Cyclization, radicals photochemical

New cyclizations via photochemically generated aminyl radicals have been reported, including further examples of the Hofmann-Loeffler-Freytag reaction.313 Intramolecular addition of an aminyl radical, generated by photochemically induced nitrogen chlorine bond homoysis, is also accompanied by cyclization as illustrated by the conversion of the unsaturated N-chloroamide 378 to the pyrrolidine 379.314 Piperidine formation can also... 

The distyrylbenzene derivative (141) is photochemically reactive on irradiation in solution. The solvent of choice is acetonitrile/benzene/water (7 2 1) saturated with ammonia. The reactions encountered with this system are derived from electron transfer initiated by p-dicyanobenzene as the electron accepting sensitizer. This process yields the radical cation (142) of the starting material and also the cyclized radical cation (143). These species are trapped by ammonia to yield the final products (144) and (145) in the yields shown. The naphthyl system [141, R-R = (CH=CH)2] is also reactive and affords the analogous products (146) and (147). A study has examined the photochemically-induced cyclization of tetraenes such as (148) under SET conditions in aqueous acetonitrile solution. A variety of electron accepting sensitizers was used. In the example cited the sensitizer (149) was effective and the cascade cyclization yielded the product (150). 

Although the synthesis of phenanthridine from iminyl radicals, photochemical processes, and microwave-mediated cyclizations has been developed, further exploration of convenient, efficient, and milder protocols are still desirable due to the broad applications of phenanthridines. Jiao group successfuUy realized an intermolecular nitrogenation reaction toward the synthesis of phenanthridines 64 (Scheme 7.43) [113]. This transformation can proceeds under metal-free... 

The photochemical reaction between the diazonium salt 1 and [Ru(bpy)3] + produces good yields of the Pschorr-type product 2, and the process is catalytic here, the reaction hrst gives a phenyl radical. This intermediate cycHzes, and then back electron transfer from the cyclized radical to Ru(lll) regenerates the catalyst complex (Scheme 4). In contrast, direct photocyclization of 1 in acetonitrile gives poor yields of the phenanthrene. The intermediate in this process now becomes the phenyl cation, and the... 

Cyclic ethers were also obtained by cyclization of alkoxyl radicals, generated in a radical chain reaction by reacting the thione 42 with (TMSfsSiH under photochemical conditions at 20 °C (Reaction 46). Regioselectivities of cyclization have been investigated and a progressive increase of the 6-endo-trig selectivity along the series R2 = H[Pg.140]

Radical cyclization is compatible with the presence of other functional groups. Treatment of XCH2CON(R)-C(R )=CH2 derivatives (X = Cl, Br, 1) with Ph3SnH and AIBN led to formation of a lactam via radical cyclization. " Cyclization of N-iodoethyl-5-vinyl-2-pyrrolidinone led to the corresponding bicyclic lactam, " and there are other examples of radical cyclization with molecules containing a lactam unit " or an amide unit. Radical cyclization occurs with enamines as well. Photochemical irradiation of A,A-dialIyl acrylamide leads to formation of a lactam ring, and in this case thiophenol was added to generate the phenylthio derivative. Phenylseleno N-allylamines lead to cyclic amines. co-Iodo acrylate esters cyclize to form lactones. " ... 

The attacking radical need not always be at carbon. Amidyl radical are known and give cyclization reactions. Aminyl radical cyclizations have been reported. Oxygen radical can be generated under photochemical conditions, and they add to alkenes in a normal manner. ... 

In cases where hydrogen atom transfer gives primarily reduced products, Bu3Sn-SnBu3 under photochemical generates the radical that can cyclize, but a halogen atom transfer agent such as iodoethane is usually present (see 15-44). 

PET reactions [2] can be considered as versatile methods for generating radical cations from electron-rich olefins and aromatic compounds [3], which then can undergo an intramolecular cationic cyclization. Niwa and coworkers [4] reported on a photochemical reaction of l,l-diphenyl-l, -alkadienes in the presence of phenanthrene (Phen) and 1,4-dicyanobenzene (DCNB) as sensitizer and electron acceptor to construct 5/6/6- and 6/6/6-fused ring systems with high stereoselectivity. 

An improved synthesis of 3,4-dihydro-2,l-benzothiazine 2,2-dioxide was reported by Togo and co-workers using photochemical conditions . Treatment of A-alkyl 2-(aryl)ethanesulfonamides 18 with (diacetoxyiodo)arenes under irradiation with a tungsten lamp at 20-30 °C afforded 2,1-benzothiazines 19 and 20. Chemical yields and selectivities were dependent upon the choice of solvents and the reactant s substituents 18 (Table 1). When THF and EtOH were used as solvents, the reactions failed to give the cyclized products, since their a-hydrogen was abstracted by the intermediate sulfonamidyl radical. Compound 20 was obtained as a major product when 1,2-dichloroethane was employed as a solvent. In contrast, in the case of EtOAc as solvent, compound 19 was obtained as the major product. 

