Of barrelene

Barrelene was obtained via a double Diels-Alder reaction from a-pyrone with methyl acrylate (H.E. Zimmerman, I969A). The primarily forming bicyclic lactone decarboxylates in the heat, and the resulting cyclohexadiene rapidly undergoes another Diels-Alder cyclization. Standard reactions have then been used to eliminate the methoxycarbonyl groups and to introduce C—C double bonds. Irradiation of barrelene produces semibullvalene and cyclooctatetraene (H.E. Zimmerman. 1969B). 

The di- r-methane rearrangement is a fairly recent reaction. One of the first examples has been reported in 1966 by Zimmerman and Grunewald with the isomerization of barrelene 8 to semibullvalene 9. This rearrangement reaction occurs in the presence of acetone as photosensitizer, and proceeds from the Ti-state. ... 

Oxepin is in equilibrium with benzene oxide by a [3,3]-sigmatropic shift. Advantage has been taken of this equilibrium to develop a short synthesis of barrelene. Outline a way that this could be done. 

The direct photolysis of barrelene to cyclooctatetraene can be pictured as follows ... 

In mechanism (8.43) the bridgehead hydrogens of barrelene should be found at the a positions of semibullvalene (2a, 0/3,0y). Mechanism (8.44) can give three different hydrogen-label distributions. If the final bond formation is concerted with bond fission, and bond fission and formation take place at the same carbon atom [mechanism (8.44A)], the label distribution should be (la, 0/3, ly). If bond formation is concerted with bond fission but with a preference for bond formation at the carbon allylic to bond fission [mechanism (8.44B)], the label distribution should be (2a, 0/3, Oy). If there is a symmetric allylic biradical which has a finite existence [mechanism (8.44AB)], then the hydrogen-label distribution should be (1.5a, 0/3,0.5y). 

A classic example of the di-7r-methane photorearrangement is afforded by the reaction of barrelene (37)<41) ... 

Early applications of this reaction are found in the conversion of barrelene to semibullvalene 324) (3.15) or in the preparation of an azabullvalene (3.16) 325). In a similar reaction azabarbaralene has been prepared from aza-bicyclononatriene 326). 

The asymmetric Diels-Alder cycloadditions of enantiopure (5)-5-(/ -tolylsulfinyl)-1,4-benzoquinones with Dane s diene under thermal and Lewis acid conditions produce tetracyclic quinones after spontaneous elimination of the sulfinyl group.The Diels-Alder reaction of barrelene with o-benzoquinone produces tetracyclo[6.2.2.2 .0 ]tetradeca-9,ll,13-triene-4,5-dione. Under kinetic control, the Diels-Alder cycloaddition of 2,3-dicyano-p-benzoquinone (98) with cyclopentadiene in MeOH produces the single cycloadduct (99) (Scheme 38). ... 

Studies of di-ir-methane photochemical rearrangements have been one of the main areas of research in organic photochemistry for many years (for reviews, see Refs. 1-4). The first example of a reaction of this type was reported by Zimmerman in 1967 in the sensitized irradiation of barrelene 1 that yields semibullvalene 2 [5] (Scheme 1). The reaction has been extended to a large number of acyclic and cyclic 1,4-dienes that yield the corresponding vinylcyclo-propanes on irradiation, in the di-ir-methane (DPM) version of the rearrangement. This reaction also takes place when a vinyl unit is replaced by an aryl group. A few representative examples of DPM rearrangements are shown in Scheme 1 [6-9]. 

Di-ir-Methane Rearrangement of Barrelene, Benzobarrelene, Dibenzobarrelene, and Related Derivatives... 

Aside from simple processes such as cis-trans isomerization, the first example of a potential energy surface derived for a photochemical rearrangement or reaction was in 1967. This was work by the Zimmerman group 44-47) in which the hypersurface for the Di-u-Methane rearrangement of barrelene to semibullvalene was obtained note Fig. 13. 

The di-TT-methane rearrangement was discovered and conceptually developed by H. E. Zimmerman (University of Wisconsin), one of the earliest examples being the photoisomerization of barrelene (1) into semibullvalene (2) (equation 2). Through intensive subsequent work, this photoreaction was shown to be quite general for 1,4-dienes. In view of the major contributions by the original author, the rearrangement is also referred to as the Zimmerman rearrangement. 

A study of the photochemical reactivity of the pyrazine derivatives of barrelene (95) and (96) has shown that the di-a-methane process is dominated by pyrazino-vinyl bondins made. Irradiation of the derivative (95, R s H) yields the products (96) by a vinyl-vinyl bridsins path and (97, R = H) and (98, R a H) froa the pyrazino-vinyl bridsins- This, (95, R a H), is the only compound in the group which shows this reactivity. Compound (95, R a CN) yields only products (97, R a CN) and (98, R = CN) from the pyrazino-vinyl path. When the more sterically crowded system (99) was used irradiation save the products (100a)and (100b).The former is produced in 76X yield and is the result of... 

Figure 1.2 Extended Htickel treatment of barrelene to semi-bullvalene reaction.

Figure 1.3 Triplet hypersurface for conversion of barrelene to semibullvalene (ellipsoids). The diamonds correspond to ground-state points. Solid triangles are selected Sj points. The ordinate is in Hartrees (-307.54 to -307.40).

The reactions of the strained benzene derivative 25 with DCA were originally studied with the aim of synthesizing planar cyclooctatetraene derivatives which are interesting compounds with respect to the question of their antiaromaticity [56]. The cyclooctatetraene derivative, 29 (Scheme 2.9) synthesized by photolysis of the barrelene derivative 26, however, turned out to be non-planar [57]. But the synthesis and the reactions of barrelene derivative 26 are interesting with respect to the utility of high pressure to control the course of reaction. The reaction of 25 with DCA at 1 bar and 127 °C produces the (1 1) Diels-Alder adduct 26 (yield, 49 %) and the unexpected dark blue (2 1) adduct 27 (yield, 14 %). At high pressure (9 kbar, 83 °C) 27 is the major product even after the low conversion of 36 % of the... 

Mixing of equimolecular quantities of barrelene and CSI in dichloromethane solution at — 78 °C followed by gradual warming to room temperature leads to the formation of N-(chlorosulfonyl)-/Mactam 154 in 74% yield. Heating the latter in DMF at 75-95 °C for 40 h gave the chloronitrile 155 in 31% yield, which on treatment with t-BuOK in DMSO-THF gave the semibullavalene 156 in 56% yield as indefinitely stable colorless needles (equation 82)77. 

The di-n-methane photorearrangement of barrelene-type molecules such as 253 has been studied extensively in solution and in the solid state (Scheme 41). Although, the asymmetric induction was very low in solution with salts... 

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