b]naphtho[1,2-d]pyran-6-one (4). Under an argon atmosphere, a 250-mL, oven-dried, round-bottomed flask, equipped with a reflux condenser, is charged with freshly distilled N,N-dimethylacetamide (DMA, 130 mL), 3,5-dimethylphenyl 1-bromo-2-naphthoate (3, 3.24 g, 9.12 mmol), palladium(ll) acetate (205 mg, 0.913 mmol), triphenylphosphine (481 mg, 1.83 mmol), and sodium acetate (1.50 g, 18.3 mmol) (Notes 4 and 5). The orange suspension is degassed three times, placed in a preheated (130°C) oil bath (Note 5), and stirred at 130°C for 12 hr (Note 6). Removal of the solvent at 40°C (0.1 mbar, 0.075 mm) gives a black oily residue, which is Chromatographed (5 x 40 cm column, silica gel 0.063 - 0.2 mm, 170 g, 1 cm of charcoal at the top of the column eluent hexane / diethyl ether 5 1), to yield 2.00 g (80%) of the lactone 4 as a slightly yellow solid. Recrystallization from diethyl ether / hexane delivers 1.63 g (65%) of colorless or pale yellow crystals (Note 7). 

Disubstituted pyran-2-ones are formed through the Ni-catalysed coupling of (Z)-3-iodopropenoates with alkynes the conesponding bromo compound yields cyclopentadienes <99T4969>. 

Pyridinium chlorochromate in dichloromethane reacts with furans to give a range of products, but they are all formed by 1,4-electrophilic attack of chlorochromate on the furan ring the fate of the resultant intermediate (57) by heterolytic cleavage of the Cr—O bond depends on the substituents at the a-positions of the substrate. 2,5-Dialkylfurans yield a, /8-unsaturated-y-dicarbonyl compounds, 5-methyl-2-furylcarbinols yield pyran derivatives, and 5-bromo-2-furylcarbinols yield 5-hydroxyfuran-2(5//)-ones (Scheme 25) (80T661). 

The reaction of 3-bromo-2-hydroxychromanone (657) with sodium methoxide also results in pyran ring cleavage followed by the formation of a furanone ring (75JHC981). 

The benzene ring of fused dihydropyrans is more easily attacked by electrophiles than that of the corresponding pyran. However, the activating effect of the carbonyl group on the 3-position of chromanones and flavanones ensures that halogenation normally occurs at that position preferentially. Thus, 3-mono- and 3,3-di-bromination is readily achieved with appropriate molar proportions of bromine but bromination of 2,3-dihydronaphtho[2,l-Z>]pyran-l-one (675) in acetic acid gave the 7-bromo derivative (676) (48JA599). 

Hydroboration and oxidation of 3,4-dihydro-2-methoxy-2//-pyran gives a mixture of cis- and rra s-5-hydroxy-2-methoxytetrahydropyran (70CJC2334). After methylation, trans-2,5-dimethoxytetrahydropyran was isolated in small amounts. The cis isomer was obtained from 3,4-dihydro-3-methoxy-2//-pyran via 3-bromo-2,5-dimethoxytetrahydropyran. 

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