Intramolecular dehydro-Diels—Alder reaction

Scheme 6.49 Intramolecular dehydro Diels-Alder reactions of conjugated enynes with a remote acetylene subunit furnishing annulated benzene derivatives, according to Danheiser and co-workers.

DFT calculations on the intramolecular Diels-Alder reaction of penta-l,3-dienyl acrylates predict stereoselectivities that are in good agreement with the experimen- (g) tal results.85 Another DFT study at the B3LYP/6-31G(d) level of the intramolecular Diels-Alder cycloaddition of 5-vinylcyclohexa-1,3-dienes has been reported. Reaction rates are influenced by dienophile twisting and substituent effects.86 The intramolecular dehydro-Diels-Alder reactions of ynamides (79) provides a new synthesis of benzo[fc]-, tetrahydrobenzo[fc]-, naphtho[l,2-/j -, and dibenzo[a,c]carbazoles... 

The diyne 66 undergoes an intramolecular dehydro Diels-Alder reaction in toluene under reflux to afford naphtho[2,3-c]chromene derivatives in reasonable yield (Scheme 20). The reaction is presumed to proceed via a [4+2]-cycloaddition of the alkyne to the aryl alkyne group to form the cyclic allene intermediate 67, which then isomerizes to the aromatic product <2003SL1524>. 

In addition to the Hopf cydization of 176, there is a second pericydic reaction leading to 162, that is, the dehydro Diels-Alder reaction of butenyne with acetylene (Scheme 6.47). The theoretical treatment of this process by Johnson et al. [59] predicted a free reaction enthalpy and a free activation enthalpy, both at 25 °C, of -13.4and 42.0kcalmol-1, respectively. Ananikov [116] arrived at a similar result for the intramolecular case of non-l-en-3,8-diyne (202) and calculated the same quantities to be -15.3 and 30.9 kcal mol-1 for the formation of the isoindane 203. As already discussed regarding Scheme 6.40, the conversion of 162 into benzene and likewise that of 203 into indane have to be considered as a sequence of two [1,2]-H shifts 116, 117], whose highest transition state has a significantly lower energy than that for the formation of 162 and 203 by the dehydro Diels-Alder reaction. 

The photo-dehydro-Diels-Alder reaction provides an access to 1-phenylnaphthale-nes, 1,1-binaphthyls, A-heterocyclic biaryls, and naphthalenophanes. The PDDA reaction proceeds through a multistage mechanism with biradicals and cycloallenes as intermediates. Again, the intramolecular photo-dehydro-Diels-Alder reaction of diyne (65) produced strained axially chiral (l,5)naphthalenophanes (66) in moderate yields (Scheme 20). " ... 

The highly twisted Ti-conjugated macrocycle (184) with two allq nyl moieties underwent intramolecular [2 + 2] photocycloaddition to give the thiophene-fused bisdehydro[12]annulene (185) in 61% yield. Photolysis of the bis(dithienyl)ethynes (186) in the presence of iodine resulted in sequential electrophilic and photochemical cyclizations to yield the tetrathienonaphthalenes (183) in one pot reactions. The compounds (187) showed a significant potential as a cruciform scaffold for nanostructured Ji-electron materials. The alkenyl-substituted bis(dithienyl)ethynes (188) underwent p-benzoquinone photosensitized double 5-exo-cyclization to give the diarylated dithienofulvalenes (189). The atylpyridinylethynes (190) and (191) in aqueous HCl solution underwent photo-dehydro-Diels-Alder reaction to afford... 

Intramolecular [4+2] cycloaddition reactions of conjugated enynes, namely, dehydro-Diels-Alder reactions, afford benzo-fused CPDNs [254]. Thus, 2-propynyldiarylacetylenes 413 undergo thermal intramolecular [4+2] cycloadditions to give benzo[fc]fluorene derivatives 414 in good yields (Scheme 6.108a). The reactivity of the reactions increases in the order hydrocarbon... 

---