Pyranones, reduction

Catalytic reduction of isocoumarins leads to 3,4-dihydroisocoumarins but LAH opens the pyranone ring to form phenethyl alcohols. 

Generally, nucleophilic attack of chromones opens the pyranone ring. However, the reduction of flavone by LAH yields 4//-flavene (2-phenylchrom-2-ene) (62CI(L)1793). 

The fused tetrahydropyran-2-one (566) is obtained from 2-methylcyclohexanone by Michael addition to methyl prop-2-enoate and reduction of the resulting keto ester (565 Scheme 216) (63JOC34). When the enamine derived from the cyclohexanone reacts with the unsaturated ester, a mixture of keto esters (565) and (567) is formed. The pyranone (568) is formed by reduction of the latter. 

Pyran-4-ones tend to give complex mixtures on chemical reduction, but catalytic hydrogenation usually gives the tetrahydropyran-4-one or the corresponding pyran-4-ol (63CR(256)1542). The influence of solvent on the nature of the product is exemplified by the reduction of 2,6-dimethylpyran-4-one, which in ethanol affords the fully reduced pyranone,... 

Sharpless catalytic asymmetric dihydroxylation of 2-vinylfuran followed by treatment of the diol product with NBS, and subsequent Jones oxidation and Luche reduction provided a highly functionalized a,(1-unsaturated y-lactone in 70% yield overall giving a key intermediate in the synthesis of phomopsolide C <02TL8195>. Similar procedures were adopted by the same authors to synthesize 2-deoxy and 2,3-dideoxyhexoses <02OL1771>. As depicted below, Sharpless kinetic resolution was also used to prepare a pyranone from a furfuryl alcohol in 38% yield, which served as the intermediate in the synthesis of (+)-isoaltholactone <02T6799>. 

D. The use of chiral oxazaborolidines as enantioselective catalysts for the reduction of prochiral ketones, imines, and oximes, the reduction of 2-pyranones to afford chiral biaryls, the addition of diethylzinc to aldehydes, the asymmetric hydroboration, the Diels-Alder reaction, and the aldol reaction has recently been reviewed.15b d The yield and enantioselectivity of reductions using stoichiometric or catalytic amounts of the oxazaborolidine-borane complex are equal to or greater than those obtained using the free oxazaborolidine.13 The above procedure demonstrates the catalytic use of the oxazaborolidine-borane complex for the enantioselective reduction of 1-indanone. The enantiomeric purity of the crude product is 97.8%. A... 

Reaction of this intermediate with base leads the anion of the newly revealed hydroxyl group to attack with the carhoxylate in what amounts to an internal transesterification. This step forms the cyclic ester and thus the requisite pyranone ring (50). Catalytic hydrogenation of 50 results in reduction of the nitro group to yield 51. Acylation of the newly formed amine with 5-trifluoromethylpyridinium-2-sulfonyl chloride affords 52. ... 

Reduction of substituted pyranones (28) leads to several products, with the distribution depending on the choice of solvent (Scheme 7). In DMF the major isolated product (40% or less) is the linear, ring-opened dimer, 29, and in a single case the ring-opened monomer, 30, which may be an intermediate in the formation of 29 [106]. Upon reduction in MeCN/H20, the product mixture contains only the tricyclic compounds, 31a-c, resulting from double coupling in an overall 2-F process [106,107]. The distribution of the... 

Reduction of the double bond in an olefinic aliphatic nitro derivatives is usually easy. The olefin linkage may be preserved only in aromatic nitro compounds [equation (j)] , or in particular structures such as hydroxycoumarins or pyranones . 

Meerwein-Poimdorf-Verley reduction of 4-pyranones with a reagent derived from MesAl and benzhydrol favors hydride dehvery from tris(diphenyhnethoxy)-aluminum on the equatorial side to give axial alcohols. 

A review of the chemistry of a-oxoketenes includes examples of pyranone and dioxinone ring construction (94S1219). A highly substituted pyran-4-one is formed by the tetramerisation of phraylketrae generated by the cathodic reduction of 2-chloro-2 phenylacetyl chloride at low current density (94H1339). 

---