Acetals Claisen rearrangement

The stereoselectivity of silyl ketene acetal Claisen rearrangements can also be controlled by specific intramolecular interactions.246 The enolates of a-alkoxy esters adopt the Z-configuration because of chelation by the alkoxy substituent. 

Protodesilylation, A new preparation of -unsaturated carboxylic acids involves the amide acetal Claisen rearrangement of a 3-(trimethylsilyl)allyl alcohol such as 1 to allylsilanes (2). Desilylation of 2 by usual methods results in formation of stereoisomers. However, use of liquid HF at low temperatures results only in (E)-3 in 88% yield. The HF-pyridine complex is not so stereoselective. Amides such as 3 can be converted into carboxylic acid esters by Meerwein s trialkyloxonium salts (—70% yield). 

An accelerated diastereoselective variant of the amide acetal Claisen rearrangement is the reaction between the lithium alkoxide of ( )- or (Z)-2-butenol with salts derived from alkylation of propanamides or fluoroacetamides with methyl triflate or dimethyl sulfate. This reaction yields directly the [3,3] sigmatropic rearrangement products of the corresponding N.O-ketene acetals at room temperature, e.g., formation of 7/8 and 12/13 (Table 8)4fiI-462. 

Table 18. Acetal Claisen Rearrangement of Cycloalkanone Derivatives522...

The key step in this synthesis is a stereoselective two-fold amide acetal Claisen rearrangement of the enantiomerically pure hematopor-... 

The mesembrane alkaloid (-)-mesembranone has been synthesized using a neat application of the amide acetal Claisen rearrangement (Scheme 82)/ ... 

The allyl ketene acetal Claisen rearrangement of uronic acid derivatives with furanoside structures has been studied. The Claisen rearrangement of alkenyl-substituted ketene acetals (84), produced in situ by selenoxide elimination from the corresponding phenylselenoacetaldehyde-derived acetals (83) of enantiomericaUy pure... 

Claisen Rearrangement. The first silyl ketene acetal Claisen rearrangement was introduced in 1972 using TMS ketene acetals. Since then, the silyl Claisen rearrangement using TBDMS ketene... 

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