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Asymmetric α‐Acetoxylation of Carboxylic Esters. Preliminary Communication
Author(s) -
Oppolzer Wolfgang,
Dudfield Philip
Publication year - 1985
Publication title -
helvetica chimica acta
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.74
H-Index - 82
eISSN - 1522-2675
pISSN - 0018-019X
DOI - 10.1002/hlca.19850680127
Subject(s) - chemistry , conjugated system , enantiomer , recrystallization (geology) , organic chemistry , enantiomeric excess , sulfonamide , enantioselective synthesis , catalysis , polymer , paleontology , biology
Using the readily accessible chiral auxiliaries 1 – 3 the sulfonamide‐shielded O ‐silylated esters 5 underwent π‐face‐selective α‐acetoxylation on successive treatment with Pb(OAc) 4 and NEt 3 HF to give after recrystallization α‐acetoxy ester 6 in 55–67% yields and in 95–100% d.e. Starting from conjugated enoates addition of RCu and subsequent acetoxylation 10 → 11 → 12 yielded α,β‐bifunctionalized esters 12 with >95% configurational control at both C α and C β . Nondestructive removal of the auxiliary ( 6 → 7 , 6 → 8 and 12 → 13 ) gave either α‐hydroxycarboxylic acids or terminal α‐glycols in high enantiomeric purity. The prepared glycols 8c and 13a are key intermediates for previously reported syntheses of the natural products 16 and 17 , respectively.