Photocyclization processes

A highly stereoselective synthesis of ( )-deplancheine has been developed (121), utilizing a photocyclization process for the preparation of enelactam 181, which was reduced by zinc in acetic acid to lactam 182. In the latter, the double bond was shifted to the 19,20 position stereoselectively with an E geometry by the use of nonacarbonyldiiron catalyst to supply 159, previously synthesized and transformed to ( )-7 by Winterfeldt et al. (112). 

In addition to the extensive information on the excited state cyclization reactivity summarized in Tables 1 -9, a wide body of reactivity data is available from studies of the direct photoaromatization process (D), the rate limiting step in such cases being the photocyclization process itself. This wealth of systematic and rather detailed excited state reactivity data is undoubtedly unique, providing numerous opportunities for theoretical studies of excited state reactivity some of which will be reviewed in Sect. VI. D. 

The influence of steric effects on the photocyclization process is clearly discernible for the ortho and meta substituted stilbenes. In these cases steric repulsion intervenes as the two C atoms forming the new bond approach. This steric repulsion opposes the stabilizing interactions which promote the cyclization process. As a result the reactivity is decreased. Other consequences of such superimposed steric... 

A. Gilhert, in W. M. Horspocl (ed.). Synthetic Organic Photochemistry, Plenum 11984). Photoaddition, photocycloaddition and photocyclization processes of aromatic compounds are all covered in this account of synthetic aspects of aromatic photochemistry. 

The photocyclization of appropriate l,2-di(pyridyl)ethylenes provides a route to all the isomeric phenanthrolines, albeit sometimes in very low yield.12 With trans-1 -(2-pyridyl)-2-(3-pyridyl)ethylene (2(5) after irradiation in benzene solution in a stream of air, the hydrated product, 1-(2-pyridyl)-2-(3-pyridyl)ethan-2-ol (27) (20%) predominated, along with 3,7-phenanthroline (8) (6%) and 1,7-phenanthroline (1) (1%). The photocyclization process proceeds from the trans isomer by way of the cis isomer.1798... 

Figure 14 Photocyclization process of 80c to 81c in the inclusion crystals with (—)-12c.

Like in other chiroptical switches (Section 5.3.1), solvent polarity was found to play an important role. Diastereoselective cyclization was observed in THF and toluene, but not in nonpolar solvents such as n-hexane. Upon photoexcitation, diarylethenes 24 (Scheme 11) can adopt a planar and a twisted conformation, and photocyclization only proceeds through the planar conformation. In the case of chiral diarylethene 27a, there are two diastereomeric planar conformations leading to the diastereomers of the cyclic product 27b. The stereoselectivity in the photocyclization process is enhanced because of a decrease in the excited state energy of the unreactive twisted form, providing a relaxation pathway for the less favorable planar diastereoisomer in more polar solvents. Chiral photochromic diarylethenes are among the most prominent photoswitches known today, featuring nondestructive read-out, excellent reversibility, and the potential for construction of switchable molecular wires and modulation of liquid crystalline phases (see Section 5.5.3).[40,411... 

The transient absorption spectra show that besides the ring-closure product (C), a new transient species was found that could be quenched by oxygen. The authors assigned the new species to an excited triplet state. The time-resolved transient absorption spectra indicated that when the time delay was increased, the excited triplet state was decreased and the ring closure product was increased slightly. The decay of the excited triplet state can be fit to first-order kinetics. The lifetime ( ) of the transient is 0.8 ps in cyclohexane. The authors noted that the photocyclization process proceeds mainly via the excited singlet state, but the excited triplet state is... 

Piperidones can also be formed through an e-hydrogen abstraction proci during the photocyclization of 4-oxo-4-phenylbutanoyl amines 163. The produ< 164 and 165 are obtained with remarkable diastereoselectivities of up to 99 [105]. When 2-amino substituents are present in 166, a large 1,4-asymmel induction is observed during the photocyclization process, which leads to corresponding lactames 167,168 (Scheme 42) [106]. 

