High Asymmetric Induction in Lewis Acid-Promoted Intramolecular Ene-Type Reactions: A diastereo- and enantioselective synthesis of (+)-α-allokainic acid. Preliminary Communication
✍ Scribed by Wolfgang Oppolzer; Christian Robbiani; Kurt Bättig
- Publisher
- John Wiley and Sons
- Year
- 1980
- Tongue
- German
- Weight
- 229 KB
- Volume
- 63
- Category
- Article
- ISSN
- 0018-019X
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✦ Synopsis
Abstract
The monocyclic amino diacid (+)‐α‐allokainic acid 1 has been prepared enantioselectively from the ester of cis‐β‐chloroacrylic acid and (−)‐8‐phenylmenthol by a series of four synthetic operations in over 15% yield. The crucial step is the intramolecular ‘ene‐type’ reaction of the (Z)‐diene 4 which on treatment with a mild Lewis acid undergoes a highly accelerated, dia‐ and enantiostereoselective cyclization to give the pyrrolidines 6 and 7 in a ratio of 95:5 (Scheme 3). Subsequent ester hydrolysis regenerates the auxiliary chiral alcohol. Similar cyclization of the (E)‐diene 5 furnished a 15:85 mixture of 6 and 7 showing an efficient reversal of the optical induction by variation of the enoate geometry.