Dehydrogenative 5-lactones

Dehydrogenative lactonization of diols is an efficient way to various lactones (Scheme 22). ° The lactone formation is found to be catalyzed by a recoverable stable dicationic iridium complex with 6,6 -dihydroxy-2,2 -bipyridine ligands, and employs a variety of benzylic and aliphatic diols in aqueous media. In comparison with the esterification of hydroxyl acids, hydroacyloxylation of olefmic acids and Baeyer-ViUiger reaction of cyclic ketones, the dehydrogenative lactonization of diols proceeds without any oxidant hence, it is more environmentally benign and atom economical. 

Fujita K, Ito W, Yamaguchi R. Dehydrogenative lactonization of diols in aqueous media catalyzed by a water-soluble iridium complex bearing a functional bipyridine ligand. ChemCatChem. 2014 6 109-112. 

On distillation with zinc dust the alkaloid gives an easily hydrogenated pyrrole base, and on dehydrogenation by platinised asbestos at 260-290 it yields (1) an amorphous dehydro-base, which forms an oxime and a methiodide, C17H23O4N. Mel, decomp. 227-8 , and contains a lactone and a methoxyl group, (2) a neutral pyrrole derivative, and (3) an acid giving a dark green colour with ferric chloride (1939). 

Flavoprotein 1,2-monooxygenases are used for the insertion of an atom of oxygen into the ring that is the first step in the degradation of both cyclopentanone and cyclohexanone before hydrolysis of the resulting lactones and dehydrogenation (Figure 3.5). There are two types of cycloalkanone... 

Aldonolactones are commercially available at low cost, when compared to most of the common monosaccharides. They are typically synthesized by selective anomeric oxidation of unprotected aldoses with bromine [6]. Usually the thermodynamically more stable five-membered lactone (y-lactone) predominates over the six-membered form, with the exception of o-gluconolactone, which crystallizes as the 1,5-pyranolactone (5-lactone) [7] (Scheme 1). Another method for the preparation of sugar lactones is the dehydrogenation of unprotected or partially... 

In contrast to linear unsaturated acids and anhydrides vide supra), 3,4,5,6-tetrahydrophthalic anhydride reacts with sulfur tetrafluoride to give mainly bicyclo products, lactone 31 and tetrafluoro ether 32, with the trifluoromethyl derivatives 33 and 34 as minor products. The ratio of products strongly depends on the reaction conditions at elevated temperature (> 150°C) cleavage of the C —O —C bond and dehydrogenation occur resulting in l,2-bis(tri-fluoromethyl)benzene (35) in high yield.252... 

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). 

Pinoresinolide (XXVI) 17) and lignenolide (XXVII) (/7, 40) are lactones which are obtained in a small yield from the mixture of intermediates. Both arise from combinations of dehydrogenated ferulic acid and Rb. They are crystalline and are responsible for the weak lactone band in the infrared spectrum of lignin. 

Dehydrogenation of S-lactones. Steroidal <5-lactones are dehydrogenated by hcn/enescleninic anhydride in good yield, but y-lactones are essentially unreactive.3 I1, x a in pie ... 

Dehydrogenation of 1,4- or 1,5-diols to lactones. This Ru-catalyzed transfer dehydrogenation can be effected with high selectivity when the 2-position of 1 is... 

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