Mixed Monosilyl Acetals and Catalyst-Dependent Chemoselective Mukaiyama Aldol Reactions

We report a direct, simple, and straightforward approach for the construction of a mixed monosilyl acetal as a new and synthetically valuable functional group by mixing an aldehyde, sodium tert-butoxide, and trimethylsilyl azide. We also demonstrate a catalyst-dependent chemoselective reaction between mixed monosilyl acetals and silyl ketene acetals through Mukaiyama aldol reactions to give different structures of O-protected β-hydroxy esters in excellent yields with high chemoselectivities. This study provided the existence of an oxonium ion intermediate and of its kinetically controlled reaction with the pre-equilibrated silyl enol ether obtained from (E)- and (Z)-isomerization.

The remarkable journey of catalysts from stoichiometric to catalytic quantity for allyltrimethylsilane inspired allylation of acetals, ketals, aldehydes and ketones

Allyltrimethylsilane inspired allylation of acetals, ketals, aldehydes, and ketones comprises one of the most viable, significant and graceful methods for the formation of carbon–carbon bonds leading to stereochemical transformation.

The domino multicomponent allylation reaction for the stereoselective synthesis of homoallylic alcohols

Stereoselective allylations of carbonyl compounds such as aldehydes and ketones are useful but challenging reactions in organic chemistry. The resulting chiral secondary and tertiary homoallylic alcohols or ethers are valuable building blocks in the synthesis of biologically active natural compounds and pharmaceuticals. Although researchers have developed several methods for the facially selective allylation of aldehydes, the stereoselective allylation of ketones still poses a severe problem. We have developed a highly diastereoselective domino multicomponent allylation reaction of a ketone and allyltrimethyl silane using the trimethylsilyl ether of a norpseudoephedrine or mandelic acid derivative as an auxiliary with a diastereoselectivity of up to 98:2. The reaction is performed at -78 degrees C in the presence of a catalytic amount of trifluoromethanesulfonic acid and leads to the corresponding tertiary ethers. The procedure can also be used for the allylation of aliphatic aldehydes with a diastereomeric ratio >99:1. Ketones give the 4,1'-syn product while the aldehydes give the reversed selectivity to yield a 4,1'-anti product. In addition, the reaction of gamma-substituted allyl silanes with ketones yields a product with two stereogenic centers and an anti diastereoselectivity of >99:1. The homoallylic ethers formed in the domino multicomponent process can be used in further synthetic transformations: the auxiliary can serve as a protecting group or can be cleaved reductively to give the corresponding homoallylic alcohols. Based on a number of both experimental and theoretical studies of the reaction mechanism, we conclude that an intermediate oxocarbenium ion is formed in the reaction of ketones. The oxocarbenium ion is attacked by the allyl silane during the stereogenic step. Using density functional theory methods, we could trace the observed stereoselectivity phenomena back to open transition states (TSs) where there is no interaction between the silane's trimethylsilyl group and the former carbonyl oxygen. On the contrary, the reaction with aldehydes forms an intermediate oxazolidinium salt, which explains the opposite selectivity. We have used the new allylation procedure in several total syntheses of natural products such as vitamin E, (+)-hydroxymyoporone, 5,6-dihydrocineromycin B, and polyoxygenated cembrenes.

Asymmetric multicomponent reactions (AMCRs): the new frontier

Asymmetric multicomponent reactions involve the preparation of chiral compounds by the reaction of three or more reagents added simultaneously. This kind of addition and reaction has some advantages over classic divergent reaction strategies, such as lower costs, time, and energy, as well as environmentally friendlier aspects. All these advantages, together with the high level of stereoselectivity attained in some of these reactions, will force chemists in industry as in academia to adopt this new strategy of synthesis, or at least to consider it as a viable option. The positive aspects as well as the drawbacks of this strategy are discussed in this Review.