The potential stereoselectivity of this photocyclization process has recently been investigated for the two rigid 2-allylanilines 70 and 71169. Irradiation of compound 70 at room temperature gives a mixture of diastereomers trans-12 and cis-12 with a little stereoselectivity (equation 23). While a poor stereoselectivity is also observed for 71, the photocyclization is regioselective, where the products trans-12 and cis-12 are minor (equation 24). However, the diastereoselectivity of trans-12 vs cis-12 is increased in the case of 71 when the temperature is changed, indicating that the reaction is significantly entropy-controlled. In addition, the observation of fluorescent exciplex formation for 70 and 71 supports the electron-transfer mechanism for the photocyclization of 2-allylanilines. 

The influence of steric effects on the photocyclization process is clearly discernible... 

Scheme 12 Asymmetric induction by chiral substituent in the photocyclization process,...

Fulgides (e.g., 53, see Scheme 15) are capable of photochromic reactions. Yokoyama et al. reported the preparation and subsequent photochromic isomerization of a fulgide which was designed to resist enantiotopomerization [103]. A sample of the resolved fulgide was irradiated (405 nm light in toluene solution) and was observed to come to a pss with a ratio of 19 81. Irradiation with visible light (X > 580 nm) led to the complete recovery of the initial conformation. An advance on this system was made by the same authors, who described the process of diastereomeric photochromism, in which a fulgide derivatized with a binaphthyl auxiliary was allowed to thermally equilibrate, and a photocyclization process carried out [104]. As a result of the relative populations of photoreactive... 

Photochemical electrocyclization of unsaturated enamides is a well-documented reaction of high synthetic value in the field of alcaloids [56]. The reaction can be viewed as a conrotatory photocyclization process leading to a dipolar intermediate which can be transformed into the final products either through a thermal, suprafacial, 1,5-sigmatropic shift or by an intermolecular protonation process. 

The complexation of Cu(hfac)2 and the isolated closed-ring isomers were performed to afford discrete 2 3 complexes closed-(4-py-DTEf)2(Cu(hfac)2 3 and closed-(4-py2-DTEf)2Cu(hfac)2, probably as the result of reduced coordination ability of the pyridine ligand upon ring-closure. Photocyclization process induced a change in the coordination structure, as demonstrated by ESR studies in the case of (4-py2-DTEf)2Cu(hfac)2. 

Hamza, K., Abu-Reziq, R., Avnir, D., and Blum, J. (2004) Heck vinylation of aryl iodides by a silica sol-gel entrapped Pd(n) catalyst and its combination with a photocyclization process. Org. Lett., 6 (6), 925-927. 

An early application of enamide photocyclizations to yohimbine alkaloid synthesis is illustrated by Ninomiya and coworkers synthesis of yohimbane (120), epiyohimbane (347) and alloyohimbane (82) (Scheme 3.57) (61, 63, 69). These compounds were prepared by a short sequence starting with har-malane (336) which was condensed with acid chloride 341 to provide enamide 342. Irradiation of 342 in benzene followed by reduction afforded a mixture of the stereoisomeric pentacyclic lactams 344, 345, and 346 which were reduced to provide yohimbane (120), epiyohimbane (347), and alloyohimbane (82), respectively. While the yield of the photocyclization process was modest, this route demonstrated how enamide photocyclizations can be used to rapidly construct pentacyclic yohimbane targets. 

Without doubt, it is possible to characterize the developments made in the area of pyridinium salt photochemistry as slow and far between. More recent investigations of this chemistry have begun to uncover its synthetic significance. However, a number of questions pertaining to the mechanism of the photocyclization process, the factors governing the rates of ring isomerization, and the regiocontrol of nucleophilic addition to the unsymmetricaUy substituted cation intermediates still need to be answered before the full potential of this unique excited-state process can be elaborated. 

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