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Konkurrenz von Endoperoxid‐ und Hydroperoxid‐Bildung bei der Umsetzung von Singulett‐Sauerstoff mit cyclischen, konjugierten Dienen. Teil I
Author(s) -
Matusch Rudolf,
Schmidt Gerhard
Publication year - 1989
Publication title -
helvetica chimica acta
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.74
H-Index - 82
eISSN - 1522-2675
pISSN - 0018-019X
DOI - 10.1002/hlca.19890720107
Subject(s) - chemistry , photooxygenation , singlet oxygen , conformational isomerism , yield (engineering) , medicinal chemistry , stereochemistry , oxygen , organic chemistry , molecule , materials science , metallurgy
Competition of Endoperoxide and Hydroperoxide Formation in the Reaction of Singlet Oxygen with Cyclic, Conjugated Dienes Rose‐bengal‐sensitized photooxygenation of (−)‐( R )‐α‐phellandrene ( 1 ) in MeOH at room temperature yielded a complex mixture of products, contrary to previous reports describing cis ‐(3 S , 6 R )‐epidioxy‐ p ‐menthene ( 2 ) and trans ‐(3 R , 6 S )‐epidioxy‐ p ‐menthene ( 3 ) as the only products. The mixture was separated by prep. HPLC (silica gel, pentane/Et 2 O 9:1). Besides the known endoperoxides 2 (yield 39%) and 3 (26%), all those hydroper‐oxides, which can be deduced from an ene reaction of 1 O 2 with 1 , were isolated, i.e. 4β‐ p ‐mentha‐2,5‐dien‐1β‐yl hydroperoxide ( 4 ) (14%), 4β‐ p ‐mentha‐2,5‐dien‐1α‐yl hydroperoxide ( 5 ) (9%), (2 R , 4 R )‐ p ‐mentha‐1(7), 5‐dien‐2‐yl hydroperoxide ( 6 ) (2,1%), (2 S , 4 R )‐ p ‐mentha‐1(7),5‐dien‐2‐yl hydroperoxide ( 7 ) (1,5%) and (1 R )‐ p ‐mentha‐3,6‐dien‐yl hydroperoxide ( 8 ; 1,5%; Scheme 1 ). Furthermore, the constant cis / trans ratio for all diastereoisomeric pairs ( 2 / 2 , 4 / 2 , 6 / 2 ) was striking. With the help of the two possible conformers 1a and 1b of the starting material a model of a common first step for endoperoxide as well as for hydroperoxide formation is developed. A photooxygenation at −50° supports this model. The absolute value of the cis / trans ratio changes in the same way for the endoperoxides and the hydroperoxides.