Nonreductive enantioselective ring opening of N-(methylsulfonyl)dicarboximides with diisopropoxytitanium α,α,α′,α′-tetraaryl-1,3-dioxolane-4,5- dimethanolate

Abstract

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).