Photochemical intramolecular ring closure

Photochemical Intramolecular Ring Closure Involving the CH Bond of Thiophene Rings... 

By Photochemically Induced Ring Closure via Intramolecular Hydrogen... 

Another possibility of meta substitotion is represented by the intramolecular ring closure reaction of suitable substituent introduced into the para-position. This approach (see Sect. 3.6), so far only rarely used in calixarene chemistry, leads to systems with condensed polycyclic arrangement. As an example we can mention the photochemical condensation of styryl-substituted cafixarene to form a compound containing a phenanthrene moiety (Scheme 3.1). 

In this chapter are summarized the photochemical reactions wherein the primary chemical event is inter- or intramolecular hydrogen transfer to the excited chromophor. In intermolecular reactions hydrogen abstraction usually implies reduction or hydrodimerization of the excited molecule intramolecular hydrogen abstraction is frequently followed by either ring closure of the diradical or fragmentation to afford unsaturated molecules. 

The initial photochemical reaction is intramolecular ortho cycloaddition leading to bicyclo[4.2.0]octa-2,4-diene derivatives. These normally are not detected because they undergo rapid thermal disrotatory ring opening to all-cis cyclooctatrienes which have large extinction coefficients and absorb a second photon to undergo electrocyclic ring closure to the photostable bicyclo[4.2.0] octa-2,7-dienes. 

Almost all triplet-state photocycloadditions studied by Wagner s group [93-103] yield ortho adducts that undergo thermal ring opening to a cyclooctatriene followed by photochemical 4-rr ring closure. Many examples of these reactions are shown in the section devoted to additions via the triplet state. Scheme 51 schematically illustrates the basic reactions for intermolecular as well as intramolecular cycloadducts. A substituted benzene has been chosen for the intermolecular reaction in order to demonstrate the two modes of ring closure. 

Scheme 51 Intermolecular and intramolecular ortho photocycloaddition followed by thermal ring opening and photochemical ring closure.

Intramolecular addition of heteroatom radicals to olefins constitutes a convenient method for the synthesis of heterocycles. The photochemical ring closure reaction of oxyl radical derived from 44 provides access to tetrahydrofuran 45 [95JOC6706]. The regioselectivity in this cyclization is excellent, however, the stereoselectivity is only modest. The stereoselectivity was dependent on the temperature of the reaction. 

The intramolecular insertion into the N—H bond of j8-lactams was used successfully in the synthesis of bicyclic ring systems. Photochemical, in contrast to Rh(II)-catalyzed, decomposition of diazo ester 62 was found to occur far less selectively. In the photolytic reaction, the imide 63 is the major product. It presumably arises by a photolytic Wolff rearrangement to a ketene intermediate, which is trapped intramolecularly. With Rh2(AcO)4 catalyst the Wolff rearrangement is suppressed and 62 undergoes ring closure to 64 nearly quantitatively (80TL31). 

Carbon attack of the carbonyl group of cyclopropyl ketones has also been achieved in dimerization reactions under McMurry and photochemical conditions. Conceivably, the reactions take place via radical anion intermediates. Photodimerization does not occur if structural features of the substrate facilitate intramolecular reactions such as ring opening, ring expansion, and ring closure. Radical... 

Osawa E, Aigami K, Inamoto I (1977) Steric effects in photochemical intramolecular [rt" + ring closure reaction of polycyclic dioleflns leading to strained cage molecules. Empirical force field calculations. J Org Chem 42 2621-2625... 

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