Earlier studies have shown that [3,3]-sigmatropic rearrangements of allyl esters are useful for the construction of fluorine-containing carboxylic acid derivatives. This paper describes the synthesis of 3-aryl-pent-4-enoic acid derivatives bearing either a pentafluorosulfanyl (SF) or a trifluoromethyl (CF) substituent in the 2-position by treatment of corresponding SF- or CF-acetates of p-substituted cinnamyl alcohols with triethylamine followed by trimethylsilyl triflate (TMSOTf). This Ireland-Claisen rearrangement delivered approximate 1:1 mixtures of syn/anti diastereoisomers due to tiny differences (<0.5 kcal/mol) both in the energy of (Z)/(E)-isomeric ester enolates and in the alternative Zimmerman-Traxler transition states of model compounds as shown by DFT calculations. Acidic reaction conditions have to be avoided since addition of the reagents in opposite sequence (first TMSOTf then EtN) led to oligomerization of the cinnamyl SF- and CF-acetates. Treatment of the corresponding regioisomeric 1-phenyl-prop-2-en-1-yl acetates under the latter conditions resulted in [1,3]-sigmatropic rearrangement and subsequent oligomerization of the intermediately formed cinnamyl esters. When EtN was added first followed by TMSOTf, no further reaction of the formed ester was detected.

Aldol reactions belong to the most frequently used C–C bond forming transformations utilized particularly for the construction of complex structures. The selectivity of these reactions depends on the geometry of the intermediate enolates. Here, we have reacted octyl pentafluoro-λ6-sulfanylacetate with substituted benzaldehydes and acetaldehyde under the conditions of the silicon-mediated Mukaiyama aldol reaction. The transformations proceeded with high diastereoselectivity. In case of benzaldehydes with electron-withdrawing substituents in the para-position, syn-α-SF5-β-hydroxyalkanoic acid esters were produced. The reaction was also successful with meta-substituted benzaldehydes and o-fluorobenzaldehyde. In contrast, p-methyl-, p-methoxy-, and p-ethoxybenzaldehydes led selectively to aldol condensation products with (E)-configured double bonds in 30–40% yields. In preliminary experiments with an SF5-substituted acetic acid morpholide and p-nitrobenzaldehyde, a low amount of an aldol product was formed under similar conditions.

New Approach to -Pentafluorosulfanyl-Substituted Carboxylic Acid Derivatives via Ireland-Claisen Rearrangements. -Allylic esters (III) undergo Ireland--Claisen rearrangements to form ,-unsaturated -pentafluorosulfanyl carboxylic acids, which are isolated as their methyl esters (V). No rearrangement is observed for the phenyl-substituted derivative (IIIc). -(DREIER, A.-L.; MATSNEV, A. V.; THRASHER, J. S.; HAUFE*, G.; J. Fluorine Chem. 167 (2014) 84-90, http://dx.

The title reaction tolerates aryl and alkyl aldehydes.