Diels-Alder reactions benzannulation

Similarly, the benzannulated enyne-allenes 172 and 173 were prepared from the propargylic acetates 171 by cuprate addition or by Pd-catalyzed addition of arylzinc chloride (Scheme 20.35) [49]. The presence of a butyl group and a p-anisyl group at the allenic terminus of 173a and 173b permits competition between a formal ene reaction and a formal Diels-Alder reaction leading to 174 and 175, respectively. 

In addition to the reaction of vinylcarbene complexes with alkynes, further synthetic procedures have been developed in which Fischer-type carbene complexes are used for the preparation of benzenes. Most of these transformations are likely to be mechanistically related to the Dbtz benzannulation reaction, and can be rationalized as sequences of alkyne-insertions, CO-insertions, and electrocycli-zations. A selection of examples is given in Table 2.18. Entry 4 in Table 2.18 is an example of the Diels-Alder reaction (with inverse electron demand) of an enamine with a pyran-2-ylidene complex (see also Section 2.2.7 and Figure 2.36). In this example the adduct initially formed eliminates both chromium hexacarbonyl ([4 -I- 2] cycloreversion) and pyrrolidine to yield a substituted benzene. 

A [4+2] benzannulation between acetylenic ketones 248 and a benzenediazo-nium 2-carboxylate proceeds effectively in the presence of a catalytic amount of AuCl, yielding functionalized anthracenes 250 in good yields (Scheme 12.67) [137]. It is suggested that the reaction involves a reverse electron demand Diels-Alder reaction between benzyne and the benzopyrylium aurate complex 249. 

Iron(lll)-catalyzed benzannulation of 2-phenacylbenzaldehyde has been reported (Scheme 16.44) [49]. This reaction provides high substrate scope and high yields of the benzannulated naphthalene derivatives. The mechanism of the reaction is shown in Scheme 16.45. It involves enohzation of 2-phenacylbenzaldehyde to give an ortho quinodimethide derivative that undergoes Diels-Alder reaction and elimination of a carboxyhc acid from the adduct to yield the naphthalene derivative. 

The Diels-Alder cycloaddition of chiral cyclic 2-amidodienes, derived from chiral a-allyl allenamides, with enones yielded optically enriched [2.2.2]bicyclic adducts with dr up to >95 5. In the presence of HOMO-raised trienamines, e-deficient l-aza-l,3-butadienes containing 1,2-benzoisothiazole-1,1-dioxide or 1,2,3-benzoxathiazine-2,2-dioxide react as dienophiles in normal-electron-demand Diels-Alder reactions rather than the corresponding inverse-electron-demand versions. The Fe(III)-catalysed benzannulation of 2-(2-oxyethyl)-benzaldehydes with alkynes produced naphthalene derivatives in high yields (99%) under mild reaction conditions. ... 

Benzannulation of pyrylium salts 7 with ethyl vinyl ether 8 was reported by Verin et al. The reaction proceeded smoothly under reflux conditions and the corresponding a-naphthyl ketone products 9 were obtained in good yields (Scheme 15.3) [2]. The reaction would proceed through [4 + 2] hetero-Diels-Alder reaction to form bridged intermediate 10, which is converted to 9 by releasing EtOH and HCIO4. Besides ethyl vinyl ether, some active methylene compounds are suitable dienophiles for this reaction. 

A plausible mechanism for the present benzannulation is shown in Scheme 15.8. As shown in Scheme 15.4, the intermediate 16" would be formed from benzopylilyum intermediate 14 and alkyne 2. Protonolysis of the C—Cu bond of 16" by a Brpnsted acid (HA) such as CF2HCO2H, followed by the attack of A to the carbon of RCO would produce 22. It would undergo the retro Diels-Alder reaction to give 19 together with the compound eliminated, 23. When the reaction was conducted in the absence of a proton source, the protonolysis of the C—Cu bond does not proceed efiflciently, and the naphthyl ketones are produced directly from 16", as in the case of the AUX3-catalyzed reaction. 

The hetero-Diels-Alder reaction (HDA) has been used extensively for the construction of spiroacetals [4], notably in the synthesis of the reveromycins [150-153]. In particular, the recent development of mild methods for the generation of ort/to-quinone methides (o-QM) in the presence of sensitive five-membered exocyclic enol ethers has revealed an alternative route to benzannulated... 

In a new benzannulation procedure, the 4 + 2-cycloaddition of lithium ynolates (131) with (trialkylsilyl)vinylketenes (132) yields, via an intermediate 3-(oxido)dienyl-ketene (133), the highly substituted phenols (134), which can be converted to ben-zofurans and benzopyrans (Scheme 37).135 The reaction of buta-2,3-dienoate with vinylketenimine yields the expected Diels-Alder adduct together with an unexpected aniline formed by a competing 2 + 2-cycloaddition.136 The first example of a... 

A palladium-catalyzed intramolecular benzannulation of bis-enynes 1135 proceeds chemoselectively to afford dihydroisocoumarins 1136 (Equation 441) <2002JOC2653>. A reaction sequence involving ruthenium-catalyzed yne-ene cross-metathesis of a polystyrene supported undecynoic acid ester followed by a Diels-Alder cycloaddition reaction with DMAD provides the basis for a combinatorial approach to dihydroisocoumarins featuring a variety of side chains at C-6 and C-8 <1999SL1879>. 

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