Dihydro-y-pyrones

A synthesis of 2-alkyl-2,3-dihydro-y-pyrones (187) from methoxybutenyne and aldehydes has been described (83TL4551). The condensation of lithiomethoxy-butenyne (184) with aldehydes at -78°C leads to the secondary alcohols 185, which form the dihydropyrones 187 via hydration of the acetylenic bond and hydrolysis of the methoxyethenyl group to the ketoenol 186 (0°C, p-TSA, THF, H2O or 30% HCIO4, 20 min) folowed by intramolecular cycloaddition. 

Cyclocondensation of dioxygenated 1,3-dienes with aldehydes. In the presence of ZnCl2 in benzene or BP3 etherate in ether, this diene 1 undergoes cyclization with a wide variety of aldehydes to afford 2,3-dihydro-y-pyrones (equation I). The products... 

As usual there has been considerable activity in the field of siloxydiene cycliza-tion reactions, but probably the most significant developments in this area over the past twelve months have been in the use of such dienes to synthesize 2,3-dihydro-Y-pyrones (Scheme 16). 

PhC=CCO Ph HjSe EtaN, MeOH (35) 2-Benzylidenc-3-oxo-5-phenyl-2,3-dihydro-selcnophene (28), 2,6-diphenyl-l-seleno-Y Pyrone (17) 179... 

DIHYDRO-3-FURANONES 2,2-Dimeth-oxymethyl-1,3-dithiane. DIHYDRO-y-PYRONES 2,2-Dimethoxy-methyl-1,3-dithiane. vic-DIOLS Titanium(III) chloride. o-DIKETONES Potassium permanganate-Acetic anhydride. Ruthenium tetroxide. 

Dihydro-y-pyrones and dihydro-3-furanones.1 A new synthesis, of dihydro-y-pyrones (5) and dihydro-3-furanones (8) involves conversion of (1) into 2-lithio-2-(2 >2 -dimethoxyethyl)-l,3-dithianc (2) by treatment with n-butyllithium in THF at -30°. Treatment of (2) with an epoxide gives (3). On treatment with p-toluenesulfonic... 

Since Danishefsky demonstrated that activated dienes, such as siloxydiene (commonly referred to as Danishefsky s diene) react with a wide spectrum of aldehydes to afford 5,6-dihydro-y-pyrones in 1982 [l],the hetero-Diels-Alder reaction has attracted a great deal of attention over the last two decades [2,3]. The use of asymmetric catalysis in these reactions is overwhelmingly associated with heterodienophiles. Especially, the cyclocondensations of activated dienes with aldehydes or their derivatives are of particular importance, providing a multitude of opportunities for the highly efficient regio- and stereoselective construe-... 

Ehnenhorst, H. Dihydro-y-pyrones in tobacco smoke. Identification of 2,3-dihydro-5-hydroxy-6-methyM//-pyran -one in cigarette smoke CORESTA 1978 Symp., Sofia, Bulgaria, CORESTA Inf. Bull., Spec. Edition 1978 Paper S05, 114—115. 

Ehnenhorst, H. and G. Nicolaus Dihydro-y-pyrone im Tabakrauch. Nachweis von 2,3-Dihydro-5-hydroxy-6 -methyl-4//-pyran-4-one in Rauch von Cigaretten aus Burley-, Vhginia-, und Orienttabaken [Dihydro-y-pyrones... 

An aq. soln. of 5-carbethoxy-2- -hexyl-2,3-dihydro-6-methyl-4-pyrone and NaOH stirred 24 hrs. at 0-2°, then acidified with HCl -> 3-acetyl-6- -hexyl-5,6-dihydro-4-hydroxy-2-pyrone. Y 77.9%. K. Sato, S. Inoue, and M. Ohashi, Bull. Chem. Soc. Japan 46, 1288 (1973). 

In addition to the considerable variety of methods for the synthesis of non-fluorinated y-pyrones [2], Tyvorskii and co-workers have described three new procedures, which produced 2-(perfluoroalkyl)-4-pyrones. One of them is a convenient two-step synthesis of 5-substituted 2-(perfluoroalkyl)-4 f-pyran-4-ones 2 by dehydration of 2,3-dihydro-3-hydroxy-6-(perfluoroalkyl)-4 f-pyran-4-ones 1 prepared... 

Lithio-2-(2,2-dimethoxyethyl)-l,3 dithian, when it reacted with ketcmes and epoxides, led to intermediates which were converted in somewhat low yield into 3-furanones and dihydro-y-pyrones, respectively. " 2-Methyl-2-lithio-dithian reacted efficiently with spiro-epoxide groups at positions 3 and 17 in steroids. The products obtained in this manner were further elaborated for example, 3a-ethyl-30-hydroxy-5a-cholestane, " 3-acetonyl-A -androstane, and both isomers of 17-acetonyltestosterone " were prepared. The carbon skeleton, including the S-lactone ring, of pestalotin... 

DIHYDRO-3-FURANONES 2,2-Dimeth-oxymethyl-l,3-dithianc. DIllYDRO-y-PYRONES 2.2-Dimetlioxy-methyl-1,3-dithianc. vrc-DlOLS Titanium(lll) chloride. 

Dimethoxy-2,5-dihydrofurfuryl alocohl dissolved in 2%-H2S04, and allowed to stand 1.5 hrs. at room temp. 2,3-dideoxy-DL-pent-2-enopyranos-4-ulose. Y ca. 100%. - This is a stage of a multi-step conversion of furan derivs. into monosaccharides. F. e. s. O. Achmatowicz, Jr., et al.. Tetrahedron 27, 1973 (1971) 6-hydroxy-2,6-dihydro-3-pyrones from furylcarbinols by oxidative ring expansion cf. Y. Lefebvre, Tetrah. Let. 1972, 133. 

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. 

Hydroxy-6-propenyl-2-pyrone heated 3 hrs. at 100° with crotonoyl diloride in tetrachlorethane in the presence of TiCl4 3,4-dihydro-2-methyl-7-propenyl-2H,5H-pyrano[4,3-b]pyran-4,5-dione. Y 70%. K. Kato, Y. Hirata, and S. Yama-mura, Chem. Commun. 1969, 95. 

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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