1,4-Additions 2-trimethylsilyloxyfuran

Likewise, addition of enol silyl ethers such as 980 to the intermediate 977 a furnish the 5-trimethylsilylmethylisoxazolidine 981 in 61% yield and 15% isoxazoline 982 [73, 74] whereas addition of 2-trimethylsilyloxyfuran 827 to 977 a affords, via the intermediates 983, on work-up with CF3CO2H, 96% yield of a mixture of lactones 984 and 985 [75] (Scheme 7.23). More recently it has also been reported that Danishefsky (trimethylsilyloxy)dienes add to intermediates such as 977 to give the corresponding products [76]. 

Triisopropylsilyloxyfurans were effective nucleophiles for the vinylogous Mannich addition to iminium ions that were formed by Rh2(cap)4-catalyzed oxidation of N-alkyl groups by THYDRO <06JA5648>. A stereoselective addition of 2-trimethylsilyloxyfurans to aryl aldehydes-derived aldimines employing a chiral phosphine/Ag complex as catalyst was developed <06AG(I)7230>. The prototypical example is shown below. 

L2909>. An organocatalytic addition of 2-trimethylsilyloxyfuran to aldehydes using 10 mol% of l,3-bis(3-(trifluoromethyl)phenyl)urea provided adducts with modest threo selectivity <06TL8507>. A syn-selective, enantioselective, organocatalytic vinylogous Mukaiyama-Michael addition of 2-trimethylsilyloxyfuran to (E)-3-... 

Scheme 6.32 y-Butenolides obtained from diastereoselective aldol addition of 2-trimethylsilyloxyfuran to aldehydes catalyzed by urea 32. 

Racemic tandem additions Mild carbon nucleophiles add to the [Os]-anisolium complex 27 exclusively at C3 to afford substituted 1,3-cydohexadiene complexes 29-32 in moderate to high yields (Table 8) [21]. Nucleophiles that have been utilized in this manner include MMTP (29 and 30), N-methylpyrrole (31), and 2-trimethylsilyloxyfuran (32). As with other tandem additions to [Os] complexes, both the nucleophile and the electrophile add to the arene face opposite to that involved in metal coordination, such that the products are those of syn addition. 

The bicyclic isoxazolidine 76 was prepared by nucleophilic addition of 2-trimethylsilyloxyfuran to A-gulosyl-nitrones and employed in a formal synthesis of Polyoxin... 

Regioselective addition of 2-methoxyfuran and 2-trimethylsilyloxyfuran to chromium(O) alkynylcarbene complexes furnished interesting dienyne and dienediyne carboxylates <07AGE2610>. The reaction likely proceeded through a formal vinylogous Michael addition adduct, as illustrated in the following example. 

A highly yvn-diastereoselective and enantioselective vinylogous Mukaiyama aldol addition of trimethylsilyloxyfurans to aldehydes was achieved by using a cinchonidine-... 

Structural characterizations of reaction intermediates and products of the addition of 2-trimethylsilyloxyfuran to naphthoquinones <1998TA1257> and benzoquinones <1999TA4357> to form furanofuranones indicated that the reaction proceeded via Michael addition, rather than Diels-Alder cycloaddition, in which the type of intermediate 9 shown in Scheme 3 was observed by proton nuclear magnetic resonance (NMR) spectroscopy. 

A ry -selective, organocatalytic, enantioselective vinylogous Mukaiyama-Michael addition of 2-trimethylsilyloxyfuran to a,/ -unsaturated aldehydes to produce 7-butenolides was achieved by using a chiral amine catalyst... 

Vinylogous Mukaiyama-Michael additions of 2-trimethylsilyloxyfuran to 3-alkenoyl-2-oxazolidinones to provide 7-butenolides were shown to be /7-selective. The reaction could be rendered enantioselective in the presence of a (T symmetric copper-bisoxazoline complex <1997T17015, 1997SL568> or a l,T-binaphthyl-2,2 -diamine-nickel(ii) complex as catalyst, as depicted in Equation (16) <2004CC1414>. 

Trimethylsilyloxyfuran reacted stereoselectively with chiral tungsten carbene complexes in a Mukaiyama-Michael addition fashion to provide -products, as shown in Equation (18) <2005AGE6583>. The metal carbene in the butenolide product serves as a useful functional group for further transformations. 

The less well-studied 3-silyloxyfuran was shown to react with aldehydes in an aldol addition manner under Lewis acidic conditions. High 5yn-diastereoselectivity was obtained with bulky aldehydes <05OL387>. 4-Alkoxy-3-lithio-2-silyloxyfurans reacted with a variety of electrophiles to form 3-substituted tetronates after acidic hydrolysis <05SL2735>. Furans and 2-trimethylsilyloxyfuran are effective nucleophiles in the organocatalytic tandem... 

Michael addition/enamine chlorination reaction <05JA15051>. 2-Trimethylsilyloxyfuran reacted readily with chiral tungsten carbene complexes in a Mukaiyama-Michael addition manner to give the anti products selectively, as depicted in the example below <05AGa)6583>. 

New Au(III)-pyridine-2-carboxylate complexes were developed to catalyze the intramolecular reaction between furan and acetylene to form phenols <04AE(G)6545>. These pre-catalysts provide higher reaction conversion than AuClj. The Lewis acid catalyzed vinylogous Mukaiyama-Mannich addition of trimethylsilyloxyfuran to aldimines, that generates 6-amino-Y-butenolide intermediates, was applied to the synthesis of piperidines... 

In the aldol reaction of 2-trimethylsilyloxyfuran with aldehydes catalyzed by (BINOL)2Ti complex, a significant impact of the product on the enantioselectivity of the catalysis was observed [124]. As shown in Scheme 14.44, the addition of 5 mol% of the product (82% ee) in the catalyst can enhance the enantiomeric excess of the product from 70% to 96%. Therefore, an asymmetric autoinduction might be involved in the catalytic system. On the basis of this observation. 

Trimethylsilyloxyfuran 338 has shown promise as a masked butenolide fragment To fuUy exploit these qualities, the threo versus erythro (339 vs 340) diastereoselectivity in aldol-type additions has to be controlled. In fact it has been shown that this is easily achieved by appropriate reaction conditions. Applying Mukaiyama conditions (i.e., using the silyl enol ether as the donor in the presence of a Lewis acid such as TESOTf to generate oxonium species) leads to threo preference for 339, presumably via an open transition state, whereas desilylation with TBAF generates the erythro-diastereomer 340, this time via a closed Diels-Alder (or Zimmerman-Traxler)-like transition state. In both cases, chelating effects can be ruled out... 

Silver(I) complexes with Tol-BINAP (270) were used by Yamamoto and coworkers for mediating diastereoselective and enantioselective Mukaiyama aldol additions. According to the authors conclusion, the mechanism does not involve transmetallation to silver enolates but follows the usual carbonyl group activation [135]. Hoveyda and coworkers used silver(II) fluoride in the presence of a dipeptide-type ligand for enantioselective additions of silyl enol ethers to a-keto esters [136]. The reaction of 2-trimethylsilyloxyfuran with aromatic and aliphatic aldehydes was catalyzed with chromium salen complex in the presence of protic additives like isopropanol [137]. Various protocols of enantioselective Mukaiyama aldol reactions that use water as cosolvent have been elaborated ... 

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