Photocycloadditions electrocyclizations

The thermal addition of dimethyl acetylenedicarboxylate to indoles, unlike the photocycloaddition (see Section 3.2.1.4.1.1.), proceeds via a polar stepwise process to yield, initially, 3,4-benzo-2-azabicyclo[3.2.0]hepta-3,6-dienes which in some cases are isolable,13 141 but which, in general, ring open in situ to give the indolylacrylates 3 and/or undergo electrocyclic ring expansion to 1-benzazepines.21... 

The photocycloaddition of aliphatic and aromatic aldehydes with 2,4,5-trimethyloxazole (131) gave bicyclic oxetanes 132 in almost quantitative yields hydrolitic cleavage led selectively to erytro a-amino-P-hydroxy methyl ketones 133 <00CC589>. The oxazolium salt 134 was converted to the azomethine ylide 136 via electrocyclic ring opening of the oxazoline 135. Intramolecular cycloaddition afforded 137 in 66% overall yield which was transformed into the aziridinomitosene derivative 138 . 

This review surveys our studies devoted to the photoswitchable molecular receptors based on photochromic crown ethers. Photochromic systems described in the review may be classified into three groups according to the reaction types E,Z-isomerization, [2+2]-photocycloaddition reactions and electrocyclic reaction. It has proved the groups to be an especially suitable basis for photochromic systems, and promising for the industrial applications. 

Of the 5-membered heteroaromatic systems, pyrrole reacts most like benzene in alkyne photocycloaddition, giving a 3,4-disubstituted azepine by 2,3-cycloaddition followed by electrocyclic ring-opening fequation 73) . Azepines with a different substitution pattern have been made by therpial 2,5-addition of an alkyne to a pyrrole, followed by photochemical ring-closure and thermal ring-opening of the tetracyclic photoproduct (equation 74) . 

Scheme 12. Different photochemical reaction pathways of czs-stilbene (1) (i) to phenanthrene (2) by conrotatory [n6] electrocyclization, and (ii) to tetraphenylcyclobutane (33) by a [2+2] photocycloaddition [67a-d, 68]...

There are a number of processes that can conpete with the di-n-rearrangement and related reactions. As indicated earlier, in the case of the parent DPM rearrangement, unproductive isomerization of the olefins within the substrate can occur when triplet-mediated conditions are applied to acyclic substrates. Electrocyclic processes can also conpete. For exanple, while acetophenone-sensitized irradiation of benzobarrelene (44) affords benzosemibullvalene (8), direct irradiation of the same substrate yields benzocyclooctatetraene (45) via a [2+2] photocycloaddition/cycloreversion sequence. Such electrocyclic processes tend to proceed preferentially from the singlet excited state and take place exceptionally rapidly. In broad terms, cyclic substrates are more likely to suffer from conpeting electrocyclic reactions by conparison with their acyclic counterparts, one reason being the entropic advantage conferred on such processes by more conformationally rigid frameworks. 

The photocycloaddition of cis-1,2-dihydrophthalic anhydride to benzene, naphthalene, or anthracene affords in each case products arising from ( 4j + 4J reaction, a type of addition unknown for cyclohexa-1,3-diene itself. Energy transfer from benzene to the diene anhydride still provided the main reaction pathway, namely electrocyclic closure to give the cis-dihydroDewarbenzene anhydride. The naphthalene photo-adduct (393) on xanthone-photosensitized irradition gave the cage compound (394) similar reaction also occurs with the benzene photo-adduct. 

The 2-I-2-cycloaddition of l,l-difluoro-2,2-bis(dimethylamino)ethene with ethyl propiolate yields a cyclobutene which readily undergoes electrocyclic ring opening to form a diene. The 1,4-diacetal-bridged cycloadduct (7), produced in four steps from the 2 -I- 2-photocycloaddition of DMAD with l,4-dichlorobut-2-ene, reacts with DMAD to provide a synthesis of the Dewar-benzene structure (8) (Scheme 3). ... 

Photocycloadditions are also exploited in the synthesis of fused polycyclic compounds. Tetracyclic (4-hetera)cyclopent[, c]acenaphthylenes can be synthesized by light-induced cycloaddition of 4-alk-l-ynylcoumarins to 2,3-dimethylbut-2-enes (Scheme 6.33). Addition of triplet-excited 4-alk-l-ynylcoumarin to the alkene affords the triplet biradical, which undergoes 1,5-cyclization to cyclopentenylcar-bene. The latter undergoes electrocyclic ring closure and [1,9]-H shift to yield the product [37]. 

Wender reported a clever combination of a [2-i-2]-photocycloaddition of unactivated diene 204 followed by a thermal electrocyclic ring-expansion to give 207 (Scheme 18.33) [137]. The reversible formation of the initial diradi-... 

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