Retro Diels-Alder reaction additional application

Examples of the application of the tandem Wittig conjugate-addition sequence to noncarbohydrate y-lactols are shown below. The tricycle 8 was prepared from the lactol 7 in good yield and highly stereoselectively. Upon thermolysis of 8. a retro-Diels-Alder reaction gave rise to furan and to the chiral dihydrofuran 9 in 94% ee36. 

Oxazoles represent the most widely recognized heteroaromatic azadiene capable of [4 + 2] cycloaddition reactions. The course of the oxazole Diels-Alder reaction and the facility with which it proceeds are dependent upon the dienophile structure (alkene, alkyne), the oxazole and dienophile substitution, and the reaction conditions. Alkene dienophiles provide pyridine products derived from fragmentation of the [4 + 2] cycloadducts which subsequently aromatize through a variety of reaction pathways to provide the substituted pyridines (Scheme 14). In comparison, alkyne dienophiles provide substituted fiirans that arise from the retro Diels-Alder reaction with loss of R CN from the initial [4 + 2] cycloadduct (Scheme 15,206 Representative applications of the [4 + 2] cycloaddition reactions of oxazoles are summarized in Table 14. Selected examples of additional five-membered heteroaromatic azadienes participatiitg in [4 + 2] cycloaddition reactions have been detailed and include the Diels-Alder reactions of thiazoles, - 1,3,4-oxadiazoles, isoxazoles, pyrroles and imidazoles. ... 

Numerous synthetic applications of the inteimolecular Pauson-Khand reaction have been reported. Pauson has reported a number of very direct tqrplications of cycloadditions of ethylene in the synthesis of prostanoids and jasmone analogs (e.g. equations 15 and 16). - This is a reliable entry to 2-sub-stituted cyclopentenones. The suitability of cyclopentene and dihydrofuran as substrates has permitted the extension of this work to the preparation of still further varieties of prostaglandin analogs (e.g. equations 27 and 51). Simple 4,5-disubstituted 2-cyclopentenones are not as directly accessible, but may be prepared from the cycloaddition products of norbomadiene (equation 45). A sequence of conjugate addition followed by retro-Diels-Alder reaction affords the product (Scheme 5). Dihydrofuran cycloadditions have been used by Billington in the syntheses of the antibiotic methylenomycin B (Scheme 6), as well as cyclomethylenomycin A (synthetic precursor to the antibiotic methylenomycin A), cyclosarko-mycin (precursor to the antitumor agent sarkomycin) ° and the iridoid Jq>anese hop ether. ... 

Other Addition Reactions.—A review on the applications of retro-Diels-Alder reactions has appeared. Reformation of the steroidal 5,7-dienes from the 4-phenyl-l,2,4-triazoline-3,5-dione adducts was achieved by heating with K2CO3 in DMSO or DME. This provides a very useful alternative to the previously reported L1A1H4 cleavage. A cation-radical mechanism was proposed... 

The application of reversible click reactions, such as Cu-catalyzed azide-aUcyne addition, Michael-type addition, and retro Diels—Alder cycloreversion, is used as a simple approach to perform a degradation process under physiological conditions. This class of reversible chck reaction is promising for predictable, tunable control of cell microenvironment properties. 

The application of thermolytic reactions (e.g. Diels-Alder and reverse Diels-Alder processes) for the synthesis of fluoro-organic compounds has been reviewed. The cycloaddition of the meso-ionic dithiolone or thiazolone compounds (209 X = S or NPh) with the Diels-Alder adduct of cyclo-octatetraene and dimethyl acetylenedicarboxylate occurs at the cyclobutene ring to give the exo-adducts (210). Pyrolysis of compounds (210) yields dimethyl phthalate and 7,9-diheteratricyclo[4,2, 1,0 ]non-3-en-8-ones (211 X = S or NPh) by retro-[4-t- 2] addition. The similar additions of (209) and related compounds to dimethyl 7-oxabicyclo[2,2,l]-hepta-2,5-diene-2,3-dicarboxylate afforded [4 -1-2] adducts which on pyrolysis suffered double thermal fragmentation e.g. loss of XCO and 3,4-dimethoxycarbonyl-furan) to afford five-membered aromatic heterocyclics. ... 

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