Irradiation cyclization

Lactams such as (258) can be synthesized from the phthalimides (259) by irradiation. Again the reactions are controlled by single electron transfer processes that are usually encountered in the photochemical reactions of phthalimides. The outcome of the reaction is a conventional proton transfer from the benzylic site within the zwitterionic biradical formed on irradiation. Cyclization within the resultant 1,5-biradical affords the final product. Griesbeck and his coworkers have studied the photochemical reactivity of the phthalimide derivatives (260). These compounds on irradiation under triplet sensitized conditions undergo decarboxylation and cyclization. The reaction involves SET and the key intermediates are shown as (261) and (262). The biradical anion (262) is the species that either cyclizes to afford (263) or abstracts hydrogen to yield (264). The reaction is controlled by a variety of factors that have been reported in some detail. Some photochemical reactions of phthaloylcysteine derivatives have been described. Typical of the processes are the decarboxylations of the derivative... 

On irradiation, 2-(cycloheptylidene)cycloheptanone isomerizes to 2-(cyclohept-l-enyl)cycloheptanone, which on further irradiation cyclizes to a mixture of all four possible isomers of the oxetan (204). An analogous mixture of oxetans was obtained by irradiation of 2-(cyclo-oct-l-enyl)cyclo-octanone. ... 

Barrelene was obtained via a double Diels-Alder reaction from a-pyrone with methyl acrylate (H.E. Zimmerman, I969A). The primarily forming bicyclic lactone decarboxylates in the heat, and the resulting cyclohexadiene rapidly undergoes another Diels-Alder cyclization. Standard reactions have then been used to eliminate the methoxycarbonyl groups and to introduce C—C double bonds. Irradiation of barrelene produces semibullvalene and cyclooctatetraene (H.E. Zimmerman. 1969B). 

Pyridazin-3(2H)-ones rearrange to l-amino-3-pyrrolin-2-ones (29) and (30) upon irradiation in neutral methanol (Scheme 10), while photolysis of 5-amino-4-chloro-2-phenylpyridazin-3(2H)-one gives the intermediate (31) which cyclizes readily to the bis-pyridazinopyrazine derivative (32 Scheme 11). 

A -Bridgehead compounds have been obtained from the photochemical cyclization of cfi-1-styrylimidazoles. For example, irradiation of the imidazole (532) in methanol in the presence of I2 resulted in cyclization at the 2-position of the imidazole ring with the formation of an imidazo[2,l-a]isoquinoline (533) (76JCS(Pl)75). Isomerization of the trans-to the c/i-styrylimidazole was followed by photodehydrocyclization, trans- 1-Styrylbenzimidazole (534) was isomerized under Pyrex-filtered light in the presence of one molar equivalent of I2. The resulting cis isomer on irradiation through quartz gave the benzimidazo[2,l-a]isoquinoline (535) in 53% yield. 

Certain substituted o-nitrotoluenes can be induced to cyclize, forming 2,1-benzisoxazoles. Bis(2-nitrophenyl)methane when irradiated gave 3-(o-nitrophenyl)-2,l-benzisoxazole. The possible intermediates including a biradical were discussed (74TL4359). 3-(o-Nitrophenyl)-2,1-benzisoxazole was prepared by the acid cyclization of bis(2-nitrophenyl)methanol (Scheme 178) (65RRC1035>. 

On direct irradiation of 1,3-pentadiene, Z-E isomerization is accompanied by cyclization to 1,3-dimethylcyclopropene and 3-methylcyclobutene ... 

A-Nor-B-homo steroids with different substitution patterns than those described above may be prepared by acid catalyzed cyclization of 3)3-hydroxy-5(10)-seco-cholest-tra/w-l(l0)-en-5-one acetate (134a) formed in 30-40% yield by mercuric oxide-iodine sensitized irradiation of cholestane-3j3,5a-diol... 

The Pictet-Spengler reaction has been carried out on various solid support materials " and with microwave irradiation activation.Diverse structural analogues of (-)-Saframycin A have been prepared by carrying out the Pictet-Spengler isoquinoline synthesis on substrates attached to a polystyrene support. Amine 20 was condensed with aldehyde 21 followed by cyclization to give predominantly the cis isomer tetrahydroisoquinoline 22 which was further elaborated to (-)-Saframycin A analogues. 

The 8-hydroxy-7-aminoquinoline-5-sulfonic acid was used to clarify the role of intramolecular hydrogen bonding on the cyclization rate of hydroxy Schiff bases in the preparation of 2-aryloxazolo[4,5-/i]quinoline-5-sulfonic acids 13. Irradiation... 

