Stereoselektivität von Photocycloadditionen, 5^[1] Photoreaktionen zwischen 2‐Cyclopenten‐1‐on und Enolethern

Abstract

Stereoselectivity in Photocycloaddition Reactions, 5[1]. – Photoreaction of 2‐Cyclopenten‐1‐one with Enol Ethers The regio‐ and stereoselectivity of [2 + 2] photocycloaddition reactions between 2‐cyclopenten‐1‐one (1) and the electron‐rich alkenes: ethyl, isopropyl, and phenyl vinyl ether (2–4) as well as 2,3‐dihydrofuran (10) were examined. In all cases the annulated cyclobutene derivatives 5–7 and 11 were isolated in yields between 70 and 99%. The regioisomeric HT/HH products were formed in a ratio of 3:1 to 4:1. For the determination of the product stereochemistry, INADEQUATE and 1H,1‐decoupling experiments were performed. Computer‐assisted investigations on the molecular geometry and 1H,1H‐coupling constants were performed by using semiempirical methods (AM1, FINITE). The HT regioisomers were formed with moderate endo diastereoselectivity, whereas in the case of the HH regioisomers both diastereomers (exo/endo) were formed in equal amounts. Photoreaction with 2,3‐dihydrofuran (10) led to almost exclusive formation of exo‐diastereomeric HH and HT regioisomers 11. The observed stereoselectivity is rationalized by the optimal geometry of the intermediate triplet 1,4‐biradicals during intersystem crossing.

Stereoselective Intermolecular [2 + 2]-Photocycloaddition Reactions and Their Application in Synthesis

“…29 In analogy to cyclic alkenes, alkyl substituted alkenes add to photoexcited cycloalkenones in an exofashion. 20,30,31 Alkoxy substituted alkenes which do not have an additional a-substituent appear to favor an endoaddition although contradicting data are found in the literature. 29,[31][32][33] Preparatively useful selectivities were reported for the cycloaddition of 2-(trimethylsilyloxy)buta-1,3-diene (12) and related silyl enol ethers to various alkenones (Scheme 6).…”

“…20,30,31 Alkoxy substituted alkenes which do not have an additional a-substituent appear to favor an endoaddition although contradicting data are found in the literature. 29,[31][32][33] Preparatively useful selectivities were reported for the cycloaddition of 2-(trimethylsilyloxy)buta-1,3-diene (12) and related silyl enol ethers to various alkenones (Scheme 6). 34,35 The reaction of compound 12 with cyclopentenone 11 gave the corresponding adduct 13 which was further transformed into a potential pentalenolactone precursor.…”

“…35 The selectivity in enol ether photocycloadditions was attributed by Griesbeck et al to the preferential intersystem crossing (ISC) geometries of the intermediate 1,4-biradicals. 31,36 For the sensitized endo-addition of an enol acetate to maleic anhydride, second-order orbital and electrostatic interactions were suggested to explain the simple diastereoselectivity. 37 Stereochemical selection rules based on a stepwise mechanism were proposed for the dioxopyrroline-alkene photocycloaddition by Sano and Tsuda.…”

Spin‐Selectivity in Photochemistry: A Tool for Organic Synthesis

“…58 The head-to-head cycloadduct 52a was the minor product, which was also observed with isopropyland the phenylvinylether 51b and 51c, respectively. 59 The major products were the head-to-tail cyclobutanes 53. All cyclobutanes were formed with moderate diastereoselectivity, the head-to-tail cyclobutanes showed (contrathermodynamic) 3:2 endo/exo-ratios (Scheme 29).…”

A Photochemical Route to 3- and 4-Hydroxy Derivatives of 2-Aminocyclobutane-1-carboxylic Acid with an all-cis Geometry

“…Neither of these approaches was appealing so we sought a more robust synthesis of the targets and endeavored to obtain them in enantiomerically pure form. [2 + 2]-Photocycloaddition reactions constitute a powerful entry to multifunctional cyclobutanes, − so we decided to examine the photochemical reaction between an alkyl vinyl ether and a cyclic derivative of maleic acid: either maleimide or maleic anhydride. ,, Our rationale was that the bicyclic photoproducts should be formed preferentially with an endo configuration, by analogy with MO calculations on the reaction of maleic anhydride with vinyl acetate, as well as the mechanistic proposal by Griesbeck for the [2 + 2]-photocycloaddition reaction between vinyl ethers and cyclopentenone, wherein the endo geometry is favored by the 1,4-biradical prior to cyclization . The photoproducts would then serve as useful intermediates leading to either all-cis -3-HACBC or all-cis -4-HACBC via controlled regioselective chemical transformations (Scheme ).…”