Pyrones reduction

The application of a sodium borohydride/cerium(III) nitrate reagent to a pyrone reduction produced results that departed somewhat from the 1,2-additions observed by Luche. Catalytic reductions of y-pyrones can lead to complex mixtures of products via nonselective processes however, the lanthanide/borohydride system provided a good yield of the specifically reduced dihydropyrone, Q, from the unsaturated precursor, while preserving the carbonyl function adjacent to the double bond (Poulton and Cyr, 1980). 

In principle, complex hydrides (NaBHj, LiAlH ) ought to react similarly with 4-pyrones and lead after treatment with Bronsted or Lewis acids to 4-unsubstituted pyrylium salts. This reaction has not been reported the reduction of 2-pyrones with LiAlH4 results in ring opening. " ... 

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... 

The lactone-peroxides 17 are derived from the corresponding ot-pyrones. Singlet oxygenation at low temperature affords the unstable a-pyrone endoperoxides which, on warming up, readily decarboxylate into 1,2-diacylethylenes. However, subambient diimide reduction leads to the desired lactone peroxides 17 (Eq. 12)29). 

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). 

Butyl)-4-hydroxy-3-ethyl-2-pyrone (Germicidin) [Reduction of a Ketone Carbonyl to a Methylene Group in a Multifunctional Compound].423 A... 

Macrocyclic 2-pyrones (113) and (114) are prepared from enamine (115) and ketene (75HCA2409). Reduction of cyano ketone (116) with LAH affords an amino ketone which spontaneously cyclizes to generate a tetrahydropyridine (117) dehydrogenation by palladium on charcoal produces the aromatized phane (118) (71TL671). 

A novel route to anthracyclinones is based on the chemistry of quinone-isobenzofuran adducts (77TL3537). The 3-methoxybenzyne-furan adduct (1) was reacted with a-pyrone to give a mixture of lactones (2). Thermolysis of this intermediate in the presence of quinone (3) gave in 93% yield the tetracyclic adduct (4) as a stereoisomeric mixture. Aromatization with sodium acetate in acetic acid gave quinone (5) which was subjected to reduction, C-ring oxidation and mild acid hydrolysis to afford a mixture of ( )-7-deoxydaunomycinone (6) and its 1-methoxy regioisomer (Scheme 1). 

Another synthesis of pterocarpans starts from the coumestan system. LAH treatment results in reductive cleavage of the a-pyrone ring (Scheme 52). Benzofuran derivative (242) is thermally cyclized to (243) (64JCS4212 see also 78IJC(B)372), which may also be formed... 

A satisfactory correlation of half-wave potentials for polarographic reduction of 2,5-diaryl-4-pyrones,221 as well as AA for the electronic absorption spectra maxima of 3-phenylazojulolidines (26) and their... 

The intramolecular thermal [5+2] cycloaddition of 3-alkoxy-4-pyrones with sulfur- (e.g., 416) or silicon- (e.g., 419) tethered alkenes has been shown to occur with complete regio- and stereochemical control to give adducts 417 and 421, respectively. The adducts can be converted by reduction and oxidation, respectively, to the bicyclic products 418 and 421 (Scheme 69) <1993JOC5585>. It should be noted that this thermal [5+2] cycloaddition has not been realized in a bimolecular mode <1977JOC3976>. This methodology serves as an alternative to the reaction of electron-deficient alkenes with pyrone-derived 4-methoxy-3-oxidopyrylium ylides <1992TL2115>. 

The protected keto-aldehyde (4) has also been converted, in several steps, into the dihydro-y-pyrones (11) and (12). A Birch reduction of the pyrone (11) with lithium in liquid ammonia afforded ( )-elaeokanine E (13) as the only product. A similar reduction of the epimeric pyrone (12) gave a product that is not identical with elaeokanine D, and it is considered to be ( )-epielaeokanine D (14).4... 

The pyrones are reduced polarographically.90-92 Four y-pyrones (1, 3, 4, 5) and a-thiathiopyrone (8)90 produced two-electron waves, which probably corresponds to the reduction of the exocyclic bond (however, y-thiopyrone gives a six-electron wave). No relation was found between the half-wave potentials (Table X) and the LFMO... 

---