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Asymmetric Evans syn ‐Aldol Reactions of Terpene‐Derived Enals: Scope and Limitations
Author(s) -
Kriening Sebastian,
Evagelou Athanasios,
Claasen Birgit,
Baro Angelika,
Laschat Sabine
Publication year - 2014
Publication title -
european journal of organic chemistry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.825
H-Index - 155
eISSN - 1099-0690
pISSN - 1434-193X
DOI - 10.1002/ejoc.201402736
Subject(s) - aldol reaction , chemistry , stereocenter , adduct , aldehyde , stereochemistry , terpene , isomerization , silylation , ether , organic chemistry , enantioselective synthesis , catalysis
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 ).