Pyridine, 2- photocyclization

Pyridines, [(l-methyl-2-indoloyl)amino]-photocyclization, 4, 204 Pyridines, [(l-methyl-2-pyrroloyl)amino]-photocyclization, 4, 204 Pyridines, nitramino-synthesis, 2, 343 Pyridines, nitro-N-oxides... 

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>. 

A photocyclization of 3-(2-pyrrolyl)-3-amino-2-alkeneimines under acidic reaction conditions leads to the formation of pyrrolo[3,2-3]pyridine as shown in Equation (22) <1999T14079>. When oxygen or sulfur heteroatoms are substituted for the pyrrole ring nitrogen, the corresponding furo- and thieno[3,2-3]pyridines are formed. All of the photocyclizations require lengthy reaction times and the yields tend to be low, especially for the furopyridine derivatives. 

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 formation of benzene and pyridine presumably occurs via the bicyclo[4.2.0] valence isomers (175) and (176), although adducts of these structures with dienophiles were not obtained on heating or during irradiation. The difference in products in thermal and photochemical reactions could be explained either by interconversion of (173) and (174) and rate-limiting isomerizations of (173) to (176) and (174) to (175), or alternatively, rate-limiting isomerization of (173) to (174) and symmetry-allowed 4ir photocyclization of (173) to (175) (79JOC1264). 

Irradiation of crude 1,2-dihydropyridine 365, obtained by the reaction of pyridine and a Grignard reagent prepared from chloromethyldimethylphenylsilane, at 300 nm in acetone gave the substituted 2-azabicyclo[2.2.0]hex-5-ene 366 in 30% overall yield (Equation 46). The low yield in the photocyclization step could be due to the competitive aromatization and/or electrocyclic ring opening of the 1,2-dihydropyridine <2005JOC590>. 

The second special case is formed by the photoreactions of 2,4-dinitro-6-(phenyliodonio)phenolate (266) with several nucleophiles759. Upon irradiation of this fairly stable zwitterion in methanol, 6-methoxy-2,4-dinitrophenol is formed in 65% yield. Photoreaction with pyridine affords 2,4-dinitro-6-pyridiniophenolate (85%) and irradiation in the presence of phenyl isothiocyanate in acetonitrile affords a mixture of two stereoisomeric 2-phenylimino-5,7-dinitro-1,3-benzoxathioles (269) (71%) which could not be separated. The reaction starts with attack of the nucleophile on the positively charged iodine atom, leading to an iodinane (267) and proceeds by expulsion of iodobenzene. The mechanism is illustrated for phenylisothiocyanate, a case in which the substitution product (268) can undergo further photocyclization to a benzoxathiole derivative (269) (equation 196). 

When the substrate has no alkyl groups on N, the dianion formed by LDA in THF also reacts under irradiation to afford oxindoles in good yields. Photocyclization of 2-chloro-3-(A-methylacetamido)pyridine by means of KNH2 in liquid ammonia or LDA in THF gave azaoxindoles in good yields (62 and 83%, respectively)338. 

When the enamine is terminally disubstituted, the photocyclization can lead to the formation of a spirocyclic ring system (equation 3)7,8. This spirocyclization proceeds in good yield both with substituted phenyl rings as well as with furan, thiophene, pyridine or indole heteroaromatic rings. 

As a further extension of photocyclization of the enamides prepared from 1-methylisoquinolines, Naito and Ninomiya (67) and Lenz (48) synthesized aza analogs of berbine, azaberbines, by irradiation of pyridine analogs of the parent enamides (27) (Scheme 57). 

Archer and co-workers (103) have also employed the Snieckus oxidative pho-tocyclization in the key step of their synthesis of the 7//-pyrido[4,3-c]carbazole ring system (Scheme 40). Thus, a Wittig condensation between pyruvate 242 and pyridine 243 gave the unsaturated ester 244. Photocyclization gave the tetracyclic ester 245. Reduction and reaction with methyl isocyanate led to carbamate 247. Oxidation of alcohol 246 to aldehyde 248, followed by a standard one-carbon homologation, gave the desired ethyl derivative 249. These chemists also synthesized the 10-methoxyl derivative of each compound. 

The photocyclization of 3-nitro-2-pyridyl-DL-leucine (354) to 2-isobutyl-imidazo[4,5-i]pyridine (355) has been studied spectrophotometrically and the quantum yield of the reaction determined as a function of pH (72JCS(P2)2218). 3-iV-Methanesul-fonamidopyridine 1-oxide (356) is acylaminated at C-2 by phenylbenzimidoyl chloride (357) and the intermediate 2-acylaminated product (358) cyclized to 2,3-diphenyl-imidazo[4,5-i]pyridine (359) (74JOC1802). 

