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Stereoselektivität von Photocycloadditionen, 5 [1] Photoreaktionen zwischen 2‐Cyclopenten‐1‐on und Enolethern
Author(s) -
Griesbeck Axel G.,
Stadtmüller Stefan,
Busse Holger,
Bringmann Gerhard,
Buddrus Joachim
Publication year - 1992
Publication title -
chemische berichte
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.667
H-Index - 136
eISSN - 1099-0682
pISSN - 0009-2940
DOI - 10.1002/cber.19921250427
Subject(s) - chemistry , stereoselectivity , diastereomer , structural isomer , stereochemistry , cycloaddition , crotonaldehyde , organic chemistry , catalysis
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 1 H, 1 ‐decoupling experiments were performed. Computer‐assisted investigations on the molecular geometry and 1 H, 1 H‐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.