5,6-dihydropyran-2-ones oxidation

Dihydro-2f/-pyran-2-one has been prepared by reductive cycliza-tion of 5-hydroxy-2-pentynoic acid [2-Pentynoic acid, 5-hydroxy-], which is obtained in two steps from acetylene [Ethyne] and ethylene oxide [Oxirane] 3 and by the reaction of dihydropyran [277-Pyran, 3,4-dihydro-] with singlet oxygen [Oxygen, singlet].4,5 2ff-Pyran-2-one has been prepared by pyrolysis of heavy metal salts of coumalic acid [2//-Pyran-5-carboxylic acid, 2-oxo-],8 by pyrolysis of a-pyrone-6-carboxylic acid [211 - Pyran-6-carboxyl ic acid, 2-oxo-] over copper,7 and by pyrolysis of coumalic acid over copper (66-70% yield).8... 

All reactions listed in Tables 5-7 were carried out under a nitrogen atmosphere, but with the rhodium or palladium catalysts no noticeable or only minor reduction in cyclopropane yields was observed when air was present. In contrast, air clearly had a yield-diminishing effect in the CuCl P(0-/-Pr)3-catalyzed reactions, especially with cyclohexene and 3,4-dihydropyran. Cyclohexene was oxidized to 2-cyclohexen-l-one, and 3,4-dihydropyran gave 5,6-dihydro-4-pyrone and 5,6-dihydro-2-pyrone, albeit in yields below 8 % 59). 

Cycloaddition of 5,6-dihydropyran-2-one with aromatic nitrile oxides leads to 3-aryl-3a,6,7,7a-tetrahydropyrano[3,4-d]isoxazol-4(47/)-ones 98. The latter react with nickel peroxide to give the corresponding dihydropyranoisoxazolones 99. Similar to 2-bromocyclohex-2-enone, 3-bromo-5,6-dihydropyran-2-one undergoes nitrile oxide cycloaddition, followed by dehydrobromination, to form regioi-someric 3-aryl-5,7-dihydropyrano 4,3-c/ isoxazol-7(4//)-ones 100 (Scheme 1.24) (242). 

The indirect dehydrogenation to pyranones has received rather more attention. The allylic bromination of 5,6-dihydropyran-2-one by NBS, followed by the elimination of hydrogen bromide, is described in detail and offers the attractions of mild conditions and easy isolation of the product (770S(56)49). A similar approach was used to synthesize the steroidal pyran-2-ones from the fully saturated lactone (354), with a combination of dehydrogenation and dehydrobromination to achieve oxidation (64MI22400). 

The (2pyrrolidine derivative 843 can catalyze a hetero Diels-Alder (hDA) reaction between aldehydes and enones 844 to form dihydropyranols 845, PCC oxidation of which affords tf //-3,4-dihydropyran-2-ones 846 in good yield and enantioselectivity (Scheme 238) <2003AGE1498>. 

The cyclopenten-2-one 847 undergoes Baeyer-Villiger oxidation to afford 3,4-dihydropyran-2-one 848 in good yield (Equation 341) <2000JOC635>. 

Chemoselective primary alcohol oxidation of the 1,5-diol 849 followed by in situ cyclization and lactol oxidation to afford 3,6-dihydropyran-2-one 850 is achieved using catalytic TEMPO in the presence of NCS (Equation 342) <20050L1853>. 

Chemoselective oxidation of the allylic alcohol in triol 865 with manganese dioxide followed by in situ cyclization and oxidation of the resulting 5,6-dihydropyran-2-ol provides the 5,6-dihydropyran-2-one subunit 866 of bryostatin (Equation 349) <20000L2189>. 

Table 43 Geometry of alkenes 930 and their oxidative cyclization to 2,6-dihydropyran-3-ones 931 (Equation 365)...

Dihydrofuran 934 undergoes an oxidative cleavage/aldol ring closure sequence without epimerization at the stereocentre providing a route to enantiopure 2,6-dihydropyran-3-ones 935 (Scheme 252) <20020L3059>. 

Oxidation of the dihydropyran 1051 bearing a vinyl borane moiety affords 6-hexyltetrahydropyran-4-one in good yield (Equation 409) <2001JA4601>. m-Hydroxylation of 2-alkyl-4-halo-dihydropyrans affords /7-2-alkyl-3-hydro-xytetrahydropyran-4-ones 1052 as a single stereoisomer (Equation 410) <20030L1979>. 

