Premium
A New Phototransformation of Methoxycarbonyl‐Substituted ( E , Z , E )‐1,3,5‐Hexatrienes: Easy Access to Ring‐Annelated 8‐Oxabicyclo[3.2.1]octa‐2,6‐diene Derivatives
Author(s) -
von Essen Regina,
von Zezschwitz Paultheo,
Vidović Denis,
de Meijere Armin
Publication year - 2004
Publication title -
chemistry – a european journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.687
H-Index - 242
eISSN - 1521-3765
pISSN - 0947-6539
DOI - 10.1002/chem.200400238
Subject(s) - ring (chemistry) , chemistry , stereochemistry , medicinal chemistry , organic chemistry
On attempting photochemically induced electrocyclizations of the previously reported 1,6‐bismethoxycarbonyl‐ or 1,6‐bistrimethylsilyl‐substituted ring‐attached ( E , Z , E )‐1,3,5‐hexatrienes 4 b , 4 c and 5 b , 5 c , equilibrium mixtures of the starting materials and their diastereomers, the corresponding ( E , Z , Z )‐hexatrienes 4 b , 4 c and 5 b , 5 c were obtained. The desired trans ‐disubstituted ring‐annelated cyclohexadienes 9 b and 10 b were formed by subsequent thermal 6π‐electrocyclization of the ( E , Z , Z )‐hexatrienes 4 b and 5 b in good yields (77–83 %). Upon treating the bissilyl‐substituted hexatrienes ( E , Z , E )‐ 5 c or ( E , Z , Z )‐ 5 c under the same conditions, 6π‐electrocyclizations did occur, but the primary products immediately isomerized to a large extent, and mixtures of the cyclohexane‐annelated cyclohexadienes 10 c – 12 c along with the dehydrogenation products 13 , 14 c were formed. When the bismethoxycarbonyl‐substituted hexatriene ( E , Z , E )‐ 5 b was irradiated for an extended period of time (4.5 h), the gradual formation of the oxabicyclo[3.2.1]octa‐2,6‐diene 17 b by a formal intramolecular hetero‐Diels–Alder reaction was observed and 17 b could be isolated in up to 69 % yield. To explore the scope of this new photochemical reaction, the new ring‐attached ( E , Z , E )‐hexatrienes 4 a , 5 a and 6 b were synthesized by twofold Heck reactions from 1,2‐dibromocycloalkenes 1 – 3 (59–66 %). While irridiation of the cyclopentene‐attached 1,3,5‐hexatrienes only led to decomposition, the cyclohexene‐ and cycloheptene‐attached hexatrienes gave the hetero‐Diels–Alder products or other photoproducts depending on the size of the cycloalkene moiety and the nature of the alkoxycarbonyl substituents at the vinyl termini. The photoreaction products 17 b and 18 b which are bicyclic acetals, underwent cleavage upon treatment with a Lewis acid such as Me 3 SiOTf to give the ring‐annelated methoxycarbonyl‐substituted troponecarboxylates 21 b and 22 b .