Ring enlargement photochemical

Another photochemical ring enlargement leading to a 1,3-oxazepine is the formation of 2-phenyl-l,3-oxazepine (13) by irradiation of 4-phenyl-2-oxa-3-azabicyclo[3.2.0]hepta-3,6-diene (11).12 It is proposed that the process involves the azirine 12 as an intermediate. 

The photochemical ring enlargement of quinoline 1-oxides provides 3,1-benzoxazepines 1 (see Houben-Weyl, Vol.4/5b, p 12971T).24... 

The differences between thermally and photochemically induced reactions of 2 are illustrated by the photolyses of cyclotrisilane 2 in the presence of disparate isocyanides. While in the thermally induced process ring-enlarged molecules can be isolated, the photolytic process results in the formation of 2,4-disila- and 3,4-disilacyclobutanediimines. A plausible explanation for the formation of these... 

When the terminal alkynes 96 are treated with the trimethylsilylalkyne 97 in the presence of HfCl4 as a Lewis acid, the silylated vinylallenes 98 are produced in acceptable yields. In an intramolecular variant of this process, 100 was obtained from the diyne 99 [32]. Vinylallenes, incorporated into a cyclic framework and hence of restricted conformational mobility, are of interest for photochemical studies [33] and are among the photoproducts in ring-enlargement reactions of polycyclic allenes [34]. 

Photochemically induced ring enlargements of alk-l-enylcyclobutanes to cyclohexenes have been reported only in a few select examples. A steroidal cyclohex-3-enone was formed in an equilibrium reaction from a 2-alk-l-enylcyclobutanone precursor.84 With a cyclohex-2-enone only the reverse reaction, i.e. ring contraction to give a cyclobutanonc, was observed.85 86 In a tricyclic system, a [1,3] shift occurred on sunlight irradiation to form a cyclohexene in 90% yield from an alk-l-enylcyclobutane.87... 

Photochemical rearrangement of 1, 4, and 64 has been examined in dichloromethane by steady irradiation and nanosecond transient spectroscopy. The ring enlargement of 1 to 292 and 4 to 293, respectively, has been postulated to occur via the singlet excited state. Phenolic derivatives 294 and 295 are formed from 1 and 4, respectively, via their triplet state. Product sensitization is proposed for the conversion of 1 and 4 to 294 and 295, respectively.167 Van Duuren et al. have found that on UV irradiation (254 nm) in methylene chloride, 4 rearranges to oxepin 293,4-hydroxypyrene, pyrene, and other unidentified products.168... 

Conversely, the photoinduced ring-opening of cyclobutenes allows the construction of medium-sized hetero- or carbocycles, starting from bicyclic frameworks. Alkenes can undergo thermal (metal-assisted) or photochemical [2 + 2]-cycloaddition to a variety of alkynes the subsequent two-carbon ring-enlargement can serve as a versatile method for the preparation of hydroazulenes and dioxacyclooctadiene derivatives (Scheme 9.44) [74, 75]. 

Mislin, G.L. and Miesch, M. (2003) Photochemical, thermal and base-induced access to hydroazulene derivatives via two-carbon ring-enlargement reactions of condensed electrophilic cyclobutenes. Journal of Organic Chemistry, 68, 433 141. 

The photochemical ring enlargement of N-phenyl-lactams does not work with four-, five-, and six-membered lactams [82]. The reaction has been observed only in the case of benzannelated /3-lactams, see Scheme VI/22 [83], Photolysis of larger N-phenyl-lactams are as follows [82] ... 

Heteroaromatic A-imines are photochemically active compounds (see Table III) On irradiation they generally yield products of (a) N-N bond cleavage, (b) ring enlargement, (c) rearrangement. However, the available data do not permit definition of the conditions necessary for selective reactions. Apparently unimportant changes in the substituents and the solvent may lead to different results. Therefore, the photochemical reactions will be discussed separately for the individual A-imines. 

Chip and Lynch, who previously42 reported the photochemical ring enlargements of the cyanoalkanones (35), have now described the work in detail.43... 

The K-region phenanthrene oxide (68) undergoes photochemical ring-enlargement to (69) via the singlet excited state. A similar photochemical rearrangement for the K-region 3,4-epoxy-3,4-dihydropyrene was also reported. 

Schonberg proposed in his book [4] that the 2+2 cycloaddition first reported by Ciamician for the case of carvone [38] should be named in honor of that scientist. Note that a non photochemical reaction, the Ciamician-Dennstedt ring enlargement of pyrroles (1881), does bear his name... 

Two groups have reported on the synthesis of the hydroxymethyl-branched carbocyclic systems 184 (B = Ad, Gua), which can be regarded as ring-enlarged oxetanocin analogues or as carbocyclic analogues of important anti-HIV agents. In one approach, the racemic unit 183 was produced by photochemical addition of methanol to the enone,2i4 whilst other workers reported routes to both 184 and its enantiomer. iS... 

Takaishi, N., Miyamoto, N., and Inamoto, Y, Ring enlargement of the photochemically generated endo-2,3-trimethylenenorborn-exo-2-ylcarbinyl cation. Occurrence of a different reaction pathway from that in sulfuric acid, Chem. Lett., 1251,1978. 

Yamamoto and coworkers have reported a photochemical ring-opening using electron transfer and the production of cation radical species. One of the electron transfer reactions of this sort is the Birch reduction. This is quite useful to bring about ring enlargement. When it is applied to the phane systems. 

---