Cycloadditions of 3,5-dibromo-2-pyrone

Notably, the cycloaddition of 3,5-dibromo-2-pyrone is more stereoselective than monobromo-2-pyrone counterparts 4 and 5. For instance, cycloadditions with acrylonitrile and benzyl vinyl ether provided the cycloadducts 44 and 51 in endolexo ratios of 76 24 and 100 0 (entries 4 and 11), respectively, while 5-bromo-2-pyrone 5 afforded the corresponding cycloadducts in ratios of 54 46 and 67 33, respectively. 

Because of the higher reactivity, 3,5-dibromo-2-pyrone can undergo the D-A cycloaddition with sterically hindered silyl enol ethers as summarized in Table 3 02TL8193 . 

The reaction of 38 with styrenes 70 provided aryl-substituted bicyclolactones 71 with good endolexo selectivity (Table 5, actual ratios not shown) 02BKCS1021 . 

Because the initial oxidative addition of Pd(0) is faster with the aryl bromide than the vinyl bromide, the vinyl groups can be selectively coupled to give 78, by using one equivalent of organo-stannane or boronic acid. Coupling with a second equivalent of organo- 

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The Diels-Alder cycloadditions of 3,5-dibromo-2-pyrone and its derivatives... 

ZnBr2-catalyzed IMDA cycloaddition of the 2-pyrone 152 (Scheme 37), tethered with a chiral enol silaketal, provided (+)-153-exo in 74% yield, along with a small amount of (-)- 5 i-endo <95JOC1617>. In this case, the -geometry of the dienophile was not preserved during the cycloaddition, as in the cycloaddition of 3,5-dibromo-2-pyrone with dimethyl maleate (cf. entry 9, Table 2), implying a stepwise reaetion mechanism. The isolated exo product (+)-153 was used as the key intermediate for the asymmetric total synthesis of a 2-alkyl-vitamin D3 analog. 

We have found that 3,5-dibromo-2-pyrone 38 undergoes both normal and inverse electron demand cycloadditions more readily than mono-bromo-2-pyrones and these reactions proceed with greater regio- and stereoselectivity (Scheme 11). Moreover, bromides can be selectively functionalized to generate an array of new 2-pyrone synthons, exhibiting much greater applicability than their mono-bromo counterparts. This review is focused on the chemistry of 3,5-dibromo-2-pyrone and derivatives thereof... 

The 3-amino-5-bromo-2-pyrones 106 were expected to favor normal electron demand cycloadditions, compared to the parent 3,5-dibromo-2-pyrone, because of the presence of the electron-donating amino group. Summarized in Table 6 are the results of the cycloadditios of... 

It is noteworthy that the stereochemistry of dimethyl maleate was preserved during the cycloaddition to give cw-disubstituted cycloadducts (entry 9). The cycloaddition of the parent 3,5-dibromo-2-pyrone with the same dienophile was shown to proceed in a stepwise fashion to produce /ra 5-disubstituted cycloadducts (cf. entry 9, Table 2). Contrary to previous results (cf. Table 2), the reactions in CH2CI2 gave uniformly better results than in toluene, in terms of... 

The iV-phenylmaleimide derivative (518), prepared in two steps from a-pyrone and nitrosobenzene, is a dienophile and yields the cycloaddition product (519) on reaction with cyclohexa-l,3-diene." ° The sealed-tube reaction of anthracene with excess l,2-dibromo-3,3-difluorocyclopropene at 120°C gave the crystalline adduct (520), presumably by way of intermediate (521) the mechanism is not known. A synthesis of the tricyclo[5,2,2,0 ]undecane skeleton related to isoeremolactone has been reported. The Diels-Alder adduct (522) was a key intermediate in this synthesis, being converted in two standard steps into an epimeric mixture of compounds... 

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