Photocyclization salts

Photocyclization of 2,4,6-triphenylpyridinium salts 64 by irradiating by 313 nm gave 7-substituted 2,4-diphenyl-6,7-dihydropyrido[2,l-u]phthalazi-nium salts 62 (94MI1). 

Brief photocyclization of pyridinium derivative 55 afforded 2,4-diphenyl-7-substituted 5,6-dihydropyrido[2,l-tf]-phthalazinium salts 56 (Equation 10) <1995IZV296>. 

The 7,8-dihydrodiol of 5-MC (30b) was prepared from 8-hydroxy-5-MC by Method IV (60,103). It was necessary to utilize the 8-rather than the Ot-phenol (i.e. 8-HO-5-MC rather than 7-HO-5-MC), since oxidation of the latter with Fremy s salt was anticipated to take place predominantly in the para position. The 8-hydroxy-5-MC was prepared in our laboratories by a modification of the procedure in Figure 18. An alternative preparation of 8-hydroxy-5-MC via photocyclization of methyl 3-phenyl-2-(l-(6-methoxynaphthyl) propenoate has also recently been described (103). 

Another interesting example of a photochemi-cally induced domino process is the combination of the photocyclization of aryl vinyl sulfides with an intramolecular addition as described by Dittami et al. [901 as intermediate a thiocarbonyl ylide can be assumed. The domino-Norrish I-Knoevenagel-allyl-silane cyclization developed by us allows the efficient stereoselective formation of 1,2-trans-subsituted five- and six-membered carbocycles.1911 A photochemical cycloaddition of enamino-aldehydes and enamino-ketones with the intermediate formation of an iminium salt followed by addition to allylsilanes gives access to novel bicyclic heterocy-des. New examples of photochemically induced... 

V-Phenylbenzylimines undergo stilbene-type photocyclizations (equation 83) to yield heterocyclic compounds143. The reactions usually take place via the iminium salt and need oxidants like oxygen or iodine. Six-electron electrocyclic reactions have been observed for 1-aza-l,3-dienes144 (equation 84). 

Single crystal-to-single crystal reactions are quite rare in solid-state organic photochemistry, and we were fortunate to discover a second example in the Yang photocyclization of the 1-phenylethylamine salt of 2-methyl-2-benzoyl-p-carboxylic acid 39 [37]. 

It is important to mention again at this point that a general feature of the solid-state ionic chiral auxiliary approach to asymmetric synthesis is that not all chiral auxiliaries lead to high enantiomeric excesses. A case in point is found in the work of Natarajan et al. on the a-oxoamide-containing salts 43 (Scheme 10) [29]. Like the nonionic a-oxoamides discussed previously (Sect. 2.2), these compounds undergo photocyclization to p-lactam derivatives, and while the prolinamide salt behaves perfectly, leading to p-lactam 44 in 99% ee at 99% conversion, the corresponding 1-phenylethylamine salt affords nearly racemic photoproduct (3% ee at 99% conversion). The reason for this difference is... 

From the foregoing discussion, it is clear that DPM rearrangements are very general for a variety of 1,4-unsaturated systems, such as, 1,4-dienes, (3,7-unsaturated aldehydes and ketones, and different 1-aza-1,4-diene derivatives. Surprisingly, the literature was devoid of studies describing the photoreactivity of the closely related 2-aza-1,4-diene derivatives. For many years, the only studies in this area were carried out by Mariano and his co-workers [60] on the photochemistry of iminium salts derived from 2-aza-1,4-dienes. The results obtained demonstrated the synthetic utility of the photocyclizations of iminium salts to different heterocycles, in reactions that are initiated by intramolecular single electron transfer [60]. 

The stereoselective glycosidation of the secoaglycon 1338 was achieved by reaction of the sodium salt of 1338 with the a-l,2-anhydrosugar 1339 to give the p-glucopyranoside 1340 in 48% yield. After the removal of the SEM protecting group with TBAF, the photocyclization of compound 1341 afforded the indolocarbazole... 

The first synthesis of a derivative of 2,8-phenanthroline was reported by Merz, Weidlich, and Fink13 in 1964. They prepared 5,6-dimethoxy-2,8-phenanthroline (34) (isolated as the dipicrate) in very low yield by conducting a double Pomeranz-Fritsch isoquinoline synthesis on 4,5-dimethoxyisophthalaldehyde. The parent compound was prepared 2 years later12 in 25-35% yield, along with 1,9-phenanthroline (4%), by the photocyclization of trans-1 (3-pyridyl)-2-(4-pyridyl)ethylene in benzene. This reaction was used by Hiinig and his colleagues15 to prepare an N,N -dimethyl diquaternary salt of 2,8-phenanthroline by methylating the crude product from the irradiation reaction. 

The synthesis of [3.3.3]cyclazinylium salts (287) by photocyclization of 1,2,6-triaryl-pyridinium cations (288) has been described <80JCS(P1)1879>. 

A new biogenetic route to the aporphines has been proposed which does not involve phenolic oxidative coupling, and which proceeds through the intermediacy of protoberberinium salts.6 The alkaloid polycarpine (44) must be derived biogenetically from palmatine (45), and indeed oxidation of (45) in vitro with m-chloroperoxybenzoic acid, followed by hydrolysis, leads to polycarpine (44). Since it is known that photocyclization of simple benzylisoquinoline... 

The reaction mechanism of photocyclization of aryl vinyl ethers was derived from results obtained by means of flash photolysis. The ground state intermediate rearranges by mono-or bi-molecular 1,4-hydrogen shifts to yield the products (Scheme 62) (81JOC978). The photocyclization of 2-aryloxyenones was used in the total synthesis of ( )-lycoramine (77JA8065). The formation of dihydrobenzo[6 ]furans by radical cyclization from o-allenyl-oxyarenediazonium salts with tri-n-butyltin(IV) hydride was successful (81CC136). 

Highly enantioselective Norrish/Yang photocyclization of cw-4-terf-butyl-1-benzoylcyclohexane 47 was also achieved in salt crystals using chiral amines like R)- + )-l-phenylethylamine or (—)-norephedrine [61,63]. Furthermore, cis-9-decalyl aryl ketones 49 in their salt crystals with chiral amines such as (/ )-( + )-bomyl amine or ( )-( — )-1 -phenylethylamine underwent highly regio-, dia-stereo- and enantioselective Norrish/Yang photocyclizations to give the cyclobutanol 50 alone in > 98% ee [64]. 

Scheme 29 Absolute asymmetric photocyclization in a salt crystal formed from two achiral components.

Photocyclization of this type of enamide is assumed also to proceed by a six 71-electron ring closure to the cyclic intermediate (A) under irradiation, followed by elimination of water to form the cyclized quaternary salt 152 (Scheme 61). 

Benzo[c]quinolizinium (ion/salt), 1,3-diphenyl-, photocyclization, 55, 304 Benzo[c]quinolizinium (ions/salts), methyl-, reactivity, 55, 349 Benzo[l,2]quinolizino[3,4,5,6-t/e/]-phenanthridinium (ion), 2-phenyl-metal complexation, 55, 352 proton nmr, covalent hydration, 55,... 

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