2 silyloxy furan

The chiral imidazolidinone 45 also catalyzes the Mukaiyama-Michael reaction between 2-silyloxy furans and a,/ -unsaturated aldehydes, affording enantiomeri-cally highly enriched y-butenolides (Scheme 4.18) [33]. For optimum catalytic performance, hydroxyl additives are necessary, and addition of 2 equiv. water proved best. 

A strategy for the assembly of various carbasugars and aminoc-arbasugars employed condensation of the reagent with 2-silyloxy-furans and 2-silyloxy-Af-protected pyrroles. The additions proceed in high yield and stereoselectivity to afford a,(3-unsaturated lactones and lactams, respectively, which were parlayed into pseu-do-D-gulopyranose, pseudo-o-xylofuranose, pseudo-o-gulopyra-nosylamine, and psuedo-o-xylofuranosylamine (eq 11). ... 

Table 5.1 Silver catalyzed vinylogous Mannich reaction of 2 silyloxy furan 17a with aromatic imines according to Hoveyda, Snapper and coworkers [7].

This protocol could be directly applied in the addition of 2 silyloxy furans to keti mines and the generation of a quaternary chiral center. Using electronically highly activated a ketoimine esters 24 as substrates, the same chiral silver phosphine catalyst (10mol%) as previously used in reactions with aldimines delivered the desired Mannich products in good yields, good diastereoselectivity, and up to 94% ee (Table 5.3) [9]. 

The vinylogous Mannich reaction of 2 silyloxy furans and imines may also be catalyzed through chiral Brpnsted acids, as shown by Akiyama et al. [10]. Previously, Akiyama [11] and Terada [12] had independently discovered that 3,3 substituted BINOL based phosphoric acids were excellent Bronsted acids for a broad range of mainly imine addition reactions via protonation of the imines and in situ formation of chiral iminium contact ion pairs. Using the slightly modified phosphoric acid 28 as catalyst carrying additional iodine substituents in the 6,6 positions, the y amino substituted butenolides 27 were obtained in excellent enantioselectivity and variable diastereoselectivity (Table 5.4). 

This protocol was successfully extended to reactions with 2 silyloxy furan 17a as the nucleophile and y aminoalkyl y butenolides 43 were obtained in high yields, good diastereocontrol, and 88 94% ee (Table 5.8). The 2 thienylsulfonyl group as N substituent proved to be important both for the reactivity and the selectivity of the reaction presumably by way of chelating the copper catalyst and thereby rigidifying the substrate catalyst complex in the transition state of the reaction. 

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