Cycloaddition, photo

Cycloadditions - Photo-Diels-Alder Reactions Using Oxygen... 

An extensive series of low-temperature matrix isolation experiments has failed to provide any evidence of oxirene formation, either by diazo ketone photolysis (82CB2192) or by attempted photo-retro-cycloaddition (82CB2202). 

Thermally unfavorable [2-1-2] cycloadditions often can be promoted photo chemically [136,137,138], some examples for fluorinated derivatives are given in equations 60-62... 

In contrast with the thermal [4 + 2] Diels-Alder reaction, the 2 + 2 cycloaddition of two alkenes to yield a cvclobutane can only be observed photo-chemically. The explanation follows from orbital-symmetry arguments. Looking at the ground-state HOMO of one alkene and the LUMO of the second alkene, it s apparent that a thermal 2 + 2 cycloaddition must take place by an antarafacial pathway (Figure 30.10a). Geometric constraints make the antarafacial transition state difficult, however, and so concerted thermal [2 + 2j cycloadditionsare not observed. 

Table 28 Photo-driven tungsten carbonyl- assisted 1,3-dipolar cycloadditions...

The single-electron transfer from one excited component to the other component acceptor, as the critical step prior to cycloaddition of photo-induced Diels Alder reactions, has been demonstrated [43] for the reaction of anthracene with maleic anhydride and various maleimides carried out in chloroform under irradiation by a medium-pressure mercury lamp (500 W). The (singlet) excited anthracene ( AN ), generated by the actinic light, is quenched by dienophile... 

Acetylchloride is a trapping agent that allows the reaction to go completion, transforming the product into a less oxidizable compound.The results of other reactions between indole (57) and substituted cyclohexa-1,3-dienes show that the photo-induced Diels-Alder reaction is almost completely regioselective. In the absence of 59 the cycloaddition did not occur the presence of [2+2] adducts was never detected. Experimental data support the mechanism illustrated in Scheme 4.14. The intermediate 57a, originated from bond formation between the indole cation radical and 58, undergoes a back-electron transfer to form the adduct 60 trapped by acetyl chloride. 

The cycloaddition of photoenol of o-methylbenzaldehyde 66 with 5-alkyli-dene-l,3-dioxane-4,6-dione derivatives 67 is an example of a photo-induced Diels Alder reaction in which one component, the diene in this case, is generated by irradiation [48]. The yields of some cycloadducts 68, generated by photo-irradiation of a benzene solution of 66 and 67 at room temperature, are reported in Table 4.14. The first step of the reaction is the formation of (E)-enol 69 and (Z)-enol 70 (Equation 4.7) by an intramolecular hydrogen abstraction of 66 followed by a stereo- and regioselective cycloaddition with... 

N3-coordination by monomeric and dimeric Zn2+-cyclen derivatives has also been shown to both inhibit the photo [2+2] cycloaddition of thymidilyl(3 -5 )thymidine, d(TpT), and to accelerate the photosplitting process of the [T-cis,syn-T] dimer (78% after 1 h of UV exposure) (38). 

More recently in 2001, Winkler and Kwak reported methodology designed to access the pyrrolidine core of the hetisine alkaloids via a photochemical [2+2], retro-Mannich, Mannich sequence (Scheme 1.3) [26]. In a representative example of the methodology, vinylogous amide 42 was photo-irradiated to give the [2+2] cycloaddition product 43. Heating cyclobutane 43 in ethanol provided enamine 44 via a retro-Mannich reaction. Exposure of enamine 44 to acidic conditions then effected a Mannich reaction, resulting in pyrrolidine 45. 

Several novel porphyrin derivatives can then be obtained by following the mentioned cycloaddition methodologies many of the new products fulfil the requirements to be considered as potential new drugs mainly for the detection and treatment (PDT) of cancer situations and in the photo-inactivation of microorganisms. 

Asymmetric synthesis of the rocaglamides was accomplished by employing [3+2] photo-cycloaddition mediated by functionalized TADDOL based chiral Brpnsted acids. The synthesis consisted of a [3+2] dipolar cycloaddition, a base-mediated a-ketol rearrangement and a hydroxyl-directed reaction <06JA7754>. Asymmetric synthesis of 1,2-dihydrobenzo /j]furans was achieved by adamantylglycine derived dirhodium tetracarboxylate catalyzed C-H insertion <06OL3437>. 

This chapter deals with [2 + 2]cycloadditions of various chromophors to an olefinic double bond with formation of a four-membered ring, with reactions proceeding as well in an intermolecular as in an intramolecular pattern. Due to the variety of the starting materials available (ketones, enones, olefins, imines, thioketones, etc.. . .), due to the diversity of products obtained, and last but not least, due to the fact that cyclobutanes and oxetanes are not accessible by such a simple one-step transformation in a non-photo-chemical reaction, the [2+2]photocycloaddition has become equivalent to the (thermal) Diels-Alder reaction in importance as for ring construction in organic synthesis. 

The [6 + 3]-cycloaddition between (cycloheptatriene)chromium(O) tricarbonyl 135 and isocyanate or ketene occurs under photo-irradiation conditions, for example, bicyclo[4.2.1]nonane-type adduct 136 was obtained in moderate yield (Equation (22)).237 238... 

The ketone (265) was prepared via photo[2 + 2]cycloaddition between the corresponding olefin and allene 91). 

The cyclobutene (268) was constructed through the photo[2 + 2]cycloaddition of acetylene to anhydromevalonolactone, which was followed by DIBAL reduction 92). 

Some of the earliest applications of allenes in natural product synthesis involved thermal- and photo-induced cycloaddition reactions. As a consequence of this and the fact that these types of reactions work so well, the use of allenes as cycloeducts in these types of reactions has been widespread. A variety of examples are showcased in the sections that follow. 

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