Michael Addition of Aryl Thiolates

The group of M. J. Kurth [36] used polymer-bound enones for the Michael-type addition of aryl thiolates (Fig. 6.20). In the first step 1,4-butanediol was attached to PS-tritylchloride resin. This was followed by an oxidation to the aldehyde, subsequent Wittig reaction and addition of aryl thiolates. Cleavage was performed with formic acid in THF. 

Nine compounds were synthesized via the split-mix method. The compounds were characterized by using GC-MS, HR-MS and by H- and °C NMR, and the isolated yields ranged from 7 % to 27 %. 

Three different pyridine syntheses starting from polymer-bound enones have been reported. The first synthesis starts from 2-alkylidene- or 2-arylidene-B-ketoesters immobilized on Wang or Sasrin resin (Fig. 6.21). These substrates reacted with enaminones in a Hantzsch reaction to 1,4-dihydropyridines which could be oxidized to the corresponding pyridines with ceric ammonium nitrate (CAN). Cleavage was performed with TFA/DCM. Sixteen compounds were synthesized, with HPLC purities of between 70% and 99%. 

By using 6-aminouracils as enaminones, pyrido[2,3-d]pyrimidines were obtained. Four of these compounds were synthesized, with excellent purities (90-100% in HPLC) (Fig. 6.21). 

Grosche et al. [33] established the classic Krohnke pyridine synthesis on the solid phase (Fig. 6.22 (A, B a, c)). The polymer-bound enones used in this reaction were obtained from polymer-bound acetophenones as well as polymer-bound aldehydes, thus providing enones with different substitution patterns (Fig. 6.19 (B and C)). 

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