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Synthesis of Medium‐ and Large‐Ring Compounds Initiated by Photochemical Decarboxylation of ω‐Phthalimidoalkanoates
Author(s) -
Griesbeck Axel G.,
Henz Andreas,
Kramer Wolfgang,
Lex Johann,
Nerowski Frank,
Oelgemöller Michael,
Peters Karl,
Peters EvaMaria
Publication year - 1997
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.19970800324
Subject(s) - chemistry , intramolecular force , decarboxylation , protonation , singlet state , photochemistry , stereoselectivity , electron transfer , amide , singlet oxygen , ring (chemistry) , stereochemistry , ketyl , reactivity (psychology) , organic chemistry , oxygen , catalysis , ion , physics , nuclear physics , benzophenone , excited state , medicine , alternative medicine , pathology
The synthesis of a variety of hydroxylactams from ω‐phthalimidoalkanoates using a triplet‐sensitized photodecarboxylation reaction initiated by intramolecular photo electron transfer is described. Ring sizes available by this method span from 4 (benzazepine‐1,5‐dione 7 ) to 26 (cyclodipeptide 26e ). Ground‐state template formation is proposed as the explanation for the high efficiency of this reaction and for the decrease in reactivity in the presence of organic bases instead of metal carbonates. The crucial step in this macrocyclization reaction seems to be the protonation of the intermediary ketyl radials ( Scheme 4 ). Spacer groups investigated were alkyl chains (C 3 ‐C 11 : 5c‐h, 11a, 12 ), ether ( 16, 18 ), ester ( 20, 22 ), and amide ( 26a‐f ) linkages. Within the detection limits, no dimeric (= decarboxylative coupling) products were observed, indicating the high preference for intra‐ vs. intermolecular photoelectron transfer. The C,C radical combination step proceeds with low stereoselectivity ( cf , products 11 and 12 ) in contrast to comparable singlet reactions. Except for the lactones 22 , all products were stable under the photolysis conditions. Prolonged irradiation of 22 led to the formation of the spiro compounds 23 , probably via an intermediary acyliminium betaine ( Scheme 8 ). One serious limitation of the decarboxylative macrocyclization is its incompatibility with the glycine spacer (as in 27a and 27b ), probably the consequence of a strong intramolecular H‐bond ( Scheme 10 ).