And retro Diels-Alder reactions

SCHEME 7. Competition between cycloadditions and retro- Diels -Alder reactions in the reaction of dicyanoacetylene and ll-methylene-l,6-methano[10]annulene... 

Uncatalysed Diels-Alder reactions usually have to be carried out at relatively high temperatures (normally around 100 °C)73, often leading to undesired side reactions and retro-Diels-Alder reactions which are entropically favoured. The Diels-Alder reaction became applicable to sensitive substrates only after it was realized that Lewis acids (e.g. A Clg) are catalytically active56. As a consequence, Diels-Alder reactions can now be carried out at temperatures down to — 100°C85. The use of Lewis acid catalysts made the [4 + 2]-cycloaddition applicable to the enantioselective synthesis of many natural compounds51,86. Nowadays, Lewis acid catalysis is the most effective way to accelerate and to stereochemically control Diels-Alder reactions. Rate accelerations of ten-thousand to a million-fold were observed (Table 7, entries A and B). 

X. DIELS-ALDER AND RETRO-DIELS-ALDER REACTIONS. 63... 

By using a sequence involving regio and diastereoselective reduction of the adducts (it only affects the carbonyl far away from the sulfinyl group, yielding y-oxygenated lactones), stereoselective AT-acyliminium addition, and retro Diels-Alder reactions as the main key steps, Koizumi et al. have synthesized chirally functionalized pyrrolidines [96, 97], pyrrolizidines [98], and indolizidines [98-100], depicted in Scheme 56. The milder conditions required for the evolution of the adducts derived from furan preserve the chirality of the substrates. 

The above-mentioned examples belong to the captivating field of covalent reversible polymers. This field includes examples like reversible polymerization of cyclic oligomers from poly (ethylene terephthalate) [62], thermo-reversible polymers based on Diels-Alder and retro-Diels-Alder reaction (see, for example [63-69]), reversible photo-polymerization in the solid state of molecules... 

Some pyrolytic reactions can be seen as a reverse (retrograde) addition. Diels-Alder reaction for example is known to be reversible and retro Diels-Alder reactions are rather common. The retro-ene reaction (retro hydro-allyl addition), for example, takes place by the following mechanism ... 

Surprisingly, existence of the Diels-Alder and retro Diels-Alder reaction in nature have been revealed as described in following sections. 

Fig. 23. Synthesis of pyrenochaetic acid A (87) through the Diels-Alder and retro Diels-Alder reactions...

In contrast to tetracyclo[4.3.0.0 .0 ]non-8-eneJ the corresponding 5-benzhydrylidene and 5-isopropylidene derivatives 15 (R = Ph, Me) do not undergo photochemical [27t + 2o-] cycloaddition but a formal [ 2 + 2 + 23] (retro-ene) reaction to furnish the substituted 5-methylenetricyclo[4.3.0.0 ]nona-3,7-dienes 16. The isopropylidene derivatives 17 also rearrange via a 1,3-sigmatropic shift and retro-Diels-Alder reaction to give the bridged cyclooc-tatrienes 18. 

Oxazoles. Acylation and pyrolysis of the hydroxy amides in decalin provides 2-substituted oxazoles by way of cyclodehydration and retro-Diels-Alder reaction. Imidazoles are similarly prepared from the vic-diamine. 

For example, 3-perfluoroalkyl-4-ethoxycarbonylfuranes 48 have been prepared from bicyclohepta-2,5-diene derivatives 49 by sequence involving cycloaddition of acetylene to tetracyclone 50 and retro Diels-Alder reaction of the cycloadduct. ... 

Fragmentation of an adduct with release of a nitrile, CO2 or N2 are most common and the latter provide an irreversible method for the formation of a new diene or aromatic compound. Cycloaddition of a pyran-2-one or a 1,2-diazine (pyridazine) with an alkyne gives an intermediate bridged compoimd that loses CO2 or N2 to generate a benzene derivative (see Scheme 3.46). Many other aromatic and heteroaromatic compounds can be prepared likewise. For example, a synthesis of lavendamycin made use of the inverse electron demand Diels-Alder reaction between the 1,2,4-triazine 116 and the enamine 117, followed by in situ elimination of pyrrolidine and retro Diels-Alder reaction, releasing N2 and the substituted pyridine 118 (3.88). 2... 

The Feist-Benary and Paal-Knorr syntheses are commonly employed in the preparation of furan ring systems. In special cases where furan derivatives are difficult to prepare by other methods, Diels-Alder and retro-Diels-Alder reactions have become important methods for their synthesis. Finally, transition metal-catalyzed cyclization and cycloisomerization reactions have recently gained significant attention for their utility in the synthesis of highly functionalized furans. Key examples of these syntheses are highlighted in the sections below. 

Wang GW, Chen Z-X, Murata Y, Komatsu K. [60]Fullerene adducts with 9-substituted anthracenes mechanochemical preparation and retro Diels-Alder reaction. Tetrahedron 2005 61 4851-6. 

Descriptive Passage and Interpretive Problems 10 Intramolecular and Retro Diels-Alder Reactions 425... 

The striking difference in product formation is a function of substrate structure and the mode of mro-Diels-Alder (loss of BnN=C=0 vs. CICN). A pioneer in azine Diels-Alder cycloadditions and retro-Diels-Alder reactions is Boger [140-144], and some examples are shown in equations 3 and 4. Both ( )-cw-(71) and ( )-tra i-triken-trin A were synthesized in this fashion [142]. 

Simple syntheses of ( )-phyllostine (384), ( )-epoxydon (385), and (+ )-epiepoxy-don (386) have been accomplished starting from either the exo- or endo-[A + 2] adducts of p-benzoquinone with 6,6-dimethylfulvene. Thus, for example, epoxida-tion of the endo-adduct gave (387) which, on treatment with formalin in the presence of diazabicycloundecene in THF at 0°C, gave (388). Pyrolytic retro-[4 + 2] reaction of (388) afforded (384), whilst partial reduction of (387) followed by hydrcxymethyla-tion and retro-Diels-Alder reaction gave (385) a similar sequence of reactions yielded (386) from the exo-adduct. 

Scheme 7.1 Diels-Alder and retro-Diels-Alder reactions...

Flow can Diels-Alder and retro-Diels-Alder reactions be used for the preparation of thermoresponsive dendrimers and dendronized hyperbranched polymers ... 

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