By a flexible three‐component synthesis, alkoxy‐substituted enamides are easily available from lithiated alkoxyallenes, nitriles and carboxylic acids (see scheme). The treatment of these versatile intermediates with trifluoroacetic acid provided 5‐acetyloxazoles in moderate to good yields. Different substituents are possible at C‐2 and C‐5 and the 5‐acetyl group is a suitable handle for further synthetic transformations.
Summary. The structures and relative energies of fundamental conformations of cyclopenta-1,2,3-triene, cyclohexa-1,2,3-triene, cylohepta-1,2,3-triene,
cycloocta-1,2,3-triene, and cyclonona-1,2,3-triene were calculated by the HF/6-31G* and MP2/6-31G*//HF/6-31G* methods. Only a C2v symmetric planar conformation is available to cyclopenta-1,2,3-triene and cyclohexa-1,2,3-triene. The calculated energy barrier
for ring inversion of the CS symmetric puckerd conformation of cyclohepta-1,2,3-triene via the planar geometry is 62.2 kJ·mol−1. The C2 symmetric twist conformation of cycloocta-1,2,3-triene was calculated to be the most stable one. Conformational racemization
of the twist form takes place via the CS symmetric half-chair geometry, which is by 60.8 kJ·mol−1 less stable than the twist conformer. The CS symmetric chair and unsymmetrical twist-boat conformations of cyclonona-1,2,3-triene were calculated to have similar energies;
their interconversion takes place via an unsymmetrical low-energy (18.4 kJ·mol−1) transition state. The twist (C2) and boat (CS) geometries of cyclonona-1,2,3-triene are higher in energy by 13.2 and 33.9 kJ·mol−1, respectively. Ring inversion in chair and twist-boat conformations takes place via a twist form as intermediate and requires 33.6 kJ·mol−1.
Corresponding author. E-mail: isayavar@yahoo.com
Received March 25, 2002; accepted April 4, 2002 相似文献
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