Cycloaddition intramolecular, photochemical

Heathcock used as concerted cycloaddition intramolecular photochemical (2 + 2) addition to create the quaternary center in his projected synthesis. A cationic rearrangement Wagner-Meerwein rearrangement) was intended as further key step but failed (162). 

This compound can be prepared by photolysis of 105, another example of an intramolecular photochemical 2 + 2 cycloaddition Lemal Lokcnsgard J. Am. Chem. Soc. 1966,88, 5934 Schafer Criegee Askani Griiner Angew. Chem. Ml. Ed. Engl. 1967, 6, 78 [Angew. Chem. 79, 54]. 

Dilling, W. L. Intramolecular photochemical cycloaddition reactions of nonconjugated olefins. Chem. Rev. 66, 373 (1966). 

The intramolecular photochemical [2+2] cycloaddition of bicyclo[3.3.0]octa. dienes has been examined by several groups.128 197, 99) A striking example of a synthetic application of this chemistry is given in Scheme 22. 

During the synthesis of the Woodward reserpine precursor (17 Scheme 4), Peailman used the protected acylacetal (13) to control the stereochemistry of an intramolecular photochemical cycloaddition to (14). The strategy for opening the cyclobutane ring employed the Baeyer-Villiger reaction to convert the 7-keto ester (15) to a 3-hydroxy ester (16), which underwent retroaldolization to (17). 

Irradiation of the diane (127) yields the cyolobutane derivative (128) as the sole product. The structure of this was confirmed by X-ray analysis. Prinzbach at aJ. have reported the intramolecular photochemical (2-I-2)-addition of the dienes (129) to afford the adducts (130). Cycloaddition is also reported to take place on irradiation of the benzene derivatives (131) where the adducts were isolated as Diels-Alder adducts of the dienes (132). Acetone-sensitized irradiation of the diene (133) results in the rapid conversion... 

Ortho addition to the arene ring also occurs in cage systems where geometrical constraints disallow other orientations of addition a common example is the intramolecular photochemical cycloaddition which can occur in... 

In the laboratory of H. Hiemstra, the synthesis of the bicyclo[2.1.1]hexane substructure of solanoeclepin A was undertaken utilizing the intramolecular photochemical dioxenone-alkene [2+2] cycloaddition reaction. The dioxenone precursor was prepared from the commercially available fert-butyl acetoacetate using the acetoacetic ester synthesis. When this dioxenone precursor was subjected to irradiation at 300 nm, complete conversion of the starting material was observed after about 4h, and the expected cycloadduct was formed in acceptable yield. 

The unsaturated 6-lactone (84) undergoes intramolecular photochemical (2 + 2)-cycloaddition to give (85)." The scope of the process has been evaluated and the 5-lactones (86-88) all behave similarly, affording the products shown in Scheme 2. The lactones (86) and (87) both cyclise in the two possible modes. Intramolecular cycloaddition reactions within polymethyldia-minebis(4-methyl-7-coumarinyl)oxyacetamides have been described. Zhu and Wu have reported that a biscoumarin system linked by a phenanthroline unit does not undergo photodimerisation. Instead, intramolecular addition of a coumarin unit to the phenanthroline occurs. 

Cyclopropanes exhibit similar modes of reactivity. [2Dipolar additions with electron-deficient alkenes and electron-donor-substituted cyclopropanes, additions of electron-rich alkenes to electron-deficient cyclopropanes, a number of radicaloid reactions and intramolecular photochemical cycloadditions are known, which may be described by the general scheme H-2 3. 

The impact of (2 + 2)-cycloaddition and (2 + 2)-cycloreversion reactions of heterocyclic compounds on organic chemistry over the last 10 years is clearly illustrated by several examples. Various members of the important /Hactam antibiotics, penicillin and cephalosporin C, as well as structurally related heterobicyclic compounds have been obtained by (2+ 2)-cycloaddition of heterocycles with ketenes (Section II,D,l).n Intramolecular photochemical (2 + 2)-cycloadditions of 2-pyrones yield 2-oxabicyclo 2.2.01hex-5-en-3-ones, which upon further irradiation afford cyclobutadienes (Section III,D,2).12 Intermolecular (2 + 2)-cyclo-additions of vinylene carbonates with olefins and with acetylenes offer a simple route to cyclobutanes and cyclobutenes, respectively (Sections III,B,3 and 5).13 (2 + 2)-Cycloaddition and (2 + 2)-cycloreversion reactions have contributed substantially to the development of the chemistry... 

