- Enantioselective Syntheses of (–)-Alloyohimbane and (–)-Yohimbane by an Efficient Enzymatic Desymmetrization Process
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Enantioselective syntheses of (–)-alloyohimbane and (–)-yohimbane were accomplished in a convergent manner. The key step involves a modified mild protocol for the enantioselective enzymatic desymmetrization of a meso-diacetate. This provides convenient access to an optically active monoacetate in multi-gram quantities and in high enantiomeric purity. This monoacetate was converted to (–)-alloyohimbane. Reductive amination of the derived aldehyde caused isomerization to the trans-product and, ultimately, the formation of (–)-yohimbane.
- Ghosh, Arun K.,Sarkar, Anindya
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p. 6001 - 6009
(2016/12/26)
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- Nonreductive enantioselective ring opening of N-(methylsulfonyl)dicarboximides with diisopropoxytitanium α,α,α′,α′-tetraaryl-1,3-dioxolane-4,5- dimethanolate
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The bicyclic and tricyclic meso-N-(methylsulfonyl)dicarboximides 1a-f are converted enantioselectively to isopropyl [(sulfonamido)carbonyl]-carboxylates 2a-f by diisopropoxytitanium TADDOLate (75-92% yield; see Scheme 3). The enantiomer ratios of the products are between 86:14 and 97:3, and recrystallization from CH2Cl2/hexane leads to enantiomerically pure sulfonamido esters 2 (Scheme 3). The enantioselectivity shows a linear relationship with the enantiomer excess of the TADDOL employed (Fig. 3). Reduction of the ester and carboxamide groups (LiAlH4) and additional reductive cleavage of the sulfonamido group (Red-Al) in the products 2 of imide-ring opening gives hydroxy-sulfonamides 3 and amino alcohols 4, respectively (Scheme 4). The absolute configuration of the sulfonamido esters 2 is determined by chemical correlation (with 2a, b; Scheme 6), by the X-ray analysis of the camphanate of 3e (Fig. I), and by comparative 19F-NMR analysis of the Mosher esters of the hydroxy-sulfonamides 3 (Table I). A general proposal for the assignment of the absolute configuration of primary alcohols and amines of Formula HXCH2CHR1R2, X = O, NH, is suggested (see 11 in Table I). It follows from the assignment of configuration of 2 that the Re carbonyl group of the original imide 1 is converted to an isopropyl ester group. This result is compatible with a rule previously put forward for the stereochemical course of reactions involving titanium TADDOLate activated chelating electrophiles (12 in Scheme 7). A tentative mechanistic model is proposed (13 and 14 in Scheme 7).
- Ramon, Diego J.,Guillena, Gabriela,Seebach, Dieter
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p. 875 - 894
(2007/10/03)
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