Acetals allylic stannanes

Allylic stannanes can be prepared from allylic halides and sulfonates by displacement with or LiSnMe3 or LiSnBu3.146 They can also be prepared by Pd-catalyzed substitution of allylic acetates and phosphates using (C2H5)2AlSn... 

Lewis acid-mediated ionization of acetals also generates electrophilic carbon intermediates that react readily with allylic stannanes.190 Dithioacetals can be activated by the sulfonium salt [(CH3)2SSCH3]+BF4,191... 

Mikami et al. also investigated the addition of ketene silyl acetals. They found that addition of the silyl enol ether of acetone and allylic silanes did not result in the synthesis of substituted l,2-dihydro[60]fullerenes [218a,220], In 1997, Mikami et al. [221] reported the photoaddition of allylic stannanes that leads to monoallylation of C6o (Scheme 13). 

Readily available functionalized aryl siloxanes are also viable cross-coupling partners for Pd(dba)2-catalyzed allylic arylations. A mixture of 5% Pd(dba)2, allylic halide, and in situ-generated aryl zinc reagent produces allylated arenes in high yield. Aryl boronic acids have been converted to allylated arenes as well. Diastereoselective intramolecular Stille-type coupling of two allylic moieties (aUylic acetate and allylic stannane) has been performed in high yield to produce the key intermediate in the synthesis of racemic 10-ep/-elemol (eq 19). ... 

Allyl stannanes have also been prepared via Claisen rearrangement (Scheme 4.85). Ritter showed that the rearrangement of the vinyl stannane glyco-late proceeded through the expected Z-silyl ketene acetal to give the syn pentenoic acid in excellent diastereoselectivity [80]. 

In the presence of Pd catalyst, Bu3SnCl-Sml2 and BusSnAlEta were effective stanna-tion reagents for allylic acetates (Scheme 3). Organodistannane (RsSnSnRj) reacted with ally lie acetates to afford allylic stannanes, which were further utilized in one-pot carbon-carbon bond-formation reactions. 

The research groups of Trost and Stille have demonstrated in related studies that allylic stannanes undergo palladium-catalysed cross-coupling with allylic acetates °° or allylic bromides to furnish 1,5-dienes in variable yields. The stannane undergoes predominant or complete allylic transposition during coupling with either type of substrate (e.g. Scheme 96). 

A solution of 1.5 mol equiv of butyllithium in hexane is added to 1.5 mol equiv of a 1 M solution of hexabutylditin in THF at 0 C under nitrogen, and the mixture is stirred for 20 min. The solution is cooled to — 78 °C and a solution of 1.5 mol equiv of diethylaluminum chloride in toluene is added. After stirring for 1 h at — 78 °C, a solution of 0.05 mol equiv of [tetrakis(triphenyl)phosphine]palladium(0) in THF is added followed by a solution of the allyl acetate in THF. The mixture is warmed to r.t., and stirred until the allyl acetate has reacted (TLC). The solution is cooled to 0°C, and an excess of aq ammonia slowly added. After an aqueous workup, the products arc isolated and purified by flash chromatography on silica gel using 1 % triethylamine in the solvent to avoid acid-induced loss of stannane. 

The Stille reaction has been successfully applied to a number of macrocyclic ring closures.207 In a synthesis of amphidinolide A, the two major fragments were coupled via a selective Stille reaction, presumably governed by steric factors. After deprotection the ring was closed by coupling the second vinyl stannane group with an allylic acetate.208... 

Suitably protected glycosyl halides or acetates, upon Lewis-acid promoted SN1 heterolysis, generate glycosyl cation intermediates that can react with electron-rich arenes, heteroarenes, Me3SiCN, enoxysilanes, enamines, allyl silanes and stannanes, acetylenyl silanes and stannanes affording C-glycosyl compounds. 

Alkylation of allylic acetates. Rcgioselccti ve monoalkylation of allylic acetates is possible by use of enol stannanes (prepared by reaction of lithium enolates with chlorotri-n-butyltin) in the presence of this Pd complex. The less substituted end of the allyl group is alkylated with formation of the (E)-isomer.4 Examples ... 

Initial reports on the use of simple enolates as nucleophiles in TT-allylpalladium chemistry met with only limited success.77 106 The enolate of acetophenone reacted with allyl acetate in the presence of Pd(PPh3)4, but gave predominantly dialkylated product.106 The use of the enol silyl ether of acetophenone gave only monoalkylated product with allyl acetate and Pd° catalysis, but substituted allyl acetates did not function in this reaction.106 Enol stannanes, however, have been found to give monoalkylated products with a wide variety of allyl acetates (equation 19).106 In situ generation of enol stannanes from lithium enolates and trialkylstannyl trifluoroacetates followed by Pd°-catalyzed allylation has been demonstrated.107... 

In the presence of Lewis acids allyl silanes and stannanes react with epoxides generally at the sterically less demanding carbon atom. Other electron-rich alkenes, such as ketene acetals, can also be used as nucleophiles. The strong Lewis acids required might, however, also lead to rearrangement of the epoxide before addition of the nucleophile can occur (last reaction, Scheme 4.72). 

Analogous to the allylation with allylsilanes and -stannanes, the transformations, vinylallylation, propargylation, allenylation, alkenylation, alkynylation, and arylation, are viable by the use of an appropriate reagent in the presence of a titanium Lewis acid these are surveyed in the review articles cited both in the Introduction and in this section. The stereochemistry of the reaction of a (vinylallyl)silane in the presence of TiCU has been reported [234]. Equation (113) shows that the major reaction of this silane and isobutyraldehyde occurred mainly in the anti sense with a ratio of anti to syn attack of 90 10 at the terminus remote from the silyl group. Essentially the same stereochemical outcome was observed for the same reaction with the corresponding trimethylsilyl derivative. The intramolecular reaction with an acetal, however, proceeded less selectively the anti syn ratio was 60 40 (Eq. 114) [234]. 

Since vinyl stannanes (chapter 18) couple with electrophiles under catalysis by Pd(0), it makes sense to create the electrophile by the palladium-catalysed reactions of allylic acetates in the same pot.50 The vinyl stannane 304 reacts cleanly with the more complex allylic acetate 305 with 0.2 equivalents of Pd(0) to give the triene 306 in good yield.51... 

The synthetic utility of (i )-enoate 392 is illustrated in the stereoselective synthesis of the bengamide E derivative 399 (Scheme 88). Silyl protection of 392, reduction with DIBAL, and Sharpless epoxidation of the resulting allylic alcohol furnishes epoxy alcohol 396 as a 95 5 anti syn mixture. Conversion of the primary hydroxyl group of 396 to an iodide under neutral conditions followed by a metallation-elimination and subsequent in situ methylation provides the ether 397. Ozonolysis, desilylation with aqueous acetic acid, and a Dess-Martin oxidation supplies the a,jS-dialkoxy aldehyde 398. This, utilizing stannane Se addition, is then converted to 399 [135]. 

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