Photochemical annulation

An alternate approach to the iridoid core, again developed by Biichi, utilized the [2+2] cycloaddition of cyclopentene 28 and tricarbonyl 29 (Scheme 6). Following photochemical annulation, the resulting cyclobutane 30 underwent a retro-aldol reaction and hemiacetal formation to provide racemic cyclopenta[c]pyran 31. Elaboration of 31 to loganin (16) required 10 additional steps. ... 

One-pot photochemical annulation of 3-acyl-2-haloindoles (209) (211) afforded fused benzo[c]carbazoles (210) (212) in high yields. Substituted dihydrexidine analogues (214) having A -benzoyl enamine units were prepared by intramolecular photocyclization. Visible light-induced intramolecular cyclization of V-3-butenylindole derivatives (215) and diamines (217) was sensitized by Ru(bpy)3 " "... 

Merlic developed a new variation of the thermally induced benzannulation reaction. The dienylcarbene complex 132 was reacted with isonitrile to give an orf/zo-alkoxyaniline derivative 135 [76] (Scheme 56). This annulation product is regiocomplementary to those reported from photochemical reaction of chromium dienyl(amino)carbene complexes. The metathesis of the isocyanide with the dienylcarbene complex 132 generates a chromium-complexed di-enylketenimine intermediate 133 which undergoes electrocyclisation. Final tau-tomerisation and demetalation afford the orf/zo-alkoxyaniline 135. 

In addition, annulated aziridine 170 was obtained in pure form by photochemical transformation of 169 (Equation 30) <2001HCA3822>. 

Dauben and coworkers produced a number of lovely examples of the reaction in the course of their studies of the photochemistry of large-ring (Cs-Cn) cyclic trienes, many of which were produced by photochemical electrocycloreversion of the isomeric annulated cyclohexadiene derivatives (cf Reference 172 and references cited therein). Two examples... 

Cycloaddition reactions with furans remain a source of new compounds. On photochemical excitation of benzofuro-annulated oxanorbomadiene 42 in the presence of dipolarophiles Ill-addition compounds are obtained. The formation of 43 constitutes one of the rare examples of the cycloaddition of a nitrile as a dipolarophile <95LA1503>. 

An interesting application of the photochemical reactivity of isoxazolin-5-ones (see also Section 12.3) has been reported for the synthesis ofa series of annulated imidazole derivatives of general structure 72, which are obtained in almost quantitative yield. The methodology consists of an initial N-functionalization of the isoxazolin-5-one with a a-chloro-azaheterocycle, leading to 2-(azahetaryl)isoxazol-5(2H)-ones 71. 

Highly strained substrates can be transformed into even more strained isomers by ODPM rearrangement. This has been shown by Murata et al. for the synthesis of a valence isomer of azulene [45]. Albeit the photochemical reaction yielded only 20-25% of the bicyclo[1.1.0]butane derivative 15, the synthesis of the precursor cyclobutene (14) is straight-forward from the bicyclo[3.3.0]octenone 13 (Sch. 18). This substrate has obviously a diverse reactivity pattern when directly excited, however, triplet sensitization reduces these competitive pathways because alkene excitation is excluded. Also benzo-annulated azulene valence isomers were generated by this approach [46]. 

Stack JG, Curran DP, Geib SV, Rebek J, Ballester P (1992) A new chiral auxiliary for asymmetric thermal reactions High stereocontrol in radical addition, allylation, and annulation reactions. J Am Chem Soc 114 7007-7018 Yang NC, Yang DDH (1958) Photochemical reactions of ketones in solution. J Am Chem Soc 80 2913-2914... 

Palladium-catalyzed cyclization of the oxime derivative 47 provided a good yield of the pyrrole 48 (Equation 9) <1999CL45>. Similar reactions have been observed in connection with cyclization studies of related ketone trimethylhydrazonium salts <2005H(65)273>. Photochemical radical cyclization of 7,5-unsaturated ketone oximes has been reported to produce 1-pyrrolines <2005TL2373>. Similar 0-acetyloximes may also be annulated to 1-pyrrolines by treatment with acetic acid in the presence of 1,4-cyclohexadiene and naphthalene-l,5-diol, possibly proceeding via a radical mechanism <2002CL144>. 

The photochemical ring opening of palustric ester (90), an example of a [l,2]-annulated cyclohexa-diene, has been achieved wherein a -1 1 photostationaiy state mixture of diene (90) and triene (91) was obtained. Upon heating for 4 h (162 °C), the photo-mixture regenerates palustric ester (90). A related interconversion was observed for a series of photochromic cyclohexadienes of the xanttenone, dibenzo-furan and acridone types, exemplified by (92) —> (93). °... 

R.L. Danheiser and co-workers generated a key vinylketene intermediate via tandem Wolff rearrangement-ketene-alkyne cycloaddition to utilize it in a photochemical aromatic annulation reaction Danheiser benzannulation) for the total synthesis of the phenalenone diterpene salvilenone. ... 

S. Aldoshin, I. Chuev, A. Utenyshev, O. Filipenko, J. L. Pozzo, V. Lokshin, and R. Guglielmetti, Specific structural features and photochemical properties of three benzo-annulated 2,2-di-phenyl[2T/] chromenes, Acta Crystallogr., Sect. C52, 1834-1838 (1996). 

If the [3-h 2]-cycloaddition reaction of a methylenecyclopropane is performed with a remote alkene or alkyne moiety within the same molecule, products 2 of the bicyclo[3.n.0]-type (n < 3) or the bicyclo-[n.3.0]-type (n > 3) can, in principle, be obtained. The former of these product types can be considered as unhkely, as an alkene or alkyne separated by only one or two atoms from the methylenecyclopropane would give rise to a strained annulated ring, i.e. a cyclopropane or a cyclobutane. It is known from investigations of photochemically initiated intramolecular [2 3-2] cycloadditions of alkenes that the minimum size required for a spacer between 7t-systems is three atoms in a reaction of a non-crossed type. There are actually a few examples of photoinduced intramolecular cycloadditions of the crossed type involving substrates with short spacers (n =... 

The known examples of the synthesis of higher-order annulated systems (exemplified by types XVI-XVn in Figure 2) involve photochemical cyclization followed by oxidation arene analogs of stilbenes. 

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