Diels-Alder reactions 4 dendralene

Scheme 27. Domino Heck-Diels-Alder reaction and formation of dendralenes by cycloisomerization of enediynes...

Sherbum reported a robust synthesis of the fascinating diene [4]dendralene (97) and its behavior in Diels-Alder reaetions with N-methylmaleimide (89, NMM). Dendralene 97 is available in one step from chloroprene and combines with three equivalents of an A-methylmaleimide-methyl aluminium diehloride complex to provide a diastereomeric mixture of 98 after three Diels-Alder reactions. ... 

The Diels-Alder reaction has a rich history of applications in tandem and cascade processes. Many of the transformations already discussed have a cascade-like nature since processes like retro-Diels-Alder reactions and aromatization reactions often occur after the initial Diels-Alder step. Clearly, the triple Diels-Alder reaction of [4]dendralene described by Sherbum (97—>98) involves a beautiful cascade of cycloaddition reactions. 

Dendralenes [24] (acyclic cross-conjugated polyenes) have been used as dienes in tandem Diels-Alder reactions, and a methodology for the synthesis of highly functionalized angularly anel-lated aromatic compounds has been developed (Scheme 16.23) [25]. A tandem double Diels-Alder reaction of DMAD with [3]dendralene followed by oxidation with DDQ gave the tetramethyl ester... 

SCHEME 16.23 Diels-Alder reaction of [3]dendralene enroute to angular aromatic compounds. 

The iron-carbonyl complexes can be viewed as a protected form of the diene, as the complexes do not undergo typical diene or alkene reactions. Complexation to iron-tricarbonyl fragments has been used in dendralene chemistry in this way. When the [3]dendralene 10.14 was complexed to iron tricarbonyl, employing a cinnamaldehyde imine as a catalyst, the two complexed alkenes lost typical alkene reactivity, while the uncomplexed alkene retained it (Scheme 10.6). Cyclopropanation, dihydroxylation and crossmetathesis of the uncomplexed alkene proceeded as expected. The monocomplexed [4]dendralene 10.19 underwent Diels-Alder reactions at the uncomplexed alkenes (Scheme 10.7). ... 

Reports depicting organocascades involving more than one Diels-Alder reaction are scarce. However, under imidazolidinone catalysis, achiral dendralenes and enals as dienophiles are able to undergo a double Diels-Alder cascade with high efficiency and enantioselectivities (Scheme 7.30) [48]. 

Two final examples of cross-coupling to furnish [3]dendralenes via C2-C3 bond formation are part of very short and efficient total syntheses, and highlight the versatility and attractiveness ofthe approach. As an extension of their work on 1,1-divinylallene (153) (Scheme 1.22), Sherburnel /. [123] synthesized allenic [3]den-dralene 188 via Kumada cross-coupUng of a metallated alkene 187 and a chiral, propargyl mesylate. A subsequent Diels-Alder reaction produced cyclic [3]den-dralene 190 en route to a pseudopterosin aglycone 191 (Scheme 1.29). Allenic [3]dendralenes are prone to decomposition [136], so the subsequent DA reaction was carried out in situ. 

Section 4.2.3. The parent [5]radialene has been synthesized at last [147]. Key to success was a low-temperature decomplexation of a [5]radialene-bis(Fe(CO)3) complex, that had been prepared from a 2,6-dichloro-3-oxa-[5]dendralene precursor. A 30 mM solution of the hydrocarbon in acetone had a half-life time of around 16 min at -20 °C. According to G4(MP2) calculations for the gas phase, a Diels-Alder reaction, which leads to dimerization/polymerization, is outstandingly facile. 

The most obvious method to install the C1-C2 alkene of a dendralene is an olefination reaction (Scheme 1.19), but it has seen very little use, because of the propensity of 2-carbonyl-1,3-butadiene derivatives to undergo rapid Diels-Alder dimerization [81]. In fact, the only successful uses of 2-carbonyl-1,3-butadienes in such processes feature substrates stabilized by 1,1-disubstitution and a 4Z substituent. Such an example is the iterative formylation/olefination sequence reported by Yoshida and coworkers (Scheme 1.20) [82]. A selective, single electrophilic formylation followed by a Wittig reaction gave hexa-substituted... 

Recently, Sherburn and coworkers [87] attempted to use Wittig olefination to synthesize 1-substituted [3]dendralenes, only to determine that [3]dendralenes featuring a l -conjugating substituent underwent rapid DA dimerization and could not be isolated. The Wittig reaction furnished only an isolated example of a IZ-phenyl substituted [3]dendralene in low yield (20%), along with a mixture of three Diels-Alder dimerization products. This led to the development of a cross-metathesis approach involving tricarbonyl-iron complexed dendralenes, which is discussed in Section 1.4. 

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