The major products from the photocyclizations of l-(oxophenylacetyl)-and A-(2-oxopropionyl)piperidine (240, 241) include the perhydro-oxazolo[3,2-a]pyridin-3-ones (242-245) [69TL371 76ACS(B)383 ... 

Durr et al. have described some novel complex cinnoline derivatives which show photochromic properties. Albini et al. have provided some new evidence on the mechanism for the photoisomerization of heterocyclic N-oxides. Simple pyridine N-oxides are exceptional. Thus irradiation of pyridine N -oxide in aqueous base affords the ring-opened product (23). Aoyama et al. have described the unprecedented photocyclization of the amide (24) to the lactam (25). The first examples of C2a+2Tr] photoreactions of a three-membered ring and an azo-compound have been described by Hunig and Schmitt. Nicolaou et al. have prepared the first stable example of a 1,2-dithiethane the procedure involves (2i +2Tr) photodimerization of C=S groups. 

It is well established that 3-alkyl pyridines are selectively reduced at N1-C2, to produce 3-alkyl-l,2-DHPs [58,59,60], 5-Alkyl-l,2-DHPs, which result from hydride addition at C-6, are potentially valuable synthetic intermediates [61]. Substituent effects on the regiochemistry of the reduction of A-carbalkoxypyridinium salts have been studied in detail by Sundberg [62]. These DHPs have served as useful dienes for the synthesis of ISQs. Methyl 2-[l-phenylsulfonyl-lH-indol-2-yl]-2-propenoate (62) served as dienophile in most of these reactions [61,62,63,64,65,66,67,68]. Palladium-catalyzed radical cyclization [63], photocyclization [64] or thermal cyclization [61,65,66,67,68] reactions have all been employed to furnish the Diels-Alder adducts (e.g., 63). 

A modest diasterodifferentiation was reported during the photocyclization of chiral divinyl amine derivatives 150 into 151 [99]. Finally, photocyclization of the 4-(l-menthoxy)-pyridin-2(lH)-one-2 into the corresponding bicyclic 3-lactams occurs only with a low diastereoselectivity [100]. 

Dihydrobenzo[b]thiophene may be obtained by reduction of ben-zo[b]thiophene (Section III,A,1) by treating 2-o-(benzylthio)phenyIethanol with SOCl2-pyridine (91%),208 and by cyclization of o-(methylthio)phenyl-carbene at 250°C (45%) the last procedure is mainly of academic interest.304 Substituted phenyl vinyl sulfides undergo photocyclization to the expected... 

Photocyclization of the azocine (6) in methanol to give the cyclobuta[c]-pyridine (7) has been reported, and is presumably the result of a concerted dis-rotatory process.8 Attempts to reverse this reaction thermally were unsuccessful. 

Interest in photochromic systems other than those based on the hexa-fluorocyclopentene moiety continues to grow. The photochemical reactivity of the two photoswitches (35) is similar, and irradiation is efficient with conversions of 85% and quantum yields of around 0.6. The novel photo-chromic systems (36) undergo reversible ring closure in a reaction analogous to that observed in the bisthienyl system. Qin et al. have studied the novel pyridyl substituted cyclopentene system (37). This undergoes photocyclization with an enhanced quantum yield when the reactions are carried out in the presence of a metal. The pyridine units are capable of co-ordinating with the metal. The photochromic dithienylethene unit tethered to 3-cyclodextrin (38) has been used as a photoswitch to control the uptake of porphyrin. A series of new photochromic molecules (39) have been synthesized and studied. These exhibit the usual cyclization on irradiation. " The terthiophene derivatives (40) exhibit reversible photochemical cyclization (at 313 nm) and reversion (at wavelengths >460 nm) reactions. The cycles can be carried out many times... 

Pyridines have also been used in cyclization reactions. Two noteworthy examples are shown in scheme 3. The reaction of substituted pyridine 32 with a nitrile affords imidazo[l -ajpyridine 33 in excellent yield <01JOC2862>. Oku and co-workers have reported the use of tetrahydroquinolizinium ylides in a 13-dipolar cycloaddition reaction (34- 35) <01JOC1638>. Sieburth has also published an account of the [4+4] photocyclization reaction of pyridones on route to fusicoccin <01S1185>. 

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. 

---