Sulfonamides prolinol

Prolinol derivatives of boron tribromide 4 give excellent results in the [4 + 2] cycloaddition of methacrolein (8a) to cyclopentadiene (exojendo ratio 99 1, 97% ee)21. Several boron catalysts derived from /V-sulfonamides of a-amino acids 7a-d have been developed. In the cycloaddition of methacrolein (8a) or croton aldehyde (8b) to cyclopentadiene these catalysts produce enan-tioselectivities ranging from medium to good8-22 23. Chiral oxazaborolidinone 6 is an excellent catalyst for the addition of 2-bromoacrolein (8e) to various dienes24 (see table overleaf)-... 

Other proline-derived catalysts promoting the selective formation of the fl/iti-diastereomer have been developed, for example, by the groups of Maruoka (axially chiral amino sulfonamides 98, symmetric chiral pyrrolidine-based amino sulfonamides 99) [66, 67] and Cdrdova (diaryl-prolinols 100) [68], among others, involving Coulombic or steric interactions (or both) as well to lock the transition state in the conformation required for stereoselective catalysis (Figure 11.3). 

Acetaldehyde 34, which is the simplest of all enolizable carbonyl compounds but highly reactive as an electrophile, is an inexpensive and versatile two-carbon nucleophile in enamine-based Mannich reactions. Mannich reactions of acetaldehyde as a donor with aryl or alkyl substituted N-Boc-imines 90 are effectively catalyzed by (S) -proline (13) in moderate yield but excellent enantioselectivity (Table 28.6, entries 1 and 2) [47]. Chemical yields are improved up to 87% when N-benzoyl (Bz)-imine is employed in the presence of diaryl prolinol silyl ether 85 with p-nitrobenzoic acid (entry 3) [48]. To suppress side reactions, such as self-aldol reactions, the moderate nucleophilicity of the axially chiral amino sulfonamide 23 is particularly useful for this type of Mannich reaction these conditions give the corresponding adducts 91 in good yield and excellent stereoselectivity (entries 4 and 5) [49]. 

The metal-free intramolecular 5 Nf-exo-trig aminohydroxylation of Al-alkenyl-sulfonamides (81) can be effected by oxone activated with Brpnsted acid as catalyst. The reaction thus proceeds via (83) and affords the prolinol derivatives (82). Although the scheme suggests that the reaction should be diastereoselective, this issue has not been addressed and there is also an incorrect use of curly arrows in this paper. 

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