Lewis acid-alkoxy combinations

The carbonylation reaction of propylene oxide in the presence of various [Lewis acid] [Co(CO)4] salts was investigated using in situ attenuated total reflection infrared (ATR-IR) spectroscopy. P-Alkoxy-acyl-cobalttetracarbonyl species were found to be key intermediates from which two reaction routes start depending on the applied Lewis acid. Labile Lewis acid-alkoxy combinations primarily favor the production of lactone products [2]. 

The combination of metal tuning and double stereodifferentiation helps to prepare chelation and nonchelation products in the imine series7. In the case of an alkoxy substituent adjacent to the aldimino, the chelation product 10 is predominantly obtained with allylmagnesium chloride, chloromagnesium allyltriethylaluminate or allylzinc bromide, while the use of allyl-boronates or allyltitanium triisopropoxide, which lack the requisite Lewis acidity for chelation, gives 11 with good Cram selectivity. 

The exchange of alkoxy groups for the cyanide function can also be achieved in acetals (cyclic types like dioxolanes and 1,3-dioxane derivatives included), 5A -acetals and ortho esters with the combination of TMS-CN/Lewis acids, or under neutral conditions with transition metal salts as catalysts (Scheme 16). Furanose derivatives may be treated with TMS-CN in a similar way. ... 

Chiral allylic silanes and chiral aldehydes. This combination of agents provides fascinating opportunities for double asymmetric induction and allows the magnitude of the various controlling features to be expressed. The sense and level of 1,2-asymmetric induction in the Lewis acid-promoted addition of chiral E-2-butenylsilanes to chiral a-alkoxy aldehydes has been examined as well (Scheme 10-18) [37j. 

The Lewis acid-promoted reaction of the allylsilane (R)-43 with either of the (5)-a-alkoxy aldehydes provides some surprising insights (Scheme I0-19) [34]. The BF3-OEt2-promoted reaction of the silane (R)-43 and either (S)-26 or (S)-46 afforded predominantly the syn homoallylic alcohols 50 and 51, even though this is presumed to be a mismatched combination of reagents. The TiCL-promoted reaction of ( )-43 with (5)-26 or (S )-46 also produces the syn homoallylic alcohols, presumably through a Cram chelate transition structure model (albeit with lesser selectivity for 46). These experiments indicate that the chirality of the R-silane re-... 

To a solulion or suspension of Lewis acid [Eu(hfc)3 or MgBr2 OEt2 (0.05 equiv or l.l equiv. respectively)] and dry CH2Cl, is added 0.5 mmol of an a-alkoxy aldehyde 1. The solution or suspension is cooled to 0 C and stirred for 5 min. 1.2-1.3 Equiv of 1,3-dimethoxy-l-trimethyisilyloxy-l,3-butadiene (7, Brassard s diene) is added dropwise, and the mixture is allowed to warm to 20 °C. After stirring for 18-24 h at 20 JC, the reaction is quenched by addition of H20 and extracted with EtOAc. The combined extracts are washed with brine, dried over MgS04. filtered and concentrated in vacuo. The crude oil is purified by flash chromatography. 

There are many examples of catalytic Nazarov cyclizations using Lewis acid catalysts in combination with a-alkoxy substituents. Tius and co-... 

Danishefsky found that alkoxy- and silyloxy-substituted dienes such as 47 (Danishefsky s diene see Scheme 17.10) sei-ve as excellent 4jr-components in cycloaddition reactions with aldehydes catalyzed by a broad range of Lewis acids [43, 44, 106, 107]. The cycloaddition between 47 and the ribose-derived aldehyde 200 in the presence of the NMR shift reagent Eu(fod)3 (201) as a mild, Lewis acidic catalyst [108] furnished 202 as the only observed product in 85 % yield (Equation 23) [109]. Danishefsky utilized this family of oxygen-substituted dienes to access natural and unnatural hexoses [44, 106, 110]. As a demonstration of the general synthetic strategy, diene 203, bearing a 1-phenmenthyl auxiliary, was utilized in combination with Eu(hfc)3 (204) to prepare L-glucose (206, Scheme 17.28). 

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