List of used abbreviations - RosDok - Universität Rostock

3-Arylthio-1-trimethylsilyloxy-1,3-butadienes 3a-m were prepared, as previously
reported, by reaction of methyl acetoacetate (1a), methyl 3-oxopentanoate (1b), and
methyl 3-oxohexanoate (1c) with various thiophenols to give methyl 3-
(arylthio)crotonates 2a-m. The latter were subsequently transformed into 3a-m by
deprotonation (LDA) and subsequent silylation. The TiCl4-mediated cyclization of 3a
with 3-silyloxy-2-en-1-one 4a, prepared from acetylacetone, afforded the 2-
(phenylthio)benzoate 5a (Scheme 1). The best yields were obtained when the
reaction was carried out in a highly concentrated solution (stoichiometric ratio: 3a/4a/
TiCl4 = 1.0/1.5/1.5). The solution was slowly warmed from −78 to 20 °C (20 h). The
formation of 5a can be explained by reaction of 4a with TiCl4 to give intermediate A.
The attack of the terminal carbon atom of 3a onto A afforded intermediate B. The
elimination of TMS-siloxane (intermediate C) and subsequent cyclization gave
intermediate D. The elimination of titanium hydroxide (before or during the aqueous
work-up) and aromatization resulted in the formation of product 5a. Due to the
symmetrical structure of A, the attack of 1a on either terminal allylic carbon atom
would result in the formation of the same product (5a).
The cyclization of dienes 3a-m with 3-silyloxy-2-en-1-ones 4a-f afforded the 2-
(thioaryloxy)benzoates 5a-v (Scheme 3, Table 1). Noteworthy, products 5e, 5h and
5k were formed with very good regioselectivity (vide infra). For 5h and 5k, this result
is in agreement with the regiochemical result of the reaction of 4f with 1,3bis(silyloxy)-1,3-butadienes.
12,15 The reaction of 1,3-bis(silyloxy)-1,3-butadienes with
4e has not been previously studied. The yields of products 5 depend on the
substituents of dienes 3. This can be seen by comparison of the yields of products
derived from the same electrophile and different dienes (or vice versa). The
comparison of the yields of 5b, 5f, and 5i (or of 5c, 5g, and 5j) show that higher
yields are often obtained for products derived from dienes containing an alkyl group
attached to carbon C-4 of the diene. However, this effect is not general. The aryl
group of the diene also has some influence. The yields of the products derived from
dienes containing an electron-rich aryl group (5a,l,o) are higher than the products
derived from dienes containing electron-poor aryl groups (5q,t,u). No clear trend is
observed for the influence of the 3-silyloxy-2-en-1-one 4.
15