127865-82-1Relevant articles and documents
A Diastereoselective Approach to α-Allyl-β-Amino Acids using the Ireland Enolate Claisen Rearrangement
Dell, Colin P.,Khan, Khalid M.,Knight, David W.
, p. 341 - 350 (2007/10/02)
Ireland enolate-Claisen rearrangements of the esters 11 derived from a range of N-alkoxycarbonyl- and related β-alanines 9 and allylic alcohols (e.g. 10) generally lead to good yields of α-allyl-β-amino acid derivatives 13, isolated for convenience as the corresponding esters 14.The N-tert-butoxycarbonyl (BOC) derivatives 15 proved to be especially useful and led to good to excellent yields of the α-allyl-β-amino acid derivatives 16 and 17, with diastereoselectivities usually in excess of 4:1.One set of optimum conditions consists of rearrangements of the N-BOC derivatives 15 by sequential treatment with lithium diisopropylamide and trimethylsilyl chloride in tetrahydrofuran at -78 deg C followed by ca. 4 h under reflux.Isolated chemical yields of the derived methyl esters 16 and 17 were generally in the range 70 - 88percent.The stereochemical outcome of the rearrangements was deduced by conversion of the initial silyloxymethyl derivatives 16d and 17d, derived from the esters (E)-15d and (Z)-15d, into the corresponding cis- and trans-butyrolactones 24 and 26, respectively.The synthetic utility was further demonstrated by conversions of the initial hydroxyethyl derivatives 19 and 20 into the valerolactones 28 and 29 and of the syn-isomer 19 into the piperidine 30.A chair-like transition state 31 is consistent with the direct relationship between the allylic alcohol geometry and the nature of the amjor diastereomer of the α-allyl-β-amino acid derivatives (16 and 17) obtained; the E-lithio enolates of the starting esters are presumably favoured due to intramolecular complexation with the enolised carbamate function.
