M. Cavazza et al., Synthesis of 1-oxaazulan-2-ones and furanotropones from troponoids: a reexamination and extension to colchicinoids, TETRAHEDRON, 56(13), 2000, pp. 1917-1922
Reactions of [3,5,7-H-2(3)]-2-tosyloxytropone (1b) in DMSO with enolates th
at, like sodium ethyl acetoacetate or sodium diethyl malonate, bear a leavi
ng group, occur at C-7, followed by either sequential protonation at C-2, 1
,6-elimination, and intramolecular heterocyclization (to give 1-oxaazulan-2
-ones), or sequential sulfinate loss and intramolecular heterocyclization (
to give furanotropones). The latter is the exclusive route with enolates th
at, like sodium acetylacetonate, do not bear a leaving group. 9-Tosyloxyiso
colchicide (15) behaves like 2-tosyloxytropone, giving the product 16 of C-
11 attack, whereas 10-tosyloxycolchicide (17) resists attack at C-8 and onl
y a very slow nucleophilic attack at C-10 by the enolate to give 18 is obse
rved. Hydroxylic solvents do not allow any of these processes. (C) 2000 Pub
lished by Elsevier Science Ltd. All rights reserved.