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Synthesis of Optically Pure Compounds by Enantiotopically Differentiating Monoacetalization of Prochiral Diketones. Part II. Fragmentation of β‐Keto‐Acetals
Author(s) -
Duthaler Rudolf O.,
Maienfisch Peter
Publication year - 1984
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.19840670324
Subject(s) - chemistry , diastereomer , acetal , decalin , medicinal chemistry , isomerization , fragmentation (computing) , benzene , organic chemistry , stereochemistry , catalysis , computer science , operating system
Abstract Treatment of β‐keto‐acetals, derived from non‐enolisable β‐diketones, with sulfonic acids in boiling benzene resuots in a smooth retro‐Claisen ‐type fragmentation. The acetal‐C‐atom is thereby transformed into a carboxylic ester via a dialkoxycarbenium ion, which is dealkylated by the sulfonate counter‐ion. Application of this reaction to the diastereomeric monoacetals 3 and 4 , derived from cis ‐9‐methyl‐decalin‐1,8‐dione ( 1 ), followed by transesterification with CH 3 OH, yields optically pure 4‐(2′‐methyl‐3′‐oxocyclohexyl)butyrate 9 ((+)− 9 from 3 , (−)‐ 9 from 4 ) and the monosulfonate of Meso ‐2,3‐butanediol (−)‐ 13 ( Scheme 2) . Unexpectedly, this cleavage proceeds as well with monoacetal 26 , obtained by acetalization of trans ‐9‐methyl‐decalin‐1,8‐dione ( 27 ) with 2,2‐dimethyl‐1,3‐propanediol ( Scheme 7) . Some attempts, aiming at an isomerization of the cis ‐ and trans ‐decalin derivatives 3 and 24 , or 25 and 26 , via the postulated carboxonium intermediate, were not successful.