Paterno-Biichi reaction intramolecular

The intramolecular equivalent of the Paterno-Biichi reaction has been observed in a number of unsaturated ketones. A series of y,S-unsaturated ketones (278) are converted in this way into the oxabicyclo[2.2.0]hexanes (279) and the oxabicyclo[2.1.1]hexanes (280), both photoproducts being the result of 1,2-cycloaddition.303... 

The reactivity and selectivity trends described for the intermolecular version of the Paterno-Biichi reaction are also valid for the intramolecular version. Special aspects of intramolecular photocycloadditions have been reviewed [157]. A crucial role for the reactivity mode of unsaturated carbonyl compounds is the tether length, i.e., the distance between reactive carbonyl and C-C double bond. Conjugated systems, i.e., oc,(3-unsaturated carbonyls, prefer photodeconjugation or photocycloaddition invovling the C-C double... 

Hydrogen bonding between the ground-state reactants dihydropyridone (259) and an aldehyde 260 was found to be responsible for a high facial diastereoselectivity (>90% de) (see also Special Topic 6.3) in the Paterno Biichi reaction (Scheme 6.107).900 901 X-ray analysis, furthermore, revealed that the corresponding lactam moieties in the racemic product 261 are intramolecularly hydrogen-bonded (dashed). 

The following tables contain a representative survey of inter- and intramolecular Paterno-Biichi reactions with the emphasis on cycloadditions of aldehydes (Tables 9 and 14) and ketones (Tables 10 and 14). In addition, examples of cycloadditions of 1,2-diketones, a-oxocarboxylic esters, simple esters, and imides are presented in Tables 11 -13. The formation of side products and further transformations to consecutive products are denoted when possible. 

The C 6-7-S-6 ring system of frondosins, a group of marine sponge-derived monoterpenoids, has been constructed by a photochemical process including the intramolecular Paterno-Biichi reaction of (24) as a key step. ... 

Irradiation of atropisomeric a-oxoamides (25) leads to intramolecular Paterno-Biichi reaction, affording bicyclic oxetanes with very high enantio- and diastereoselectivity. Remarkable differences in product selectivity are observed in solution and in the crystalline state. ... 

The photocycloaddition of triplet benzophenone to norbornene was originally reported by Scharf and Korte. The photoproduct 101 that is formed in high exo-selectivity could be thermally cleaved to the 5,e-unsaturated ketone 102, an appHcation of the carbonyl-olefin metathesis (COM) concept. The 1,4-biradical formed in the interaction of norbornene with o-dibenzoyl-benzene was trapped in an intramolecular fashion by the second carbonyl moiety. A highly regioselective reaction of triplet benzophenone was reported with 5-methylenenorborn-2-ene, with preferential attack toward the exo CC double bond. A number of publications have discussed the photocycloaddition reactions of triplet carbonyl compounds to norbornadiene and quadricyclane, as weU as the competition between the Paterno-Biichi reaction and the sensitized norbornadiene/quadricyclane interconversion. Oxetane formation has also been reported for the photoreaction of biacetyl and para-quinones with benzvalene. ... 

The low quantum yield indicates that intermediate complexes and diradicals decay unproduc-tively. In general, Norrish type II photoreactions and other hydrogen abstraction processes must be overcome in order to achieve successful cycloaddition. Only in a few reported cases is intramolecular hydrogen abstraction a serious competitive reaction path. For example, the cycloheptenyl-substituted ketone 5 yields an oxetane 6 in addition to a cyclobutanol derivative 758, whereas the unsaturated cycloheptanone 8 only gives oxetane products 9 and 10 on irradiation59. The main product 10 was converted in two steps to azulene in 25 % overall yield. The reaction sequence 11 - 12 - 13 also demonstrates the high synthetic potential of the intramolecular Paterno-Biichi reaction61. 

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