Thiourea derived catalysts vinylogous

S. Oxindole Derivatives. Most recently, Curti et al. [140] disclosed the first example of a direct, organocatalytic asymmetric vinylogous Michael addition of 3-alkylidene oxindole to nitroalkenes. Bifunctional cinchona alkaloid/thiourea catalyst 69 could effectively promote the reaction, solely aHbrding the 7-substituted 3-alkylidene oxindoles 146 with excellent regio-, diastereo-, and enantioselectivities (Scheme 5.71). Importantly, both aromatic and aliphatic substituted nitroalkenes were applicable for such a reaction. 

The stereoselective vinylogous Michael addition-cyclization cascade of alkylidene malononitriles and 3-alkylideneoxindoles was effectively catalyzed by rosin-derived thiourea 36 to give spirocyclic oxindoles with excellent stereoselectivity (Scheme 35) [61]. Although simple thiourea 20 (see Scheme 16) provided low stereoselectivity (dr = 3 1, 29% ee), introduction of the rosin unit into the thiourea core significantly improved the diastereoselectivity to >20 1 the pyrrolidine structure was essential for high enantioselectivity. The effect of the chirality of the rosin component on the absolute stereocontrol was negligible and, thus, the major proposed role of this structural unit was to increase the steric hindrance of the catalyst. One of the salient features of this protocol is the... 

The utility of the alkylidene oxindole-derived vinylogous enolates was demonstrated in the y-selective asymmetric conjugate addition to nitroolefins (Scheme 38) [67, 68]. Dihydroquinine-derived thiourea 10 earned distinctirm as the most effective catalyst in terms of catalytic efficiency and stereocontroUing ability. The tertiary amine and the thiourea functionalities both appeared to be essential for ensuring catalytic activity. It should be noted that not only perfect y-selectivity but also very high /Z-selectivity were observed under the optimal conditions. A wide variety of nitroolefins and substituted alkylidene oxindoles were amenable to this protocol. 

A vinylogous Michael addition of 3-alkylidene oxindoles (441), normally regarded as good Michel acceptors, to nitroalkenes, has been attained in the presence of the cinchona-derived thiourea catalyst (372). This rare example of organocatalytic umpol-ung, presumably proceeding via the hydrogen-binding stabilized enol (442), afforded the adducts (443) in a >99 1 y.a ratio with 10 1 to >20 1 Z/E) selectivity and 97 to >99%... 

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