Addition of lithiated methoxyallene 2 to optically active N‐benzyl‐Boc‐protected amino aldehydes 12–14 and to aldehyde 16 provides products 17–20 with good to excellent diastereoselectivity. These adducts are subsequently cleaved by ozonolysis to give α‐hydroxy‐β‐amino acid derivatives 21–25 in good overall yield. By conversion into an oxazolidone derivative, the configuration of the major diastereomer was determined to be anti (2S,3S). Thus, the additions of 2 follow the course proposed by the Felkin‐Anh model and are not ruled by chelation effects. The diastereoselective synthesis of 22 constitutes one of the simplest routes to a protected norstatine derivative.
Lithiated methoxyallene 2 adds to chiral aldehydes such as 2-phenylpropanal and .ZV/JV-dibenzylated -amino aldehydes 4-8 to give products 9-14 in good yields and with excellent anti-selectivity. The stereochemical outcome of these reactions can be explained by the Felkin-Anh model in a straightforward manner.The crude reaction products can either be transformed to enones by hydrolysis with acid or be converted into 2,5-dihydro-3-methoxyfuran derivatives 19-24 by treatment with potassium tert-butoxide in DMSO. The latter can be hydrolyzed to give 3 (2H)-dihydrofur anones 25-29. The diastereoselectivity of the initial addition step is transferred to the dihydrofuran derivative without a major change in the isomer ratio. Compounds derived from -amino aldehydes 5, 6, and 8 are assumed to be enantiomerically pure. Sodium borohydride reductions of and Grignard additions
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Der vielseitige Synthesebaustein Methoxyallen zeigt sich hier von einer neuen Seite: Das aus ihm erhältliche Titanderivat 1 reagiert mit Aldehyden unter Allenylumkehr in γ‐Position. So entstehen mit α‐N, N‐Dibenzylaminoaldehyden 2 in hoher anti‐Selektivität die Methoxyalkine anti‐3, die als aktivierte Carbonsäurederivate z.B. durch Hydrolyse in die Homostatinlactone anti‐4 überführt werden können. R = CH3, CH2Ph, CH2CHMe2.
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