Siloxyfurans

A palladium catalyst is used in the transformation of a siloxyfuran to a phenyl substituted furanone <00JCS(P1)3350>. Similar products, furan-2(5//)-one derivatives, are afforded through the reaction of tetra-n-butylammonium fluoride with the corresponding substituted 2-siloxyfuran <00S1878>, as well as the oxidation of 3,4-disubstituted furans by singlet oxygen . 

Under similar conditions, the same authors were able to control two stereogenic centers in an asymmetric vinylogous Mannich reaction. Indeed, treatment of imines derived from aryl a-ketoesters with siloxyfuran under related conditions gave functionalized y-butenolides with high diastereo- and enantioselectivities (Scheme 10.21 ).40... 

Trimethylsilyloxy)furan can also be used as a functionalized silyl enol ether for the asymmetric catalytic aldol-type reaction. Figadere has reported that the reaction of aliphatic aldehydes with the siloxyfuran catalyzed by BINOL-derived titanium complex provides the diastereomeric mixtures with high enantioselectivity (Sch. 42) [107], The addition reaction proceeds at the y position of the siloxyfuran to give butenolides of biological and synthetic importance. 

Chiral cycloadduct 134 assembled from a tethered 2-siloxyfuran was treated with PPTS to reveal the hydroxycyclohexenone 136 (Scheme 8) [143]. Other natural products that have been synthesized employing this strategy include triptonide and triptolide [151]. 

Recendy, the first total synthesis of taxodone was accomplished via this strategy [152]. Cycloadduct 153, readily available from the Diels-Alder reaction of siloxyfuran 152 and methyl acrylate, was treated with acid to induce ring opening and dehydration to afford phenol 154, Eq. 103. 

Scheme 13 Rhodium-catalyzed oxidative Mannich reaction of tertiary amines with 2-siloxyfurans...

Doyle and co-workers demonstrated the use of 2-siloxyfurans 22 as a nucleophile for the oxidative coupling reaction with tertiary amines 1 using dirhodium caprolactamate (Rh2(cap)4) as catalyst (Scheme 13) [29]. 

Michael reaction. This carbon acid (1) is highly effective for inducing the Michael reaction between 2-siloxyfurans and enones. 

Substituted butenolides. 2-Siloxyfurans react with aldehydes and with imines at C-5 to give chiral products. The hydroxyalkylbutenolides formed can be incorporated into the catalyst system thus rendering the aldol reaction autoinductive. 

The intramolecular [4+3] cycloaddition reaction towards seven-membered rings involving furans and allylic cations have been reviewed <01ACR595>. A review on the use of 2-siloxyfurans as butenolide precursors has appeared <01T3221>. Organometallic compounds of furans and their benzoannulated derivatives have also been summarized <01 AHCI>. 

In the total synthesis of (+)-Greek tobacco lactone (14SL1888), the authors utilized the previously reported asymmetric Mukaiyama—Michael reaction of acrolein with a 2-siloxyfuran to prepare the key aldehyde intermediate in one step (14CEJ5983). This process was catalyzed by tmns-2,5-diphenylpyrrolidine via an iminium intermediate, with high enantioselectivity. The natural product was obtained from the aldehyde intermediate in a remarkable 34% overall yield in only four steps. 

Qin T, Johnson RP, Porco, JA Jr (2011) Vinylogous Addition of Siloxyfurans to Benzopyryliums A Concise Approach to the Tetrahydroxanthone Natural Products. J Am Chem Soc 133 1714... 

The enantioselective vinylogous Mannich reaction of siloxyfurans with ketimines catalyzed by a cinchona alkaloid amide/Cu(OAc)2 combination afforded furan-2(5fJ)-one containing contiguous tetra- and trisubstituted stereocenters with high selectivity (13AGE5557). 

The enantioselective vinylogous Mukaiyama—Michael addition of 2-silyloxyfurans to cyclic unsaturated oxo esters afforded a variety of compounds bearing a y-butenolide or y-lactone connected to a cycloalkane or cycloalkene moiety (13JOC2289). The reaction of siloxyfurans with aromatic aldimines could be catalyzed efficiently by silver(I) complexes of the Ar-BlNMOL-derived chiral monophosphine (13T8777). 

Reaction of cw-2-acetyl-2,3-diphenyloxirane 221 with ethyl trifluoroacetate afforded 3-hydroxy-2-phenyl-5-trifluoromethylfuran 222. Oppositely, tranx-isomer was cyclized under the same conditions into pyran derivative 223 [141]. Silylation of trifluoromethylated butenolide 224 gave 2-siloxyfurans 225 in 63-71 % yield [142],... 

Chiral imidazolidinones like 20 serve as chiral organocatalysts for the enantioselective reactions of 2-siloxyfuran with a y9-unsaturated aldehydes through oc,fi-unsaturated iminium species (Scheme 3-83)J The Mukaiyama-Michael reaction with nitroolefines provides an access to 1,3-dicarbonyl compounds... 

Scheme 3-83. Enantioselecrive reaction of 2-siloxyfuran with o /0-unsaturated...

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