ISSN:
0018-019X
Keywords:
Chemistry
;
Organic Chemistry
Source:
Wiley InterScience Backfile Collection 1832-2000
Topics:
Chemistry and Pharmacology
Notes:
Kinetic and equilibrium studies show that typical xenicanes such as dictyolactone (1) and 4-hydroxydictyolactone (3) undergo slow conformation medium-ring flipping between the predominant trans-(1a or 3a; Me(20) trans to H-C(3)) and the minor cis-conformers (1b or 3b; Me(20) cis to H-C(3); see Scheme 1). The formation of the latter is inhibited in heterocyclic-ring-opened congeners such as 18-acetoxy-4-hydroxydictyo-19-al (7). Molecular-mechanics calculations suggest that typical-xenicane cis-conformers are disfavoured by mainly C(4)-C(5) torsional strain. This is confirmed by the observation of two sizably populated cis-and trans-conformers for the unnatural 4-oxoxenicanes 10-12. Unusually facile thermal (E)→(Z) isomerization of xenicanes 1,3,10-12, and 7 is also observed (→ 13-17 and 9, resp.; Scheme 3), reflecting great strain relief in the transition state. Conflicting results in the literature now fit into this scheme which provides a basis for unravelling recognition phenomena with these biologically active systems.
Additional Material:
3 Ill.
Type of Medium:
Electronic Resource
URL:
http://dx.doi.org/10.1002/hlca.19940770503
