46 アレン酸ジエステル類の新規合成法とアルカロイド類合成への応用(口頭発表の部)

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Allene-1,3-dicarboxylates are useful for the Michael addition and [4+2] or [2+2] cycloaddition. The recognized methodology for their synthesis has, however, not been satisfactory in terms of the yield and the reaction-steps required. Focusing on the excellent dehydrating ability of 2-chloro-1,3-dimethylimidazolinium chloride (DMC), we exploited a novel efficient synthetic method for allene-1,3-dicarboxylates (2) from acetone-1,3-dicarboxylates (1) in one step. A typical reaction of dimethyl acetone-1,3-dicarboxylate (1a) with DMC (1.2 eq.) in dichloromethane at room temperature for 1 h in the presence of triethylamine (3 eq.) afforded dimethyl allene-1,3-dicarboxylate (2a) in 90 % yield. Optically active di-L-menthyl (R)-allene-1,3-dicarboxylate (2f-R) has been prepared by Kanematsu et al. through optical resolution. In order to improve this synthetic method, we examined the epimerization of di-(-)-L-menthyl allene-1,3-dicarboxylates (2f-R and S) with the assistance of triethylamine. As a result, we exploited a new crystallization-induced asymmetric transformation for the synthesis of di-L-menthyl (R)-allene-1,3-dicarboxylate: crystallization from a diastereomeric mixture (R: S=4: 5) of di-(-)-L-menthyl allene-1,3-dicarboxylates (2f) in pentane with 0.05 equivalent of triethylamine at low temperature (-20℃) gave 2f-R in 90 % yield (>98 %de). The former process opened a new expeditious route to a 1-azabicyclo[3.3.0]octane skeleton of pyrrolizidine alkaloids from dimethyl acetone-1,3-dicarboxylate and bis(2-chloroethyl)amine via a Michael addition and a novel tandem cyclization. The latter process was applied to a formal asymmetric synthesis of (-)-epibatidine utilizing the highly diastereoselective Diels-Alder reaction of 2f-R with N-Boc-pyrrole as a key reaction.
天然有機化合物討論会の論文
1998-08-31

46 アレン酸ジエステル類の新規合成法とアルカロイド類合成への応用(口頭発表の部)

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