Fig. 18 Crossings of in-plane and out-of-plane frontier MOs in radical-anionic Bergman and C1C5 cyclizations (crossings for photochemical, dianionic and radical-cationic cyclizations involve the same MOs but differ in the number of electrons).

In a combination of photochemical cyclization and a radical reaction Yoshimatsu et al synthesized 2-azabicyclo[33.0locta-3,7-diene 169 from the trienal hydrazone 166.1891 The domino process was initiated by irradiation of 166 at 400-500 nm in benzene. The transformation may include an intermolecular [2+2]-cyclization, followed by ring opening to give... 

Birch reduction-alkylation of 5 with 2-bromoethyl acetate was carried out with complete facial selectivity to give 57. This tetrafunctional intermediate was converted to the bicyclic iodolactone 58 ( > 99% ee) from which the radical cyclization substrate 59 was prepared. The key radical cyclization occurred with complete regio- and facial-selectivity and subsequent stereoselective reduction of the resulting tertiary radical gave 60 with the required trans BC ring fusion.The allylic alcohol rmit of (+)-lycorine was obtained by a photochemical radical decarboxylation, 62 63. 

The cation-radical version of diene synthesis, in which the diene is in a strongly electron-deficient state, is characterized by an unusual high endoselectivity. In this case, endoselectivity is sig-nihcantly higher than that of thermal or photochemical initiation of a neutral molecule (cf. Mlcoch and Steckhan 1987). As follows from the charge diagram depicted in Scheme 7.21, when a cation-radical and neutral molecule approach each other, not only the C(l)-C(6) and C(4)-C(5) interactions are bonding (indeed, these interactions result in cyclization), but C(2)-C(7) and C(3)-C(8) interactions are also bonding. As a result, the endo product is formed. 

In qualitative terms, the rearrangement reaction is considerably more efficient for the oxime acetate 107b than for the oxime ether 107a. As a result, the photochemical reactivity of the oxime acetates 109 and 110 was probed. Irradiation of 109 for 3 hr, under the same conditions used for 107, affords the cyclopropane 111 (25%) as a 1 2 mixture of Z.E isomers. Likewise, DCA-sensitized irradiation of 110 for 1 hr yields the cyclopropane derivative 112 (16%) and the dihydroisoxazole 113 (18%). It is unclear at this point how 113 arises in the SET-sensitized reaction of 110. However, this cyclization process is similar to that observed in our studies of the DCA-sensitized reaction of the 7,8-unsaturated oximes 114, which affords the 5,6-dihydro-4//-l,2-oxazines 115 [68]. A possible mechanism to justify the formation of 113 could involve intramolecular electrophilic addition to the alkene unit in 116 of the oxygen from the oxime localized radical-cation, followed by transfer of an acyl cation to any of the radical-anions present in the reaction medium. 

Enamine carbaldehyde 76 (Scheme 20) by sequential photochemical cycloaddition and iminium ion-propargylsilane cyclimtion furnishes allenes 77a-c in good yield and with high diastereoselectivity (92X2081). (Gas chromatographic fR values and thin-layer chromatographic Rp values have been reported.) Radical cyclization of ca-iodoalkyl isoquinolone 78a under... 

Intramolecular cyclization of diphenylamines to carbazoles is one of the most versatile and practical methods. This has been achieved photochemically, thermally in the presence of elemental iodine at 350°C, or with platinum at 450-540°C, via free radicals with benzoyl peroxide in chloroform, or by using activated metals such as Raney nickel or palladium on charcoal. Most of these methods suffer from low to moderate yields, and, in some cases, harsh reaction conditions (8,480). 

Tin glycolates or o-diphenolates undergo photochemical or thermal radical cyclization (Equation (31)) to 1,3,2-dioxastannolanes or benzo-l,3,2-dioxastannolenes in high yields <86JOM(303)87>. 

Radical [6 + 0] cyclization is of some importance for the preparation of polycyclic azines, but is of no significant synthetic utility for the preparation of mono- or bi-cydic compounds. Photochemical oxidative cydizations of aromatic Schiff bases (equation 49) and azo compounds (equation 50) constitute important procedures for the preparation of phenan-thridines and benzodnnolines. These reactions proceed by initial formation of the dihydro compounds and aromatization is effected with either oxygen or, preferably, iodine (present in the reaction mixture). 

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