Photochemical ring expansion

H-Benzo[a]carbazole, 4,4a,5,l 1,1 la,l Ib-hexahydro-synthesis, 4, 283 Benzo[b]carbazole, N-acetyl-photochemical rearrangements, 4, 204 Benzo[/]chroman-4-one, 9-hydroxy-2,2-dimethyl-synthesis, 3, 851 Benzochromanones synthesis, 3, 850, 851, 855 Benzochromones synthesis, 3, 821 Benzocinnoline-N-imide ring expansion, 7, 255 Benzocinnolines synthesis, 2, 69, 75 UV, 2, 127 Benzocoumarins synthesis, 3, 810 Benzo[15]crown-5 potassium complex crystal stmcture, 7, 735 sodium complex crystal stmcture, 7, 735 Benzo[ 18]cr own-6 membrane transport and, 7, 756 Benzo[b]cyclohepta[d]furans synthesis, 4, 106 Benzocycloheptathi azoles synthesis, 5, 120... 

Perfluorotetramethylthiadiphosphanorbornadiene and bis(trifluoromethyl) thiadiphosphole can be prepared by thermolysis of an adduct of methanol and hexakis(trifluoromethyl)-l,4-diphosphabarrelene with sulfur [113] (equation 23) Pyrolysis of the adduct of hexafluorinated Dewar benzene and phenyl azide results in ring expansion giving azepine, which photochemically yields an intramolecular 2-1-2 adduct, a good dienophile for the Diels-Alder reaction [114, //5] (equation 24) Thermolysis of fluonnated derivatives of 1,5-diazabicyclo-... 

The photochemical ring expansion of A-oxides when applied to isoquinoline 2-oxides leads to 1,3-benzoxazepines 5.2122 The yield was reported for 5b only. 

Functionalized silacyclobutanes 16 result from photochemical decomposition of [azido-, isocya-nato- and isothiocyanato-bis(tert-butyl)silyl]diazoacetates 15. They undergo a remarkably facile ring-expansion reaction to cyclic O-silyl ketene acetals 17 even at 60°C. 

Nitrones derived from 2-azabicyclo[5.3.0]decane give quinolizidine compounds by photochemical Beckmann rearrangement which implies simultaneous ring expansion and ring contraction reactions. Intramolecular Schmidt reactions in 2(4-azidobutyl)-cyclopentanones also give quinolizidinone derivatives by ring expansion. Examples of both types of reactions are given in Sections 12.01.11.1 and 12.01.11.3, respectively. 

It was also found that the ring expansion could be accomplished photo-chemically, from either phenyldiazomethane or triplet phenylcarbene.7 Both the thermal and photochemical ring expansions were found to be reversible,5c, thus providing rare examples of carbene-to-carbene interconversions. One remarkable example of this reversibility is the interconversion of the isomeric tolylcarbenes upon pyrolysis — the ultimate products of which include styrene and benzocyclobutene (Scheme 3).6,8,9... 

Earlier, Dunkin and Thomson had observed that matrix-isolated triplet 10a did not undergo photochemical ring expansion.83 However, Morawietz and Sander have recently provided evidence for photochemical conversion of 310a and 310b to the corresponding fluorinated azirines (Scheme 19).48d This represents a rare instance where an azabicyclo[4.1.0]heptatriene, the putative intermediate in the ring expansion of a phenylnitrene, has actually been observed. 

A series of experiments on ring expansions under photochemical conditions has been published by Wentrup et al. <1996CC813, 1998J(P1)2247, 20040BC246, 20040BC1227>. Schematic representation of these transformations is shown in Seheme 6. 

Biscarbene 34 was characterized by IR and UV/vis spectroscopy [49], The analysis of the experimental data showed that these are compatible with the presence of two phenylchlorocarbene (6) subunits in 34. This interpretation was further supported by the reactivity behavior of 34, which, like 6, is unreactive toward oxygen under conditions where triplet carbenes react fast. In contrast to its para isomer (22), 34 appears to undergo photochemical ring expansion analogous to that of 6[105]. In addition, the computed [RHF/6-31G(d)] IR spectrum of 34, which is in good agreement with the observed one, is based on the wave function for the singlet (cr /cr ) biscarbene (54 of Fig. 9). 

Compounds 217 and 218, which are the hydrated forms of 118 and 182, give the same products when treated with 8% NaOH (Equation 16 see Equation 3 for comparison) <1992T4545>. The ring expansion of 201 (Equation 17) <2007S225> and photochemical ring contractions of benzothiazines (Scheme 23) <1994TL3365> have also been reported. 

Silyl substituted lf/-phosphirene 40 undergoes photochemical ring expansion to the 1,2-dihydro-1,2-phosphasilete 41 with cleavage of a silicon-silicon bond <99MI1581>. 

It is important to note that a possible photochemical ring expansion by [1,3]- or [l,5]-acyl shifts in the 5-membered vinyl lactams 191 has not been observed,4,116 nor have [l,5]-shifts of acyl groups involving an aromatic ring been reported for either enol esters or enamides. However, an example of a similar [l,5]-alkyl shift, 193 -> 194, in related ketene dialkyl acetals is known.24... 

Photochemical Ring Expansion of Cyclic Ketones Via Cyclic Oxacarbenes P. Yates and R. O. Loufty, Acc. Chem. Res., 1975, 8, 209-216. 

Photochemically induced rearrangement of the cycloadduct (251) results in either ring expansion to give benzazepines or ring contraction to aminomethyleneindenes, depending upon the reaction conditions (B-77MI30500). 

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