Bruno Frei

13C nmr spectroscopy of carbonyl compounds : 2. Conformational amd configurational dependence of the (z)-epoxyenone/furan isomerization

Abstract The furan derivative 4 which has been isolated upon photolysis of the epoxyenone compound (E)- 1 results from a thermal isomerization of the intermediate (Z)-isomer (Z)- 1 The latter reaction can best be understood as a multi-centre process implying a nucleophilic attack of the carbonyl oxygen upon the oxirane ring. Inspection of related epoxyenones (e.g. 2 and 3 ) reveals that the ease of the above isomerization sensitively depends upon the conformation with respect to the C-6/C-7 single bond. The conformational properties of a series of epoxyenones as well as of related dienones and epoxydienes have been studied by dynamic 13C NMR spectroscopy. The chemical and spectroscopic information is combined to describe the stereochemical course of the epoxyenone/furan isomerization. As a consequence we can control the thermal reactivity of the title compounds by creating an appropriate conformational behaviour.

Photochemistry of 5.6-epoxy-1,3-dienes : Influence of a 7-hydroxy substituent

Abstract Triplet sensitization of the 7-hydroxy-5,6-epoxy-1,3-diene 10 causes cleavage of the C(5),0 epoxide bond followed by the formation of the three isomers 11 – 13 or induces scission of the C(6), C(7) bond followed by the formation of the aldehyde 14 and the bicyclo[3.2.0]heptanols 15 and 16 . However, irradiation of the corresponding acetate 18 gives only C(5),0 epoxide bond scission ( 18 → f ) followed by the cyclization process f → 19 , the 1,2-Me shifts f → 20 , 21 and the γ-H abstraction f → g leading to the cyclopropane formation g → 22 .

13C NMR of carbonyl compounds -4. Solution conformation of β-ionone and related dienones

Abstract The steric factors relevant for the conformations of β-ionone and structurally related compounds were studied by dynamic n.m.r. and 1 H, 1 H-NOE measurements.

13C NMR of carbonyl compounds. 3. Structure and dynamics of protonated enones and dienones

Abstract The unsaturated ketones 1 1 – 10 10 were protonated at low temperatures and studied by dynamic 13C NMR spectroscopy. Four interconverting stereoisomers were detected which differ in their conformation with respect to the CO and CC (enone) single bond.

13C-nmr of carbonyl compounds 1. Stereodynamic behaviour of acyclic and cyclic dienones

Abstract The conformational mobility of a series of conjugated dienones is investigated by low-temperature NMR- and IR-spectroscopy and interpreted in terms of substituent effects.