Strained ring compounds photochemical cycloaddition

Cyclic azoalkanes continue to be of active interest because they serve as precursors to interesting diradicals and as synthons for the preparation of highly strained ring systems and sterically crowded structures. One of the most important syntheses of the azoalkanes involves the cycloaddition of TADs to a suitable substrate to give urazoles by a method mentioned in preceding parts of this review. These methods include Diels-Alder, homo Diels-Alder, and domino Diels-Alder addition, as well as the ene reaction, 1,2-cycloaddition, or other types of cycloaddition reactions. These adducts are transformed into cyclic azoalkanes by hydrolysis an oxidation. The azoalkanes are very often used for thermal or photochemical decomposition to cyclic compounds. This sequence is outlined in Scheme 79. 

Cycloaddition of carbon-carbon double bonds can also occur intramolecularly. Direct irradiation of simple dienes leads to cyclobutanes. This is a singlet-state process and is concerted. The stereochemistry of the cyclobutane can be predicted on the basis of orbital-symmetry rules (Part A, Section 10.1). Nonconjugated dienes can also undergo photochemical cyclization employing mercury or carbonyl compounds as sensitizers. Cyclobutane formation is usually unfavorable with 1,4-dienes because it would result in a very strained ring system. When the alkene units are separated by at least two carbon atoms, cyclization becomes more favorable sterically ... 

There are so many different examples of photochemical dimerizations and cross-cycloadditions 8-11,13-17) 0f olefinic compounds that one is not surprised to find several variations of mechanistic patterns. Simple olefins, including dienes and strained small ring, bicyclic olefins and styrene derivatives form a class of compounds that undergo such reactions sensitized by triplet energy donors. Some examples axe given in Eqs. 19—23, where only cyclobutane products are depicted. Theory... 

Fused aziridines are interesting compounds owing to the fact that the strained three-membered ring can easily open and cause dipolar cycloaddition reactions as well as their photochromic properties. Therefore, most of this chapter covers the chemical and photochemical properties of bi- and tricyclic aziridines. Some properties of aziridinyl ketones are also reviewed, in particular, reactions leading to aziridinyl anils. 

In the proposed set of events, it was envisioned that the strategically positioned C—C double bonds in 26 could be induced to participate in a photochemically allowed [2+2] cycloaddition, thereby affording tetracyclic intermediate 25. Due to the particular spatial arrangement of the carbonyl group and the E-ring nitrogen atom in this strained product, this new compound was not expected to... 

Unless they have considerable strain in their ring systems, cyclophanes can be formed by most conventional bond-forming reactions. The first photochemical approach was applied by Schonberg et al. to prepare cage compound 1, a cyclophane, in 1968. Moreover, styrene photocyclodimerization (Scheme 1) was also published in the same year, > although it took almost two decades before we first successfully applied the reaction to cyclophane synthesis." The mechanism (Scheme 1) of [2 + 2] cycloaddition with direct photoirradiation of vinylarenes and their derivatives has been published."... 

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