Pd-catalysed oxidative heterocyclisation features in a synthesis of substituted dihydropyran-4-ones from P-hydroxy-ynones. The process involves domino Wacker - Heck coupling of the ynones with ethyl acrylate and, although yields are only moderate, the stereochemistry of the ynone is retained in the product (Scheme 24) <06JOC8390>. 

The oxo-HDA reaction of 2-carbonyl derivatives of pyridine 1-oxide with electron-rich dienes catalysed by bisoxazoline Cu(II) complexes proceeds by a Mukaiyama-aldol route and affords dihydropyran-4-ones 33 in good yield and excellent enantioselectivity <07JOC240>. 

An acidic solution of 2,4-dinitrophenylhydrazine reacts with N-p-chlorophenyl-sulfonyl-3-ethoxy-l,2-thiazetidine 1-oxide to give (80%) the bis-2,4-dinitrophenyl-hydrazone of glyoxal. The adduct of A-sulfinyl-p-chlorophenylsulfonamide with dihydropyran is inert to catalytic hydrogenation and bromination. Treatment of two l,2-thiazetidine-3-one 1-oxides (e.g., 421) with hydriodic acid results in ring-cleavage and loss of sulfur. They are not oxidized to 1,1-dioxides by peracetic acid, ° but m-chloroperbenzoic acid accomplishes this oxidation. The unstable adducts with ketene decompose to amides with loss of hydrogen sulfide and sulfur dioxide or may be trapped by reaction with aromatic amines as shown for thiazetidine 1-oxide 422.An aldol-type condensation has been reported for A -cyclohexyl-1,2 thiazetidine-3-one 1-oxide and p-(A(A"-dimethylamino)benz-aldehyde. " Sulfur monoxide is lost in the flash-vacuum thermolysis of 422a. ... 

We protect one hydroxyl terminus of the commercially available 1,8-octane diol 1 by reaction with dihydropyran to give the monoalcohol, 8-tetrahydropyranyloxyoctanol 2. The protected alcohol 2 is oxidized to the aldehyde with pyridinium chlorochromate to give 8-tetrahydropyranyl-oxyoctanal 3. 1-Heptyne 4 is coupled with propargyl bromide 5 in a copper catalyzed reaction to produce the diacetylenic 1,4-decadiyne 6. 

A general route to 2,3-dihydropyran-4-ones is based on the Pd-mediated oxidative cyclisation of p-hydroxyenones given the correct conditions there is no evidence of Michael addition products. The cyclisation occurs without racemisation (Scheme 18) <04OL91>. 

Dihydro-2//-pyran-2-one has been prepared by reductive cycliza-tion of 5-hydroxy-2-pentynoic acid [2-Pentynoic acid, 5-hydroxy-], which is obtained in two steps from acetylene [Ethyne] and ethylene oxide [Oxirane] and by the reaction of dihydropyran [2Jf-Pyran,... 

From a synthetic point of view, indolines are used to provide different reactivity properties from the indoles. Once a required sequence of reactions is achieved, the indoline can then be oxidized readily to the indole. One of the most effective methods for the conversion of indolines to indoles involves treatment with benzeneseleninic anhydride, together with three equivalents of a sacrificial scavenger such as indole or dihydropyran to remove the phenylselenating by-product the method has been used in the synthesis of ergot alkaloids (Equation (100)) <85TL4183, 90JCS(pi)707>. 

Dihydropyran derivatives can be synthesized facilely by a smooth oxidative Mukaiyama-Michael addition followed by a cyclization with silyl enol ethers in the presence of Dess Martin periodinane (DMP) and pyridine under mild reaction conditions from MBH adducts in a one-pot process (Scheme 4.95). Notably, these dihydropyrans were obtained exclusively as cw-isomers in good yields. Moreover, all the reactions worked very well, irrespective of whether MBH adducts were derived from aliphatic or aromatic aldehydes, and silyl enol ethers were derived from acetophenone, cyclohexanone or cyclopentanone. 

The lactonization of MBH derivatives to give 3-arylidene-3,4-dihydropyran-2-one derivatives has been realized by treatment with TFAA in CH2CI2 at room temperature.As shown in Scheme 4.102, ot-arylidene-8-lactones 317 were obtained in 50 83% yields. Subsequent oxidation of 317 with PCC afforded the desired a-pyrones 318 in 51-64% yields. By application of this synthetic strategy, tricyclic compound 320, previously reported by Basavaiah and Satyanarayana, " and bicyclic compound 319 could be prepared from easily available MBH adducts. 

---