Stereoselektive Umsetzungen von rac ‐, ( R ), oder ( S )‐5‐Alkyliden‐2‐ t ‐butyl‐3‐methyl‐4‐oxo‐1‐imidazolidincarbonsäure‐ t ‐butylestern (chirale 2,3‐Dehydroaminosäure‐Derivate) und Herstellung einiger nichtproteinogener Aminosäuren
1991; Wiley; Volume: 1991; Issue: 7 Linguagem: Inglês
10.1002/jlac.1991199101119
ISSN0170-2041
AutoresDieter Seebàch, H. Bürger, Christof P. Schickli,
Tópico(s)Asymmetric Hydrogenation and Catalysis
ResumoStereoselective Conversions of t ‐Butyl rac ‐, ( R )‐, or ( S )‐5‐Alkylidene‐2‐ t ‐butyl‐3‐methyl‐4‐oxo‐1‐imidazolidinecarboxylates (Chiral 2,3‐Dehydroamino Acid Derivatives) and Preparation of Some Nonproteinogenic Amino Acids The reacivity of the alkylidene derivatives 1–11 specified in the title and prepared in two operational steps from commercially available Boc‐BMI and aldehydes is investigated. – According to high‐field 1 H‐NMR analysis of the crude products, additions of HH and DD (catalytic hydrogenation, products 12–20 , of RH (BF 3 ‐activated R 2 CuLi/LiI and protonation, products 21–29 ), and of Cl 2 C (CHCl 3 /NaOH, products 33, 34 ) occur with complete selectivity from the face opposite to the t ‐butyl group. The doubly unsaturated carbonyl derivative 11 reacts with dibutyl cuprate in the δ position, again with formation of a single diastereomer 32 , while trifluoroethylidene‐Boc‐BMI 3a is reduced to the difluoro analogue 30 by this cuprate. – Li dienolates are generated by deprotonation with LDA of the ethylidene ( 2a ) and butylidene compound 4a ; subsequent alkylations (with primary and secondary halides, products 35–41 ) and hydroxyalkylations (with aldehydes, products 42–44 ) lead to single products of electrophilic attack in the a carbonyl position (C‐5 of the imidazolidinone ring) under kinetic control. On the other hand, reaction of the 2a ‐derived dienolate with benzaldehyde under equilibrating conditions furnishes the four possible ( E/R, E/S, Z/R , and Z/S ) γ adducts 45–48 . – A combination of methods – 1 H‐ and 13 C‐NMR spectroscopy, NOE measurements, spectroscopic analogies, chemical correlation (with authentic samples or by spectroscopic or optical comparison), and X‐ray analysis (Table 1, Figure 1, Schemes 1 and 2) – is used to assign the product configurations and thus the stereochemical course of the reactions. Some unusual amino acids ( 50–52, 54, 55 ) are prepared by hydrolysis of the corresponding imidazolidinones.
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