2-Pyrones, 4-methoxy-, reaction with

Recently, the reaction of masked ortho-benzoquinone [92] with C60 was tested [93]. The [4+2] cycloaddition reaction of such electron-deficient dienes with fullerenes resulted in the formation of highly functionalized bicyclo [2.2.2] octenone-fused fullerenes. The reactants were generated in situ by the oxidation of the readily available 2-methoxy phenols with hypervalent iodine agents. For the several different masked ortho-benzoquinones that were tested, it was found that the yield of the cycloadducts depends on the nature of the starting materials and the reaction conditions. Other Diels-Alder reactions of such electron-deficient dienes with electron-poor fullerenes involved tropones [94], 1,3-butadienes substituted with electron-withdrawing groups [95], and 2-pyrone [96]. 

Silica gel has been reported to accelerate Diels-Alder reactions of 3-methoxy-carbonyl-2-pyrone and a related chiral dienophile with butyl vinyl ether or benzyl vinyl ether (Scheme 4) [31]. Related dienophiles, 4,6-disubstituted 2-pyrones, were investigated in reactions with naphthoquinone or A-phenylmaleimide catalyzed by silica or other heterogeneous catalysts in the absence of solvent [32], An example is included in Scheme 4. 

Carbene complexes of Fe and Co carbonyls are also prepared. Unlike the Cr carbene complexes, no cyclopropanation of alkenes occurs with these carbene complexes. Furans are formed by the reaction of alkynes involving rearrangement of methoxy group. The 2-aminofuran 323 is formed by the reaction of the dimethylaminocarbene complex 319 of Fe carbonyl, via rearrangement of the amino group. Under CO pressure, pyrone 324 is the main product [97]. In these reactions, the... 

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 reaction of N-(terf-butoxycarbonyl)leucinal 2-41 a by Danishefsky et al. with l-methoxy-3-trimethylsilyloxy-l,3-butadiene 2-10 gave the pyrones 2-42 and 2-43 with an induced diastereoselectivity of 9 1 in favour of the syn-com-pound in the presence of Eu(hfc)3 [96]. Later Garner [97] used a N-Boc-serine derived aldehyde 2-41 b and Danishefsky s diene 2-10. In both cases a chelation-control forming a complex between the nitrogen and the oxygen could explain the obtained selectivity. In the presence of HMPA chelation is minimized to give a higher extent of the anfi-product 2-43 (Fig. 2-12) [97]. 

Hetero-Diels-Alder reactions have been employed with the reagent to afford pyrones that have been elaborated into o-sugars. Thus, l-methoxy-3-(trimethylsilyloxy)buta-l,3-diene reacted with the reagent under Lewis acid catalysis to afford the pyrone in 72% yield (eq 9). The pyrone was converted to 2-deoxy-o-ribonolactone to establish conclusively the stereochemistry of the newly formed center. ... 

Marko and co-workers applied chiral Yb catalysts to enantioselective Diels-Alder reactions of electron-deficient dienes (Table 10) [35]. When the reaction of 3-carbo-methoxy-2-pyrone with phenyl vinyl sulfide was conducted in the presence of THE (5 mol equiv. to Yb), the bicyclic lactone was obtained in 92% yield and in more than 95% ee (entry 5). Vinyl ethers could also be used as dienophiles, affording the corresponding products with excellent selectivities. 

Interestingly, the reactions of (i )-2,3-0-isopropyUdene-D-glyceraldehyde (35) with l-methoxy-2,4-dimethyl-3-(trimethylsiloxy)butadiene, catalyzed by (-f)-34 and (-)-34, respectively, involved a high degree of double stereodifferentiation. The matched combination of (-)-34 with R)-35 gave one of the four possible diastereomeric pyrone products in 93.1% selectivity. On the other hand, the mismatched pair showed almost no selectivity (Scheme 21) [38]. 

The 5-phenyl analogue has been obtained by a type of Homer-Emmons reaction. 4-Methoxy-6-phenyl-2-pyrone in tetrahydrofuran was added at -10 C to dimethyl methylphosphonate in tetrahydrofuran previously treated at -10°C with n-butyllithium in hexane and reacted for 30 mins. Further butyllithium was introduced and after reaction during 1 hour at ambient temperature and work-up by acidification, 5-phenylresorcinol monomethylether was isolated in 88% yield (ref.16). 

To a flask equipped with a condenser were added 70 g 2-methyl-5-methoxy y-pyrone (0.2 mol) and 31 g dimethyl sulfate (0.25 mol). The mixture was heated at 50°C for 2 h, and was then poured into 3 eq. of ice-cooled 20% HCIO4. After 2 h, the pyronium perchlorate salt was filtered and added to 175 mL 10% ammonium carbonate solution saturated with ammonium sulfate. After the reaction, the product was extracted with EtOAc. The product was obtained via evaporation of solvent and vacuum distillation, b.p. 75-80° C (1 mmHg). The product can also be converted to hydrochloride salt, m.p. 164°C. 

When, diphenylthiourea was allowed to condense with malonyl dichloride, l,3-diphenyl-2-thiobarbituric acid (XLV) was formed together with a 25 per cent yield of 4,5-dioxo-l,3-diphenyl-7-hydroxy-2-thiopyrano[2,3-d]pyrimidine (XLIV) [31]. It is possible that (XLIV) is produced by the reaction of an excess of malonyl dichloride on (XLV), formed initially. On methylation with diazomethane, the 7-methoxy derivative of (XLIV) is formed [31]. When heated in pyridine, the pyrone (XLIV) decomposes to give 5-acetyl-l,3-diphenyl-2-thiobarbituric acid [31]. 

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