3- Methoxycarbonyl-2-pyrone, cycloaddition

H 2]-Cycloaddition occurs between 3-methoxycarbonyl-2-pyrone 309 and a trimethylenemethane Pd[(/-PrO)3P]4 complex to give the cycloadduct 310 in 71% yield (88JA1602). (See Scheme 59.) Reactions of 309 and 3-acetylcoumarin 236a with cyclopropenone acetal derivative 312 afford regioselectivity the cyclopentenone acetal compound 313 and 314, respectively (92JA5523). 

A conceptually different [4-1-2] cycloaddition catalyzed by a chiral lanthanide complex has been disclosed. The inverse electron demand Diels Alder reaction of 3-methoxycarbonyl-2-pyrone (67) and enol ethers or sulfides [135] was catalyzed by a chiral ytterbium(III) triflate-binaphthol complex in the presence of diisopropylethylamine (Scheme 51) [136]. Thermal decarboxylations of bicyclic lactones such as 68 are known to yield dienes which may undergo subsequent pericyclic reactions [137] thus, the adducts of this process are potentially useful chiral building blocks. The nature of the substituent on the 2k component was found to be crucial for the realization of high enantioselectivity. 

Under appropriate conditions, even unactivated alkenes will take part in intermolecular cycloadditions with 3- and 5-bromo-2-pyrones and with 3-methoxycarbonyl-2-pyrone. Reactions can be conducted at 100 °C, or at room temperature under 10-12 kbar and with zinc chloride catalysis. 

Various other nonconjugated dienes have been used as the dieno-phile component in tandem Diels-Alder processes. In these cases, the first cycloaddition occurs as usual. Then cycloreversion with loss of COj is followed by intramolecular Diels-Alder reaction with the remaining pendant alkene to yield bicyclo[2.2.2]octene structures. For example, methyl coumalate (5-methoxycarbonyl-2-pyrone 26) reacts with 1,5-cyclooctadiene under thermal conditions to give the cage compound 27. ... 

Methoxycarbonyl-2-pyrone. It has been reported that the [4+2] cycloaddition reactions of 5-methoxycarbonyl-2-pyrone 41... 

Markd and co-workers reported low levels of stereocontrol in the cycloaddition of 3-methoxycarbonyl-2-pyrone and butyl vinyl ether when TADDOL ligands are used in conjunction with YbfOTOj. ... 

Markd and co-workers also explored the use of binol complexes as Lewis acid catalysts for [4-t-2] cycloadditions of 3-methoxycarbonyl-2-pyrone. Marko reported modest to excellent levels of stereocontrol in the cycloadditions of a variety of vinyl ethers with this pyrone (Table 1 i)."b-d,6i gjjjj better results are reported when vinyl thioethers function as the dienophile. It is important to note that these impressive stereochemical results are obtained with catalytic amounts (0.1-0.2 equivalents) of Lewis acid. 

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