Pyrones, synthesis

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Pyrones, synthesis via transition metal-aided cycloaddition reactions of alkynes 88CRV1081. 

Scheme 6 2-pyrone-synthesis from 3-hexyne and CO2 under supercritical conditions. 

Other references related to the Mentzer pyrone synthesis are cited in the literature. ... 

This reaction is related to the Mentzer Pyrone Synthesis. 

Our scheme of a-pyrone synthesis involves the esterification of homoallylic alcohols with acryloyl chloride, followed by ring-closing metathesis (RCM) using Grubbs ruthenium catalyst (Scheme 6) (44). We have identified a large number of such pyrone-containing natural products that can be readily synthesized via our allylboration strategy. Representative examples of our syntheses are described below. 

Pyrone synthesis by cycloaddition of CO2 to terminal alkynes (1-hexyne, 1-propyne) has also been investigated. This process can be catalytically promoted, albeit with low yield and selectivity, by Co [74] and Rh [75] complexes. Rh(dppe) (Ti -BPh4), in acetonitrile, at 390 K, catalyzed the formation of 4,6-dimethyl-2-pyrone from 1-propyne and CO2 (1 MPa) with a TON of 50 [75]. The Rh-catalyzed reaction has been proposed to proceed through a mechanism (Scheme 5.15) not involving an oxametallacycle intermediate species. The CO2 insertion into the Rh-C(sp )-o-bond of a Rh-alkenyl intermediate, obtained upon propyne dimerization, affords a linear unsaturated carboxylate which is converted into the pyrone. 

As important hetero Diels-Alder reactions catalyzed by aluminum Lewis acid, two kind of reactions, namely, [4 + 2] cycloaddition of Danishefski s diene with carbonyls and [4 + 2] cycloaddition of nitroalkenes with electronic rich alkenes, have been well known. The former reaction provides highly functionalized pyrones. As the first example of pyrone synthesis through catalytic asymmetric hetero Diels-Alder reaction with chiral aluminum complexes, in 1987, Quimpere and Jankowski reported the reaction of oxomalonate with 1-methyl-1,3-butadiene using Koga s catalyst (48). However, the asymmetric induction and chemical yield were quite poor (Scheme 6.147) [175]. 

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