Photochemical cyclisation

The photochemical cyclisation of p.y-unsaturated ketoximes to 2-isoxazolines, e.g., 16—>17, has been reported <95RTC514>. 2-Isoxazolines are obtained from alkenes and primary nitroalkanes in the presence of ammonium cerium nitrate and formic acid <95MI399>. Treatment of certain 1,3-diketones with a nitrating mixture generates acyl nitrile oxides, which can be trapped in situ as dipolar cycloadducts (see Scheme 3) <96SC3401>. 

The photochemical cyclisation of 2-aryl-3-cycloalkenyloxychromones results in the formation of a spiropyran unit in a tetracyclic array <96TL8913>. 

The synthesis of 9-phenylphenanthrene (Expt 6.14) illustrates the formation of the phenanthrene system by the cyclisation of a 1,2-diphenylethylene (stilbene). The process involves an allowed photochemical cyclisation which gives initially a dihydrophenanthrene. This is readily dehydrogenated in situ by molecular oxygen in the presence of iodine. 

Presumably the silyl enol ether of 37 adds in a conjugate fashion to the unsaturated ester 39 and the intermediate enolate then cyclises onto the cation 40 to give 38. This will happen only if the stereochemistry of 40 is the same as that of the product 38 as the 4/5 and 4/6 ring fusions must both be cis. This suggests that the first step is reversible. The formation of the cyclobutane requires that particular relationship between ketone and unsaturated ester so this kind of reaction is less versatile than photochemical cyclisation. Asymmetric versions of these reactions are also known.14 Probably the most versatile thermal method to make cyclobutanes uses ketenes and is the subject of the next chapter. 

E. Crundwell, R. R. Hill, and J. Hudec Photochemical cyclisation of Diels-Alder-adducts. J. chem. Soc. [London] 1964, 3062. 

The photochemically-exdted hexatriene system has one of its r-electrons promoted to the first excited r-orbitaL The terminal symmetry of this orbital (13) is the reverse of that depicted in (9), and requires bonding overlap to develop from a "con-rotatory displacement of end-groups, so that orbital envelopes on opposite faces of the molecule can interact [10]. This results in photochemical cyclisation of the B-seco-trienes (2) and (3) giving products with anti configurations about the C(9> C(io) bond, corresponding to ergosterol (9a,iOjS) and lumisterol (9/ ,ioa) respectively. The photochemical process... 

Enamides of various structural types can be photochemically cyclised to give alkaloid ring systems this year, for example, it has been reported that the dimethoxyprotoberberines (190) have been prepared by irradiation of (191) in which the enamide is an N-vinyl benzamide which cycllses non-regioselectively onto the non-equivalent orfho-positions of the benzamide phenyl ring. In the... 

An electron-transfer mechanism is proposed to account for the photochemical cyclisation of the fluorinated aryl amines (36) and (37). Photochemical oxidative cyclisation has also been reported as an efficient path to some benzo[h]thienothienoquinolines. A study of the photochemical reactivity and degradation of diclofenac and meclofenamic acid has been published. The bis(diphenylamino)butane (38) is photochemically reactive and irradiation at 313 nm in an air-saturated solution brings about cyclisation which yields the biscarbazole (39). It is clear from the results obtained that the reaction is stepwise with initial cyclisation affording (40). The quantum yields for the cyclisation of (38) to (40) is 0.3 while that for (40) to (39) is 0.02. 

The photochemical cyclisations of the imines (46) is a typical 6e process and irradiation in the presence of HBF4 gives conversion into the corresponding cyclic compounds. The yields obtained from the processes are moderate and there is little doubt that cyclisation involves the protonated imine. Several products are formed on irradiation of the naphthylalanine derivatives (47). ... 

Irradiation of the thiopyran derivative (292a) results in extrusion of HNCS and the formation of the pyridine (292b, 63.3%) as the major product. The photochemical reactivity of the pyranthione (293) is concentration dependent in 3-methylpentane as solvent. At low concentrations the thione reacts with the solvent but at higher concentrations the main reaction is the production of thiyl radicals. Laser flash photolysis has been used to identify that the triplet excited state of (294) is involved in addition reactions which occur to electron deficient alkenes such as acrylonitrile giving (295), for example. Other compounds related to thiourea are also photochemically reactive. Thus the photochemical cyclisation of (296) to afford (297) has been reported. ... 

R =OMe, R R =CH2) respectively, followed by y-methylation. (Kessar et al.. Tetrahedron Letters, 1977, 1459 Takao et al.. Heterocycles, 1981, 1, 221). Dihydrochelirubine and cheli-rubine have also been prepared by photochemical cyclisation of the amide (386), which involves the loss of a methoxy group, followed by reduction and dehydrogenation of the resulting di-hydro-compound to chelirubine (Ishii et at., Chem.Pharm.Bul1., 1978, 864). 

As an alternative to the photochemical cyclisation of ring opened berberines, such as (402, R =R =H) and (402, R R =0), cyclisations have been achieved by the action of hydrochloric... 

A Michael Reaction Coupled to a Photochemical Cyclisation Copper Again Michael Additions of Heteroatom Nucleophiles... 

There are many examples of asymmetric addition of organo-metallic reagents to aldehydes (chapter 27) and the one that works here uses Seebach s TADDOL 51 as a ligand for zinc.8 The TAD-DOL 51 is prepared from tartaric acid and used as its titanium (IV) complex to catalyse the addition of the organozinc compound to acrolein. After photochemical cyclisation in the presence of Cu(I), the bicyclic alcohol 53 was isolated in good yield and >98% ee. 

The electrocyclic step is a photochemical cyclisation of 210 under reducing conditions to give a mixture of diastereoisomers of 212 in about a 7 1 ratio. These were further reduced and the i V-ben/oyl group replaced to give a separable mixture of amines the required diastereoisomer 213 was isolated in 61% yield. 

Narciclasine and narciprimine, two alkaloids of biosynthetic interest whose structures had been incorrectly assigned, are now shown to be (21 R = OH) and (22 R = OH) respectively. The latter has been synthesised by a standard photochemical cyclisation route (23 R = = OCH2Ph) - (22 R = OH). 

Another photochemical cyclisation of an o-quinomethide derived from a... 

Synthesis of a cyclic 1,3- dicarbonyl compound from an acyclic 1,2-dicarbonyl compound may be achieved in two steps by a photochemical cyclisation followed by a cationic rearrangement in which the cyclobutanone acyl group migrates preferentially [equation (40)].141... 

Structural aspects of carbapenem antibiotics have been reviewed <01H497>. Photochemical cyclisation of A -methylacryloylthiobenzanilide 81 in the solid state gave 82 <01T6713>. 

A photochemical cyclisation was used as a key step in the synthesis of a novel type of cyclonucleoside, , 5 -anhydroadenosine-8-phosphoric acid (14) (Scheme 4). ... 

In case of butadiene, the photochemical cyclisation reaction produces two products—cyclobutene and bicyclobutane. The product obtained depends on geometry of butadiene. 

In some case, the photolysis products of dienes and trienes depends on the excited state in which photochemical cyclisation occurs (singlet or triplet). To populate triplet state, suitable triplet sensitizer are used if required. The. 3-Methylene-l, 5-hexadiene gives a cyclobutadiene derivative through singlet excited state and tricyclic ring compound from triplet excited state. This difference of photochemical process is because of life-time of the excited states. Higher the life-time of reacting intermediate more will be possibility to form more stable radicals. 

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