Cycloadditions photochemical, intramolecular

As discussed, 3-oxaquadricyclanes are generally obtained from 7-oxanorbornadienes by a photochemical intramolecular [2+2] cycloaddition. When the irradiation is carried out at ambient temperature, the 3-oxaquadricyclanes cannot be isolated but react further to oxepins... 

The effect of substitution of the ethane linkage on the isomerization quantum yields is less dramatic, though the course of the photochemical reaction can be affected by the nature of the substituents (cf. also Section II.C). Both l,2-di-9-anthrylethanol (0F = 0.04) and its acetate (F = 0.03) undergo photochemical intramolecular An+An cycloaddition with a quantum yield of 0.35 [51], However, irradiation of meso-l,2-di-9-... 

The photochemical intramolecular 2 + 2-cycloaddition of diphenylbicyclo[3.2.0]oct-3-ene-2,5-diones (15, 16) produced pentacyclotetradeca-10,12-diene-2,7-diones (17,... 

Nishio et al. performed similar photochemical intramolecular [2+2] cycloadditions of 2-allylindan-l-thione derivatives <1996LA117>. The reaction provides access to tetracyclic thietanes in mostly satisfactory yields (Table 3). 

Although quinazolin-4(3//)-one itself does not undergo photochemical intramolecular [2 + 2] cycloaddition to alkenes, its 2-(trifluoromethyl) analog does perform such a reaction. Thus, irradiation of 2-(trifluoromethyl)quinazolin-4(3i/)-one in methanol under bubbling ethene gives the [2 + 2] cycloadduct, 2a-(trifluoromethyl)-2,2a-dihydro-l/y-azeto[l,2-a]quinazolin-4(377)-one as the sole product. In several cases, the initially formed adduct of 2-(trifluo-romethyl)quinazolin-4(3/7)-one with alkenes is further transformed under the irradiation conditions." ... 

As pointed out in the introduction, if the two chromophores of the system are suitably chosen, an intramolecular cycloaddition process can occur. The photochemical intramolecular cycloaddition... 

A photochemical intramolecular alkene-oxirane cycloaddition can occur when the C—C bond of the oxirane and the alkene n-system are properly aligned, as in compound (193) (Scheme 43) <90T3409>. 

Various cage compounds have been obtained by photochemical intramolecular (2 + 2)-cycloadditions of heterocyclic compounds. The 2,7-dioxatricyclo[6.2.0.03,6]deca-4,9-diene derivative 208 (R = Me) underwent intramolecular (2 + 2)-cycloaddition upon irradiation.244 The di-237 Woodward and Hoffman,22 p. 1)1. 

Of special interest are the photochemical intramolecular (2 + 2)-cycloadditions of 2-pyrones and 2-pyridones because the resulting 2-oxa- or 2-azabicyclo[2.2.0]hex-5-en-3-ones are precursors of cyclobutadiene and stable complexes of cyclobutadiene.12,238-261 In 1964 Corey and Streith238 reported the photochemical conversion of 2-pyrone (222) and N-methyl-2-pyridone (223) into the corresponding bicyclic isomers (224 and 225) at —20°. More recent studies performed at a temperature as low as 8°K with 2-pyrones in a matrix of argon or nitrogen showed that the intramolecular (2 + 2)-cycloaddition is only a minor photochemical pathway.12,260-262 The major reaction proved to be... 

Photochemical intramolecular (2 + 2)-cycloaddition within the framework of a heterocyclic seven-membered ring has further been observed in l//-l,2-diazepines, 1,3-oxazepines, and 1,2-phosphazepines. 

An intramolecular photochemical cycloaddition suggested for inclusion in the undergraduate laboratory curriculum couples a ground-state Diels-Alder reaction with the sunlight-induced cage formation of (13a) from the crystals of the thermal adduct (14a),7 as depicted in Scheme 1. Marchand and Allen8 have reported an improved synthesis of the pentacycloundecane (15) using the photochemical intramolecular cycloaddition of the dienedione (14b), which was achieved in 86% yield by acetone-sensitized irradiation. 

Intramolecular cyclization of (76) in the presence of NaH gave exo (77) and endo (78) products (Equation (9)) <90SL584). Allene-based electrophile-mediated cyclization afforded iV-tosyl iodohexa-hydroazocines <94JCS(Pl)3549>. Photochemical intramolecular cycloaddition of phthalimide in acetone afforded the ring-opened product <82TL498l>. An unstable enaminone azocine derivative (81) was formed after the desulfurization of the bicyclic aminal (80), which in turn was synthesized from... 

A new photochemical intramolecular para 4 + 2-cycloaddition of arenes with allenes has been described. The bicyclo[2.2.2]octadiene cycloadducts are produced in low... 

