Indoles thioureas derived from

Ricci and co-workers introduced a new class of amino- alcohol- based thiourea derivatives, which were easily accessible in a one-step coupling reaction in nearly quanitative yield from the commercially available chiral amino alcohols and 3,5-bis(trifluoromethyl)phenyl isothiocyanate or isocyanate, respectively (Figure 6.45) [307]. The screening of (thio)urea derivatives 137-140 in the enantioselective Friedel-Crafts reaction of indole with trans-P-nitrostyrene at 20 °C in toluene demonstrated (lR,2S)-cis-l-amino-2-indanol-derived thiourea 139 to be the most active catalyst regarding conversion (95% conv./60h) as well as stereoinduction (35% ee), while the canditates 137, 138, and the urea derivative 140 displayed a lower accelerating effect and poorer asymmetric induction (Figure 6.45). The uncatalyzed reference reaction performed under otherwise identical conditions showed 17% conversion in 65 h reaction time. 

Some bifunctional 6 -OH Cinchona alkaloid derivatives catalyse the enantioselective hydroxyalkylation of indoles by aldehydes and a-keto esters.44 Indole, for example, can react with ethyl glyoxylate to give mainly (39) in 93% ee. The enan- tioselective reaction of indoles with iV-sulfonyl aldimines [e.g. (40)] is catalysed by the Cu(OTf)2 complex of (S)-benzylbisoxazoline (37b) to form 3-indolylmethanamine derivatives, in up to 96% ee [e.g. (41a)] 45 Some 9-thiourea Cinchona alkaloids have been found to catalyse the formation of 3-indolylmethanamines [e.g. (41b)] from indoles and /V-PhS02-phenyli mines in 90% ee.46 Aryl- and alkyl-imines also give enantioselective reactions. 

The simply obtainable thiourea compounds 142-145 were the first organo-catalysts for the catalytic conjugate addition of indoles with nitroalkenes to yield optically active 2-indolyl-l-nitro derivative as 2.R-50 in fairly good yields and enantioselectivity. The simple thiourea-based organo catalyst 145 could be easily accessed in both enantiomeric forms from the commercially available materials. At the same time, the extremely simple methodology has proved the new approach useful for the synthesis of optically active target... 

As with most groups of compounds, silica gel has generally proved to be the most widely used adsorbent, although cellulose, alumina, and polyamide layers have sometimes had important uses. The main contributions have been summarized in Table 22. The /f/values for alkyl, hydroxyindoles, and a series of 3-substituted derivatives on silica gel are listed in Table 23 with the solvents A to E (197,203), Table 24 records the ) /values for some of the same compounds on cellulose with the solvents A and B shown. The / /values of a group of 3-substituted indoles on silanized silica gel with and without the addition of //-dodecylpyridinum chloride are shown in Table 25. Apart from these types of layers, polyamide has been employed (205). Biogenic amines, including serotinin, have been examined on silica gel in 23 different solvents with detection by means of thiourea (206). Spray... 

Phosphoric acid 25a was successfully employed for mediating enantioselective synthesis of 1,3-disubstituted isoindolines from electrophilic bifunctional substrates (containing an imine and a Michael acceptor site) and indoles [33]. Catalyst 7a was used effectively in an organocatalytic asymmetric F-C alkylation/cyclization cascade reaction between 1-naphthols and a,(i-unsaturated aldehydes to give chro-manes in good yields and select vities (Scheme 3 5.18) [ 10]. Furthermore, 2-naphthols and p,y-unsaturated a-keto ester also reacted in a F-C alkylation/dehydration sequence, in the presence of a thiourea catalyst and a catalytic amount of concentrated sulfuric acid, affording optically active naphthopyran derivatives [54]. 

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