Acid-induced Reactions

During Swern oxidations, adventitious HC1 may be present either due to the use of impure oxalyl chloride, or due to the hydrolysis of some chlorine-containing chemical, caused by employing wet DMSO. Adventitious HC1 may cause acid-induced side reactions on sensitive substrates.174,246 

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Allenylstannancs also provide sw-products stereoselcctively from their Lewis acid induced reactions with a-branched aldehydes86. 

Good Cram selectivity is observed for Lewis acid induced reactions between allylstannanes and aldehydes with alkyl-substituted a-chiral centers66,87. This enhanced Cram selectivity may be due to the effect of the Lewis acid on the trajectory of nucleophilic attack on the aldehyde66. 

For Lewis acid induced reactions between a-alkoxyaldehydes and allylstannanes, either Cram or chelation control is observed depending on the choice of Lewis acid and the O-substituent88. 

Chiral amides of 2-(tributylstannylmethyl)-2-propenoic acid show useful stereoselectivity in their Lewis acid induced reactions with aldehydes, and the products have been converted into optically active a-methylenelactones95. 

Useful Cram selectivity is observed for Lewis acid induced reactions between 3-alkoxyallylstan-nanes and aldehydes with alkyl substituted a-stereogenic centers116. 

The Lewis acid induced reaction of silyl enol ethers and silyl ketene (thio)acetals with 4-acetoxyazetidinones is often used for introduction of a carbon substituent in the 4-position of the jS-lactam ring. Numerous examples are known, both with and without substituents at nitrogen, some of which are shown. 

Lewis-Acid-Induced Reactions, Increasing the Electrophilicity of the Electrophile... 

Acid-induced reactions producing mostly rearrangement 376... 

Lewis Acid Induced Reactions of Methyl 2-Siloxycyclopropanecarboxy-... 

A wide variety of RCCo3(CO)9 complexes may be prepared (R = H, halogen, alkyl, aryl, alkenyl, acyl, alkoxycarbonyl, aminocarbonyl, triorganosilyl, dialkyl phosphino, alkoxy, dialkylamino, etc.), either by direct reactions of octa-carbonyl dicobalt with organic tri- and dihalides or by acid-induced reactions of... 

Scheme 7. Figure 19 finally provides the quantitative basis for the qualitative model (Scheme 11), which has been employed for deriving conditions that allow the selective synthesis of 1 1 products by Lewis acid induced reactions of alkyl halides with alkenes. 

Lewis acid induced reactions of a-carbonyl epoxides have been extensively studied, but protic acid reactions in general have not found favor. An exception is shown in equation (19). The protic acid mixture shown was found to be superior to BF3 etherate for the desired rearrangement. ... 

The aldolization reaction is certainly the most popular reaction for creating carbon-carbon bonds and much effort has been made to achieve the reactions in smooth conditions with a high degree of stereoselectivity. The reaction involves activated carbonyl compounds (enol, enolates, various enol ethers) which add on another carbonyl compound or various electrophiles in related reactions. As most of the activated compounds are compatible with water, it is not surprising that the aldolization in aqueous conditions became an efficient process in view of the negative activation volume of the reaction. The diastereo-selectivity depends greatly on the reaction conditions. Thus, in a intramolecular aqueous aldolization, the acid-induced reaction of a keto aldehyde provided a syn hydroxy ketone while the base-catalyzed reaction led to the anti isomer (Scheme 35) [179]. 

Acid-induced reactions of silicon-containing unsaturated compounds with A -acyliminium intermediates have proven particularly useful. Allylsilanes lead to the corresponding allyl-substituted products. This reaction proceeds well even for -lactams (equation 49). 5.86 j, g penicillin-derived 4-acetoxyazet-idinone (91) reacts with excess allylsilane in refluxing dichloroethane, catalyzed by TMSOTf, to produce as the sole product trans compound (92). A -Acyliminium ions generated from cyclic ureas and carbamates (93) likewise yield allylated products with virtually complete trans stereoselectivity (equation SO). Allylstannanes react under somewhat milder conditions than allylsilanes, requiring only a catalytic amount of BFsZ EtjO at room temperature for a 3-lactam system (equation 51). This allylstannane procedure was applied to cyclic carbamate systems for the stereoselective synthesis of several isomers of statine. Thus, ethoxycarbamate (94) reacts with methallyltributylstannane in excellent yield and dia-stereoselectivity (equation 52). ... 

For the synthesis of compactin, Konoike et al. performed an intramolecular cycloaddition of the chiral (Z)-129 to give the dccalin derivatives 130 and 131 [62] (Scheme 8.31). The best results were obtained under high pressure at 1.0 GPa and 20 °C resulting in 53 % of 130 and 8 % of 131. Whereas the Lewis acid-induced reaction gave a lower selectivity of 42 % of 130 and 16 % of 131, the thermal reaction showed much worse results with 20 % of 130 and 5 % of 131. However, it is not clear whether these results are due to a pressure-induced improvement of selectivity or due to an isomerization of the starting material caused by the Lewis acid or the higher temperature. 

Attempts to effect intramolecular aldol condensation using a variety of bases under a range of conditions were unsuccessful. Examination of acid-induced reactions led to the isolation of a crystalline product 196. The physical properties of this material indicated that it was most certainly not the desired pentacycle 193. ° Single crystal X-ray structure determination confirmed the material as the dihydroanthracene 196. The formation of this material was explained by a complex stepwise acid catalyzed transformation. 

A 2,5-disubstituted C g tetrahydrofuran fatty ester [13] was obtained from methyl ricinoleate by addition of bromine to the isomerized substrate, followed by hydrogenation over palladium on charcoal (68). Free radical and Lewis acid-induced reactions involving flie double bonds of unsaturated fetty esters have been conducted by Metzger et al. (69-74) these have resulted in the production of a large number of functionalized, cyclic, and branched fetty ester derivatives (e.g., [14], [15]). The synthesis of methyl rac-2-dodecyl-cyclopentane carboxylate from methyl 2-iodo-18 1(6Z) is presented in Scheme 5. 

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