Miscellaneous Diels-Alder Reactions

MA = maleic anhydride BQ = -benzoquinone DMM = dimethylmaleate DMF = dimethyl fumarate NQ = naphthoquinone AC = acrolein AN = anthracene IS = isoprene 2,3-DB = 2,3-dimethylbutadiene 1,3-CH = 1,3-cyclohexadiene. 

Base-catalyzed Diels-Alder reactions are rare (Section 1.4). A recent example is the reaction of 3-hydroxy-2-pyrone (145) with chiral N-acryloyl oxazolidones 146 that uses cinchona alkaloid as an optically active base catalyst [97] (Table 4.25). Only endo adducts were obtained with the more reactive dienophile 146 (R = H), the best diastereoselectivity and yields being obtained with an i-Pr0H/H20 ratio of 95 5. The reaction of 146 (R = Me) is very slow, and a good adduct yield was only obtained when the reaction was carried out in bulky alcohols such as t-amyl alcohol and t-butanol. 

Diels-Alder reaction of dienophiles, N-allylic enamides and a,/l-unsaturated lactam derivatives with open chain and inner ring dienes is promoted by iodine [98]. Thus the cycloaddition of N-benzyl-N-methallyl acrylamide 147 with cyclo-pentadiene (1) proceeds smoothly in DMF at —78 °C in the presence of I2 (2 eq.) to give a prevalence of endo adduct l Vd) in 88% yield (Equation 4.17). 

Among special chemical methods that facilitate the Diels-Alder reaction can be included the temporary metal connection strategy [101] that is illustrated in Table 4.27. Si, Mg and A1 are used as temporary connectors of diene and dienophile moieties. The cycloaddition occurs easily due to its intramolecular nature and because the dienophilic component of reagent is now formally a vinyl carbon ion (i.e. a vinyl carbanion in 154 with M = AlEt ). Thus the metal-tethered 154, prepared from lithium alkoxide of 153 with the suitable metal vinyl halide, gives, by heating, the cycloadducts 156 and 157, through the 

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Syntheses with Miscellanous Hetero Diels-Alder Reactions... 

Miscellaneous. There are several other reports on the application of this ligand to catalytic asymmetric reactions, although enantioselectivities are modest. Those reports include the Mukaiyama-Michael reaction, allylation of aldehydes, asymmetric Diels-Alder reaction, Mukaiyama-Aldol reaction of ketomalonate, aziridination reaction of a-imino esters, and asymmetric hetero-Diels-Alder reaction. ... 

The first part of this chapter deals with the effects of high pressure on cycloaddition reactions, particularly the Diels-Alder reaction, which is the most important cycloaddition reaction. The second part will illustrate applications of pressure to nucleophilic substitutions, condensations and other reactions (miscellaneous reactions), such as Mannich, Heck, ene, SeAr, Wittig, Horner-Wadsworth Emmons and multicomponent Strecker reactions. 

The emphasis in this work is on the scope and preparative synthetic utility of the hetero Diels-Alder reaction. No attempt has been made to carefully define or delineate the important mechanistic questions, many of which are as yet unanswered, of the various [4 -I- 2] cycloaddition reactions other than to try to provide a rationale for the facility with which the cycloadditions proceed and to provide a basis for the stereo- and regio-chemical observations. We have purposely excluded reactions of singlet oxygen as a dienophile, since extensive surveys are available elsewhere. Many miscellaneous heterodienophiles and heterodienes have not been covered if in our opinion they are not of general synthetic value. A comprehensive treatment of all recorded hetero [4 + 2] cycloadditions is beyond the scope of this monograph. However, we do hope to provide a broad survey of this reaction type as it exists today in order to furnish a foundation for continued development. 

Diels-Alder Reactions of Vinylthiophenes and Related Compounds 2.11.6.5.1 Miscellaneous cyclizations... 

Several methods for construction of diazine core of chain-fluorinated pyrimidines do not fall into any of the mentioned above categories. One of such approaches is discussed in Sect. 7.8, namely, inverse-electron-demand Diels-Alder reactions with fluorinated sym-triazines. Other methods that fall into category miscellaneous are too different to discuss them systematically therefore, selected examples of them are listed in this section. 

Koreeda and Luengo have reported the most reeent of these miscellaneous approaches which resulted in the first synthesis of the 6,11-diepitrichothecene skeleton (89). Their sequence, shown in Scheme 22, is based on an intramolecular version of the Diels-Alder reaction. Preparation of the two dienol ethers (210) and (211) was carried out from the common intermediate (209). While heating the -isomer (210) produced no cyclized materials, the Z-isomer, upon heating, produced the 6,11-diepitri-chothecene (212) in an 80% yield. It should be recalled that this trichothecene isomer could not be prepared via a group 3 type cyclization (see Scheme 14). Obviously, the extension of this approach to more complex trichothecene skeletons remains to be explored. 

Miscellaneous Applications. The chemoselective oxidation of alcohols and diols using T 0-i- r)Jt-Butyl Hydroperoxide has been reported. The title reagent has also been employed as a catalyst in Diels-Alder reactions and as an additive in the palladium-catalyzed reaction of aryl-substituted allylic alcohols with zinc enolates of p-dicarbonyl compounds (eq 24). The latter... 

Miscellaneous Reactions. Some hydantoin derivatives can serve as precursors of carbonium—immonium electrophiles (57). 5-Alkoxyhydantoins are useful precursors of dienophiles (17), which undergo Diels-Alder cycloadditions under thermal conditions or in the presence of acid catalysis (58). The pyridine ring of Streptonigrine has been constmcted on the basis of this reaction (59). 

This chapter addresses chirally modified boron Lewis acid complexes, in which there has been increased interest because of their capacity to induce chirality. They have been successfully used for Diels-Alder, aldol, and a variety of other miscellaneous reactions. I will describe and analyze here the different types of catalyst and classify them according to their efficiency, selectivity, and flexibility. 

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