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Stereoselectivity of the Diels‐Alder Reaction of ( E )‐γ‐Oxo‐α,β‐Unsaturated Thioesters with Cyclopentadiene
Author(s) -
Tsai ShihHwa,
Chung WenSheng,
Wu HsienJen
Publication year - 1996
Publication title -
journal of the chinese chemical society
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.329
H-Index - 45
eISSN - 2192-6549
pISSN - 0009-4536
DOI - 10.1002/jccs.199600041
Subject(s) - chemistry , cyclopentadiene , stereoselectivity , diels–alder reaction , organic chemistry , medicinal chemistry , catalysis
The stereoselectivity of the Diels‐Alder reaction of ( E )‐γ‐oxo‐α,β‐unsaturated thioesters 3a‐3d with cyclopentadiene is greatly enhanced in the presence of Lewis acids favoring the endo acyl isomers 4a‐4d . In the absence of Lewis acid, Diels‐Alder reaction of 3a‐3d with cyclopentadiene at 25 °C gave two adducts 4a‐4d and 5a‐5d in a ratio of 1:1 respectively. In the presence of Lewis acids, Diels‐Alder reaction of 3a‐3d with cyclopentadiene gave 4a‐4d and 5a‐5d in ratios of 75‐94:25‐6 respectively. The stereoelectivity was enhanced to ratios of 95‐98:5‐2 with lowering the reaction temperature. The stereochemistry of the cycloadducts 4 and 5 was confirmed by iodocyclization. Reaction of the endo ‐thioester 5c with I 2 in aqueous THF at 0 °C gave the novel methylthio group rearranged product 6c in 80% yield, the first example of iodo‐lactonization of endo ‐thioesters. Reaction of the endo ‐acyl isomer 4b with I 2 under the same reaction conditions gave an isomeric mixture of 7b and 8b in 1:2 ratio. The stereochemistry of the thioester group in 8b was proved by X‐ray single‐crystal analysis. The solvent effect on the endo selectivity of (Z)‐γ‐oxo‐α,β‐unsaturated thioester 2b was also examined.