Photocycloaddition cyclooctadiene

Another unique example was observed for the recently isolated marine natural product, alteramide A (209), isolated from a symbiotic bacteria (Alteromonas sp.) found on the sponge Halichondria okaclai96. It was found that the tetraene core of this compound underwent intramolecular [4 + 4]-photocycloaddition upon exposure to sunlight (equation 9). Deliberate irradiation led to a quantitative conversion to cyclooctadiene 210. 

The dimerization of thioformylketene was investigated by B3LYP and G3MP2B3 methods. The 4 + 4-pathway has the lowest energy barrier and calculations suggest that the reaction is pseudopericyclic.231 The stereospecific intramolecular 4 + 4-cycloaddition reaction between cyclohexadiene iron tricarbonyl complex and appended dienes (198) generates cyclooctadiene tricyclic adducts (199) (Scheme 56).232 The first example of an asymmetric intermolecular 4 + 4-photocycloaddition reaction in solution between 9-cyanoanthracene and chiral 2-methoxy-l-naphthamides has been reported. The frozen chirality is effectively transferred to the optically active product.233... 

An intramolecular [4+4] photocycloaddition of a 2-pyridone with a furan ring yields the complex 1,5-cyclooctadiene 231 <20060L3367>. The proposed transition state conformation leading to the realized (and desired) f vsy -product is shown (Equation 110). The isopropyl group on the cyclopentane of the pyridone demonstrated stereocontrol and the... 

Photodimerization of simple 1,3-dienes in a 4 1 + 4 r cycloaddition process is typically an inefficient process . This is not surprising, given the highly ordered transition state for [4+4]-cycloadditions, and the predominance of the unreactive s-trans conformation . As a result, as noted above [2 + 2]-cycloadducts are often the major product, accompanied by varying amounts of vinylcyclohexenes and cyclooctadienes. Crossed photocycloadditions employing 1,3-dienes with substituents at the 2- or 3-positions can furnish greater amounts of cyclooctadiene products (equation This presumably results from a perturbation of the diene conformational equilibration to provide a higher proportion of the s-cis conformer. 

Oxabicyclo[3.2.0]heptanes are also produced in the CuOTf-catalyzed photocycloadditions of allyl 2,4-hexadienyl ethers (eq 24). The CuOTf-catalyzed photocycloadditions of bis-2,4-hexadienyl ethers are more complex. Thus UV irradiation of 5,5 -oxybis[( )-l,3-pentadiene] in THF for 120 h produces vinylcy-clohexene and tricyclo[3.3.0.0 ]octane derivatives (eq 25). However, shorter irradiations reveal that these products arise by secondary CuOTf-catalyzed rearrangements of 6,7-divinyl-3-oxabicyclo[3.2.0]heptanes that are the primary photoproducts (eq 26). UV irradiation of the divinylcyclobutane intermediates in the presence of CuOTf promotes formal [1,3]- and [3,3]-sigmatropic rearrangements to produce a vinylcyclohexene and a 1,5-cyclooctadiene that is the immediate precursor of the tricyclo[3.3.0.0 ]octane. 

CuOTf-catalyzed photocycloaddition of the tetraene 96 produces a mixture of the compounds 98,99, and 100 (Scheme 25). The 1,2-divinyl cyclobutanes 97 initially formed from [2 + 2]-addition of the tetraene 96 undergo further reaction on prolonged irradiation in the presence of CuOTf to form these products. The tricyclic compound 100 arises from intramolecular 2jt + 2Jt addition of cyclooctadiene derivative 99. In fact, the transformation of cis,c/s-l,5-cyclooctadiene 101 to the tricychc compound 102 on irradiation in the presence of CuCl was the first example of an intramolecular Cu(I)-catalyzed photocycloaddition reaction. ... 

Intramolecular [2+2]-photocycloadditions of cycUc a,P-unsaturated enones with remote double bonds have been extensively used to synthesize a variety of interesting compounds, including natural products. An analysis of the mechanism of the additions has also been carried out. 2-Pyrones having pendant enes and dienes undergo synthetically useful photocycloaddition processes to give tricyclic lactones and lactone-bridged cyclooctadienes by [2+2]-and [4+4]-cydoadditions, respectively. The photochemical reactions... 

When the 1,3-dienes are largely or exclusively in the s-trans-conformation, such as with diene 83, pyridone 18 leads to three products, 87, 88, and 89." The two cyclobutane products 87 and 88 derive from Cope rearrangement of the highly strained [4-1-4]-photocycloaddition adducts 84 and 85, a mechanism precedented by photocycloadditions of 1,3-dienes with other aromatic species. Pyridine 89 may be formed from oxetane 86. When the 1,3-diene is delivered intramolecularly (90), the only photoproduct is 91, presumably "via an intermediate relating to 85. In contrast with the chemistry of the cis-pyridone dimers (Scheme 3), cyclobutane 91 thermally rearranges to the cyclooctadiene 92. ... 

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