An intramolecular 2,2 -couphng in certain l,2-bis(indol-3-yl)ethenes has been reported to occur on irradiation, e.g., leading to the indolocarbazole 39 after photoinduced cyclization of the E) compound 40 (Scheme 7). The substrate 40... 

In a recent communication, the parent system 2 has been obtained in poor yields (16% and 10%, respectively) from the double cyclization of N-diphenyl-1,3-phenylenediamine either by using two equivalents of palladium acetate in refluxing acetic acid or by irradiation in methanol in the presence of a catalytic amount of iodine (00SC3651). All the available approaches sununarized so far were marred by harsh reactioi conditions or troublesome-to-prepare starting materials, leading to low overall yields of the desired products and difficulty in introducing sensitive substituents. 

The fragmentation/cyclization ratio is determined by the relative orientation of the respective molecular orbitals, and thus by the conformation of diradical species 2. The quantum yield with respect to formation of the above products is generally low the photochemically initiated 1,5-hydrogen shift from the y-carbon to the carbonyl oxygen is a reversible process, and may as well proceed back to the starting material. This has been shown to be the case with optically active ketones 7, containing a chiral y-carbon center an optically active ketone 7 racemizes upon irradiation to a mixture of 7 and 9 ... 

A similar result is obtained for the thermal electrocyclic ring-opening of 3,4-dimethylcyclobutene. The trans isomer yields only (2 ,4 )-2,4-hexadiene when heated, and the cis isomer yields only (2 ,4Z)-2,4-hexadiene. On UV irradiation, however, the results are opposite. Cyclization of the 2E,4 isomer under photochemical conditions yields cis product (Figure 30.4). 

Carbon-centered radicals generated by Barton s thiohydroxamate method can also participate in ring-forming reactions (see Scheme 26).52b,s3 For example, irradiation of 129 results in the formation of compound 130 (82% yield). The outcome of this transformation is reminiscent of Stork s elegant radical cyclization/trapping processes (see Schemes 7 and 8), in that/botn alkene carbon atoms have become functionalized. / I... 

On the basis of model studies, it appears that a preexisting ring(s) in the cyclization substrate (e. g. 39) is necessary for the success of this reaction type. Moreover, irradiation at elevated temperatures leads to cleaner reactions and higher yields, suggesting that thermal energy may be necessary to achieve the proper conformation for coupling to occur.22... 

Cyclization of bromostilbenes 6 to dibenz[/ ,/]oxepins 7 can be achieved by irradiation in excellent yield.102 103 Under solvolytic conditions (60% NaOH, EtOH), the yield is greatly reduced. When a silver salt (AgOAc) is used, however, complete conversion to the tricyclic system is accomplished.102 Tribenz[/>,catalyzed cyclization of biphenyl-2-yl 2 -chlorosulfonylphenyl ether in 44% yield.260... 

EPR spectroscopy is usually used to calibrate the clock (i.e., to determine kc). The method described here uses EPR to detect the two radicals. These are the parent (R1 ) and the product (R2 ) of its reaction, be it cyclization, decarbonylation, decarboxylation, rearrangement, or whatever. The radical R1 is produced photochemi-cally in the desired inert solvent by steady and usually quite intense light irradiation of the EPR cavity. Typically, R1 and R2 attain steady-state concentrations of 10-8 to 10 6 M. 

The use of diamine 27, bearing a fluorous-Boc protecting group, has been used with microwave irradiation in an Ugi/de-Boc/cyclization strategy for the synthesis of benzimidazoles 28 and quinoxalinones 29 [64]. Compared to the original procedures, which take 1-2 days, this approach avoids the use of... 

A one-pot synthesis of thiohydantoins has been developed using microwave heating [72]. A small subset of p-substituted benzaldehydes, prepared in situ from p-bromobenzaldehyde by microwave-assisted Suzuki or Negishi reactions, was reacted in one pot by reductive amination followed by cyclization with a thioisocyanate catalyzed by polystyrene-bound dimethyl-aminopyridine (PS-DMAP) or triethylamine, all carried out under microwave irradiation, to give the thiohydantoin products in up to 68% isolated yield (Scheme 16). 

Manganese(III)-promoted radical cyclization of arylthioformanilides and a-benzoylthio-formanilides is a recently described microwave-assisted example for the synthesis of 2-arylbenzothiazoles and 2-benzoylbenzothiazoles. In this study, manganese triacetate is introduced as a new reagent to replace potassium ferricyanide or bromide. The 2-substituted benzothiazoles are generated in 6 min at 110°C imder microwave irradiation (300 W) in a domestic oven with no real control of the temperature (reflux of acetic acid) (Scheme 15). Conventional heating (oil bath) of the reaction at 110 °C for 6 h gave similar yields [16]. 

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