Photocyclization of Stilbenes and Related Molecules

Abstract

On irradiation in solution with ultraviolet (UV) light, cis ‐stilbene undergoes reversible photocyclization to give trans ‐4a,4b‐dihydrophenanthrene, an intermediate that can be trapped oxidatively with hydrogen acceptors such as iodine, oxygen, or tetracyanoethylene to give phenanthrene in high yield. This type of photoreaction occurs with a wide range of substituted stilbenes and related molecules, including various polycyclic and heterocyclic analogs; in addition, certain systems with a single heteroatom (nitrogen, oxygen, or sulfur) in place of the central π bond undergo photocyclization, creating a new five‐membered heterocyclic ring. In some of these photocyclizing systems, the transient dihydroaromatic intermediates are trapped without added oxidants by the elimination of suitable leaving groups or by various hydrogen shifts. Photocyclization is the preferred method for the synthesis of many different polynuclear aromatic systems. The discovery and early development of stilbene photocyclization has been surveyed, and several other general reviews have appeared.

Photoisomerization and Photocyclization of 5-Styryloxazole

“…22 While the P spectrum displays a slightly structured shape, the DHP spectrum shows a typical bellshaped band shifted to the red at 380 nm, characteristic of an increased conjugation in the closed form. 4,5 The presence of the N heteroatom (not implied in the aromatic sextet) as a new site of basicity decreases Figure 1. Absorption spectra of the two geometrical isomers (E and Z) of 5-StOx in CH together with the DHP intermediate and its oxidation P-type product.…”

“…In the case of the model molecule stilbene, photocyclization leads to an unstable coloured 4a,4b-dihydrophenanthrene (DHP) which, in the presence of an oxidant, loses the two hydrogen atoms in trans stereochemistry to give the stable polynuclear phenanthrene-type (P) arene. [4][5][6] At least (see later) four components (E, Z, DHP and P) are thus generally found in an irradiated solution of the E or Z isomers.…”

“…The process implies an intramolecular 1,n-hydrogen shift 5 which can be base-catalysed, e.g. by a primary amine, the latter operating as the transferring agent.…”

Photocyclization of 3-styrylcyclohex-2-enones to 2,3-dihydro-1H-phenanthren-4-ones

“…As model compounds we selected substituted 5,5-dimethyl-3-styrylcyclohex-2-enones (3) and investigated their photochemical cyclization to 2,2-dimethyl-2,3-dihydro-1H-phenanthren-4-ones (4). Whereas the photoisomerisation and oxidative cyclization of stilbenes to phenanthrenes is a well known process (reviews: 3,4 ), the photochemical behavior of the phenylsubstituted dienones such as 3a-e has not yet been investigated in detail. As required for the photocyclization of stilbenes, a trans/cis-photoisomerisation has to precede the photocyclization reaction.…”

Formal (4+1) Cycloaddition of Methylenecyclopropanes with 7‐Aryl‐1,3,5‐cycloheptatrienes by Triple Gold(I) Catalysis

“…Thus, compound 5 f can be transformed into a cyclopenta derivative of benzo [g]chrysene (6) by a one pot photo-induced isomerization/oxidative Mallory cyclization. [13] Tetrasubstituted MCP 3 m reacted with 1 a to give only the product of cyclopropanation 7 (Scheme 3 and Figure 1), whose structure was confirmed by X-ray diffraction (Figure 1). [12] Given that 3 m does not undergo ring-expansion, the isolation of spiro derivative 7 strongly suggests that the cyclopropanation of MCP is not the initial step in the formal (4+1) cycloaddition and that cyclobutenes are likely intermediates in this transformation.…”