2- Pyrones 5,6-dihydro

Lia DJ, Mariko K, Hiromitsu T, Sjamsul AA, Norio A. A 6-substituted-5,6-dihydro-2-pyrone from Cryptocarya strictifolia. Phytochemistry 2000 54 989-993. 

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). 

Dimethoxyoxetanes.1 In the presence of ZnCl2, this ketal reacts with 0-hydroxy aldehydes to give 2,2-dimethoxyoxetanes, which can be converted into 4-hydroxy-3-methyl-8-lactones and 5,6-dihydro-2-pyrones. In one case, an optically active lactone was obtained in 80% ee from an optically active y-hydroxy aldehyde. 

Hydrogenolysis of 2-pyrone derivatives to give open-chain products occurs especially readily when phenyl group is substituted in the 6 position.237,238 4-Ethoxy- and 4-hy-droxy-6-phenyl-5,6-dihydro-2-pyrones were hydrogenated to give 3-ethoxy-5-phenyl-2-pentenoic acid and 3-hydroxy-5-phenylvaleric acid, respectively, over Raney Ni at room temperature and 0.3 MPa H2 (eq. 12.126).237... 

Carlson and Olger have reacted the dilithium salt of propargylic acid with oxiranes and shown that the resulting 8-hydroxycarboxylic acids could be cyclized wiA solvation to 5,6-dihydro-2-pyrones (Scheme 14). The method was subsequently used in a neat synthesis of pestalotin (R = Bu"CHMe). 

Pyrone itself can be prepared via Prins alkylation of but-3-enoic acid with subsequent lactonisation, giving 5,6-dihydro-2-pyrone, which, via allylic bromination and then dehydrobromination, is converted into 2-pyrone. Alternative manipulation of the dihydropyrone affords a convenient synthesis of a separable mixture of the important 3- and 5-bromo-2-pyrones (see 11.2.2.4). 

Pyrone forms unstable adducts with bromine which give the 3-bromo-2-pyrone on warming. 5-Bromo-2-pyrone is prepared by A -bromosuccinimide bromination, then dehydrobromination of 5,6-dihydro-2-pyrone. With nitroni-um tetrafluoroborate, the electrophile is assumed to attack first at carbonyl oxygen leading subsequently to 5-nitro-2-pyrone. 

Crude methyl 6-benzyloxy-5-hydroxy-2-decynoate added to a soln. of Na in methanol and stirred 22 hrs. at 25° -> 4-methoxy-5,6-dihydro-6-(l-benzyloxy-pent-l-yl)-2H-pyran-2-one. Y 82%. F. e., also from the acids, s. R. M. Carlson and A. R. Oyler, Tetrah. Let. 1974, 2615 5,6-dihydro-2-pyrones s. a. J. Org. Chem. 40,1610 (1975). 

P-Siloxy-5-isoxazolidones from 5,6-dihydro-2-pyrones Asym. induction by Michael addition of hydroxylamines... 

Chloro ulfonyl isocyanate and isoprene in ether give 5-hydroxy-3-methyl-2-pentenoic acid lactone (4-methyl-5,6-dihydro-2-pyrone, XII-98) and 5-amino-3-methyl-3-pentenoic acid lactam (4-methyl-3,6-dihydro-2-pyridone, XII-99) by hydrolysis of intermediate N-sulfochlorides that were not isolated ... 

Franca N C, Gottlieb O R, Suarez AMP 1973 6-Phenylethyl-5,6-dihydro-2-pyrones from Aniba gigantifolia. Phytochemistry 12 1182-1184... 

Substituted butyrolactones are formed in moderate yields by the photoalkylation of alcohols to 5,6-dihydro-2-pyrone. ... 

Acyl-4-hydroxy-5,6-dihydro-2-pyrones from 5-carbalkoxy-2,3-dihydro-4-pyrones... 

Dihydro-2-pyrones and 4-hydroxytetrahydro-2-pyrones from, d-dihydroxycarboxylic acid tert- mty esters... 

The conversion of sugar-derived 5,6-dihydro-2-pyrones by tin(IV) chloride into 3-acyloxy and 3-acylamido-2-pyrones has appeared (Scheme 15). For example (24) gave (25). On prolonged reaction time pyrone (26) gave an almost quantitative yield of halogenated pyrone (27). 

Shao L, Kawano H, Saburi M, Uchida Y. Asymmetric hydrogenation of 3,5-dioxoesters catalyzed by Ru-binap complex a short step asymmetric synthesis of 6-substi-tuted 5,6-dihydro-2-pyrones. Tetrahedron 1993 49 (14) 1997-2010. 

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