Diastereoselective Intramolecular meta Photocycloaddition of Side-Chain-Substituted 5-(2-Methoxyphenyl)pent-1-enes

Abstract

Irradiation of a series of 5‐(2‐methoxyphenyl)pent‐1‐enes substituted with a hydroxy or trimethylsilyloxy group at the α‐, β‐, or γ‐position of the side‐chain yields in all cases meta photocycloadducts, in which the configuration at the substituted carbon atom is mainly endo. This indicates that the diastereoselectivity originates from minimization of steric interactions between the side‐chain substituent and the ortho‐methoxy group at the arene unit. Hydrogen bonding does not seem to be involved. The introduction of the side‐chain substituents also influences the regioselectivity of the addition: The linear to angular adduct ratios are significantly increased compared to the case of the parent compound.

The Double [3+2] Photocycloaddition Reaction

“…Scheme 1.8: Bryce-Smith"s advocated biradical species, called a "prefulvene" 33, the precedent species to benzvalene (34), the meta photoadduct (35) and fulvene (36).…”

“…A further illustration of this was conducted by Jan Cornelisse et al 35 in his studies toward "diastereoselective control of intramolecular meta photocycloadditions of side-chain-substituted 5-(2-methoxyphenyl)-pent-1-enes". They also reported that when there is an -substituent attached to the side-chain, the stereochemistry at that centre results in stereoselectivity in the photoadduct, such that the endo varieties (66 and 67) are predominant (Scheme 1.1.5).…”

“…The anti periplanar route seemed to hold true in this case, and hydrogen-bonding was thought to "not play a role in determining the diastereoselectivity of the reaction". 35 …”

“…35 However this was only achieved in a disappointing 22% yield and our own repetition of this Grignard reaction resulted in no significant improvement.…”

“…This compound had previously been prepared by Cornelisse, Lodder et al 35 A magnetically stirred solution of ketone 256 (3.00 g, 15.8 mmol) in MeOH (20 ml) was cooled to 0 °C.…”