Thermal or photochemical intramolecular cycloaddition reactions of diaUenes afford cyclobutenes. Heating of vinylallene at 170 °C affords 10 % of methylenecyclobutene and mixtures of C10H12 dimers. The thermal reaction of bisaUenyl 6 (1,2,4,5-hexatetraene) at 200 °C/35-65 torr affords a quantitative yield of 3,4-dimethylenecyclobutene 7 . 

This chapter deals with the main photochemical reactions of aromatic compounds, including photoisomerization, photoaddition and cycloaddition, photosubstitution, intramolecular photocyclization, intra- and inter-molecular photodimerization, photorearrangement and related photoreactions. 

SCHEME 17 Photochemical intramolecular cycloaddition of 2,4-cyclohexadienone Diels-Alder dimers. 

Fig. 25 Photochemical intramolecular [2+2] cycloaddition of cinnamate units of a pseudogeminal substituted [2.2]paracyclophane [126]...

As final examples, the intramolecular cyclopropane formation from cycloolefins with diazo groups (S.D. Burke, 1979), intramolecular cyclobutane formation by photochemical cycloaddition (p. 78, 297f., section 4.9), and intramolecular Diels-Alder reactions (p. 153f, 335ff.) are mentioned. The application of these three cycloaddition reactions has led to an enormous variety of exotic polycycles (E.J. Corey, 1967A). 

The reaction is illustrated by the intramolecular cycloaddition of the nitrilimine (374) with the alkenic double bond separated from the dipole by three methylene units. The nitrilimine (374) was generated photochemically from the corresponding tetrazole (373) and the pyrrolidino[l,2-6]pyrazoline (375) was obtained in high yield 82JOC4256). Applications of a variety of these reactions will be found in Chapter 4.36. Other aspects of intramolecular 1,3-dipolar cycloadditions leading to complex, fused systems, especially when the 1,3-dipole and the dipolarophile are substituted into a benzene ring in the ortho positions, have been described (76AG(E)123). 

UV irradiation. Indeed, thermal reaction of 1-phenyl-3,4-dimethylphosphole with (C5HloNH)Mo(CO)4 leads to 155 (M = Mo) and not to 154 (M = Mo, R = Ph). Complex 155 (M = Mo) converts into 154 (M = Mo, R = Ph) under UV irradiation. This route was confirmed by a photochemical reaction between 3,4-dimethyl-l-phenylphosphole and Mo(CO)6 when both 146 (M = Mo, R = Ph, R = R = H, R = R" = Me) and 155 (M = Mo) resulted (89IC4536). In excess phosphole, the product was 156. A similar chromium complex is known [82JCS(CC)667]. Complex 146 (M = Mo, R = Ph, r2 = R = H, R = R = Me) enters [4 -H 2] Diels-Alder cycloaddition with diphenylvinylphosphine to give 157. However, from the viewpoint of Woodward-Hoffmann rules and on the basis of the study of UV irradiation of 1,2,5-trimethylphosphole, it is highly probable that [2 - - 2] dimers are the initial products of dimerization, and [4 - - 2] dimers are the final results of thermally allowed intramolecular rearrangement of [2 - - 2] dimers. This hypothesis was confirmed by the data obtained from the reaction of 1-phenylphosphole with molybdenum hexacarbonyl under UV irradiation the head-to-tail structure of the complex 158. 

Recent synthetic applications of the photochemical [2 + 2] cycloaddition of unsaturated sulfones have been noted. Musser and Fuchs84 have effected an intramolecular [2 + 2] addition of a 6-membered ring vinyl sulfone and a five-membered ring vinylogous ester in excellent yield, as part of a synthetic approach to the synthesis of the mould metabolite, cytochalasin C. The stereospecificity of the addition was only moderate, however, and later problems with this synthetic approach led to its abandonment. Williams and coworkers85 have used the facile [2 + 2] photoaddition of 73 and... 

Bicyclic [6.4.0]dodecane systems have been prepared [17] by catalyzed and photochemical intermolecular cycloaddition of the cyclooct-2-en-l-ones 10 and 1,3-butadiene (1) and by catalyzed intramolecular cycloaddition of trienone 11 (Scheme 3.4). 

Photochemical 2 + 2 cycloadditions can also take place intramolecularly if a molecule has two double bonds that are properly oriented. " The cyclization of the quinone dimer shown above is one example. Other examples are... 

Numerous examples of intermolecular and intramolecular photocycloaddition to heterocyclic systems (including the dimerization of individual heterocycles) have now been reported. Two types of cycloaddition can readily be effected photochemically, namely, [n2 + 2] and [ 4 + 4] additions. Although concerted suprafacial additions of this type are allowed photochemical processes, in reality many cycloadditions occur via diradicals, zwitterions or exciplexes. 

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