Asymmetric Evans syn ‐Aldol Reactions of Terpene‐Derived Enals: Scope and Limitations

The ( E )‐ and ( Z )‐terpene‐based aldehydes 6b and 6c with a silyl ether function in the γ‐position were prepared and investigated in boron‐mediated asymmetric Evans aldol reactions. Screening experiments of chiral N ‐acylated oxazolidinones 7 , which are conveniently accessible from 5‐methyl‐5‐hexenoic acid and Evans oxazolidinone auxiliaries, with various boron triflates and terpenoid neral ( Z )‐ 6a as aldehyde component, provided conditions in which highly selective formation of syn ‐aldol adduct 5a occurred and competing C=C double bond isomerization to 10 was completely suppressed. Applying the optimized conditions to O ‐silylated aldehydes 6b and 6c and N ‐acyloxazolidinone derivative ( R )‐ 7a confirmed the syn ‐selectivity and gave the appropriate products syn ‐ 5b , c and syn ‐ 21b , c in good yields. In the case of neral‐derived syn adduct 5a , the configuration of the new stereogenic centers C‐2/C‐3 could be assigned as (2 R ,3 S ).