Bis(methoxycarbonyl)-7-oxa- and 7-azabicyclo[2.2.1]hepta-2,5-diene (201 R = COOMe) isomerized upon direct or sensitized irradiation to give the corresponding 7-oxa- or 7-azaquadricyclenes (202 R = COOMe) in yields of 60-90%.14 22 -234 These intramolecular photochemical (2 + 2)-cycloadditions have up to now been regarded as some of the few examples of concerted (2 + 2)-cycloadditions.235 However, recently Kaupp has provided evidence for the occurrence of a two-step process.236 The 7-oxa- or 7-azaquadricyclenes (202) were thermally far less stable than the 1-azaprismane (200) and the quadricyclene. They isomerized at 100° (X = O, R = COOMe)22 231 or even at room... 

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. 

Oldroyd DL, Weedon AC (1992) Intramolecular photochemical cycloadditions of N-alke-nyloxycarbonylindoles and N-alkenoylindoles. J Chem Soc Chem Commun 1491-1492... 

The 1980s witnessed a revival of interest in synthetic photochemistry, largely based on the successful application of intramolecular arene-olefin and enone-olefin photochemical cycloaddition reactions to the synthesis of polycyclic natural and unnatural products. The potential application of intramolecular arene-amine addition to the synthesis of alkaloids was suggested in 1973 by Btyce-Smith et al.," based on their observation of intramolecular photochemical 1,4-addition reaction of the (N,A/-dimethylaminoalkyl)benzenes I and 2 to yield 3 and 4, respectively. Intramolecular addition reactions have subsequently been observed for a large number of (aminoal-... 

This chapter s final example of caged hydrocarbon synthesis is one that further eii5)hasizes the importance of cycloaddition reactions in creating substrates for the quasi-Favorskii rearrangement. This synthesis also showed, as many polycyclic hydrocarbon syntheses have, the limits that exist in intramolecular photochemical [2+2]-cycloaddition processes. 

Wiesner carried out pioneering studies on the photocycloaddtion of vinylogous imides in the 1960s, and he first applied the intramolecular photochemical [2 + 2] enone-olefin cycloaddition reaction in natural product synthesis. In his landmark synthesis of 12-e/>i-lycopodine, compound 186 is irradiated to give a 70% yield of photoadduct 187. Protection of the ketone, epoxidation of the exocyclic double bond, and reduction of the epoxide give ketal alcohol 188. Deprotection of the ketal and spontaneous refro-aldol fragmentation lead to the diketone 190. Of note is that the bond cleaved in this retro-a do pathway (i.e., 189 to 190) is different from that of typical de Mayo reactions. Diketone 190 transforms to the tetracyclic alcohol 191 via aldol reaction, and this alcohol is converted to 12-e/7/-lycopodine in four steps. 

It is also worthwhile comparing the intramolecular photochemical cycloaddition reactions of ethylenic aldehydes and ketones with free radical intramolecular additions. For instance, irradiation of 5-hexen-2-one (470) (Scheme 161) in the gas phase gives the oxetane 471 as only cyclized product, as expected from the known photochemical intermolecular reaction between olefins and ketones. If the irradiation is conducted in solution 470 gives 471 (26%) and 472 (18%). With other y,< -unsaturated ketones, the bicyclic compound analogous to 472 may become the major product. With 2-allylcyclanones such as 473 (Scheme 161) bicyclic compounds are obtained (80% yield) as a mixture of 474 and 475, with 475 being the major product, but such compounds are difficult to isolate. " In the same manner, selective irradiation of the carbonyl group of 2-acyl-2,3-dihydro-4/f-pyrans (476) leads exclusively (23% yield) to exo-brevicomin (477) (a sex attractant), neither oxetane formation nor Norrish type II reaction being observed. The formation of the compounds 472, 475, and 477 which was considered as unexpected... 

In cases where the diene is unstable, a dihalo starting material may be used with the iron carbonyl acting as a reducing agent. Both the trimethylene methane complex 10.5 (Scheme 10.3) and the cyclobutadiene complex 10.7 (Scheme 10.4) have been made in this way. The cyclobutadiene-iron complex 10.7 is a convenient storable form of this highly unstable diene. It can be liberated by oxidation and, if this is done in the presence of a dienophile, the Diels-Alder product is obtained. The Diels-Alder reaction with 2,5-dibromobenzoquinone gave the expected e do-product 10.8. An intramolecular photochemical [2-1-2] cycloaddition, followed by a Favorskii reaction, gave a cubane dicarboxylic acid 10.10. ... 

Intramolecular photochemical [2+2] ene-allene cycloaddition of 10 or homo-logue 11 generates the tricyclic methylene cyclobutanes 12 and 13 in 60-99%... 

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