Photocyclization reaction

Methacrylates also undergo 2 + 2 photocyclization reactions with a variety of substrates (35,36). 

The photocyclization reaction is in contrast to the photodimerization of 58 in 59. In the case of 61, the photodimerization of 58 is probably prevented by steric hindrance. It is almost certain that one optical conformer of 58 is included in 61, but a real proof of this fact requires an X-ray structural study in the future. However, the formation of the optically active 62 is valuable, because photoreaction of 58 in solution does not give any intramolecular photocyclization product34). 

On the other hand, stereo- and enantioselective photocyclization reactions of N-benzoylformylpiperidine (74a), N-benzoylformylmorpholine (74b), and N-benzoyl-formylhexamethyleneimine (74c) were achieved by irradiation of the respective inclu-... 

X-Ray crystal structural studies32) (Fig. 13 and Scheme 8 which refers to the crystal structure) showed that one molecule of 93 is held in a fixed conformation determined by two hydrogen bonds and by neighboring host molecules which prevent free rotation about the CO—CO single bond in 93. Free rotation about this bond would enable the production of the two possible enantiomers. The fixed conformation of the guest molecule by the chiral host molecule causes the least molecular motion during the photocyclization reaction and the high enantioselectivity. 

Photochemistry has also been a prominent theme this past year. Intermolecular photocyclization reactions involving pyridones have been reported . An unusual photocyclization between 1-cyanonaphthalene and substituted pyridines was also reported . In similar fashion the reaction between benzofurans and substituted pyridines was reported by Sakamoto <00CC1201>. 

Scheme 8 Asymmetric induction in the Yang photocyclization reaction.

There has been considerable interest in various photocycloaddition reactions over the last years which not only broadened the number of useful photochemical applications but also revealed further mechanistic insight into these reactions [76,77]. Among these reactions, reports focusing on either the [2 -h 2] or the [4 -I- 2] cycloaddition, are numerous. Also the efforts toward the enantiodifferentiating photosensitization in photocyclization reactions have to be mentioned [78],... 

Leibovitch M, Olovsson G, Scheffer JR, Trotter J (1998) An investigation of the Yang photocyclization reaction in the solid state asymmetric induction studies and crystal strac-ture-reactivity relationship. J Am Chem Soc 120 12755-12769... 

An example is the photopolymerization of 2,5-distyrylpyrazine (DSP) (Fig. 17), with polymerization occurring via intermolecular [2-1-2] photocyclization reactions at each end of the monomer molecule. Although this reaction is regarded as a classic solid-state reaction, having been studied extensively around 40 years ago [121,122], structure determination of the polymeric product phase was only carried... 

In the crystal structure of the polymer phase (Fig. 17a), the polymer chains are aligned along the c-axis and the distance (3.71 A) between the centres of adjacent cyclobutane and pyrazine rings corresponds to half the c-axis repeat of the unit cell. For comparison between the monomer and polymer structures, an overlay plot of these structures is shown in Fig. 17b. It is clear that the solid-state reaction is associated with only very small atomic displacements at the site of the [2-1-2] photocyclization reaction (the displacement of the carbon atoms of the C=C double bonds of monomer molecules on forming the cyclobutane ring of the polymer is only ca. 0.8 A for one pair of carbon atoms and ca. 1.6 A for the other pair). Such small displacements are completely in accord with the assignment of this solid-state reaction as a topochemical transformation [124—127] (in which the crystal structure of the reactant monomer phase imposes geometric control on the pathway of the... 

Methylisoxazolo[5,4-. ]pyridine, 132, reacts with vinyl ether in a photocyclization reaction to give a mixture of mono- and bicyclic products, including a moderate yield of the dihydropyrrolo[3,2-3]pyridine derivative as one of three major products shown in Equation (65) <2002EJ04211>. 

The selectivity is caused by the electron distribution in the excited diarylethylene. Several reactivity parameters for the photocyclization have appeared to be valuable. The most simple one is the sum of the free valence numbers of the atoms r and s involved in the cyclization in the excited state (E F s) (Fr = /3 — IP, P = bond order)13). The following rules could be derived from a large number of photocyclization reactions 14) ... 

Besides from 1,2-diarylethylenes it is possible to prepare helicenes from proper bis-arylvinyl aromatics by two subsequent photocyclization reactions e.g. hexa-helicene can be prepared from 2,7-distyrylnaphthalene (1+2+1) (60%), and trideca-helicene from 3,6-bis(2-benzo[c]phenanthrylvinyl)-phenanthrene (4+3 + 4) (52%). 

F. B. Mallory and C. W. Mallory, Organic Reactions, vol. 30. Wiley (1984). This is an extensive compilation and discussion of the photocyclization reactions of stilbene and related systems. 

If the irradiation of 68 (n = 0, R = CH3) is closely connected to photoxidation processes which will be described later, the photocyclization reaction is the source... 

The photocyclization reaction is more efficient with p-anomers, and the new C—C bond created at the anomeric center is obtained by preferential axial anomeric hydrogen abstraction followed by carbocyclization. Norrish II reactions generally result from hydrogen abstraction at the y-position according to Scheme 31 to give cyclobutanols or degradation products. For 68, (n = 1) photoabstraction at the 5-position of the anomeric hydrogen leads to cyclopentanols 69 (Scheme 37) [67]. 

Figure 106 Illustration of thymidine 2+2 photocyclization reaction to form a cyclobutane ring. Note that the reaction can occur between neighboring thymidines on the same DNA strand (intrastrand) or between two different strands to form interstrand cross-links.

B. Enantioselective Intramolecular Photocyclization Reactions of Achiral Molecules in Inclusion Complex With a Chiral Host Compound... 

When the photocyclization reaction of tropolone alkyl ethers 62a and 62b, which produces the corresponding cyclization product 63 and its ring-opened derivative... 

Intramolecular [2 + 2] photocyclization reactions of 2- /V-(2-propcnyl)amino]cy-clohex-2-enones (67) are also controlled enantioselectively by carrying out irradiation in inclusion complex with a chiral host compound. When inclusion crystals of 67 with 12 are irradiated in the solid state, optically active 9-azatricyclo[5.2.1.0]-decan-2-ones (68) were obtained in the chemical and optical yields indicated in Table 3 [30],... 

Photocyclization reaction of the V-(aryloylmcthyl)-8-valerolactams (80) occur stereoselectively and enantioselectively in their inclusion crystals with optically... 

The photocyclization reaction of acrylanilide to 3,4-dihydroquinolinone was first reported in 1971, and its application to alkaloid synthesis has long been studied [44], Although stereo- and enantio-control are important in this reaction, no such attempt has been reported, except one enantioselective photocyclization of 1-(methylacryl)-A-methylanilide (85) in benzene-ether containing (i )-bis(/>tolu-oyl)tartaric acid to give ( )-3-methyl-A-methyl-3,4-dihydroquinolinone (87) of 12-16% ee [45],... 

Optically pure P-lactam derivatives can also be prepared by an enantioselective photocyclization reaction of 2-pyridone in its inclusion complex with a chiral host. For example, irradiation of the 1 1 inclusion complex of 1 and 4-methoxy-/V-inethyl-2-pyridone (33) in the solid state gave (—)-34 of 100% ee... 

Figure 1 Enantioselective photocyclization reactions of 31 in a 1 1 inclusion compound with 1.

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