Sultams, acyl aldol reactions

An interesting asymmetric aldol reaction utilizing enantiomerically homogeneous bomane sultam derived boron enolates has recently been reported by Oppolzer et al. The reaction of aldehydes with boron enolates (57), generated from acyl sultams (58) under standard enolization conditions (Pr 2NEt/Bu2BOTf/0 C), provides syn aldol products (59) with extremely high ratios of (59) to (60) as shown in Scheme 29. Results from the aldol reactions with representative aldehydes are summarized in... 

Table 4 Aldol Reactions of Boron Enolates Derived from Enantiomerically Pure Acyl Sultam (58) ...

Introduction. The toluene-2,a-sultams are recently introduced relatives of the well established 10,2-Camphorsultam chiral auxiliary and have been designed to provide similar high levels of face discrimination in reactions of pendent prochiral functionality. Feamres that distinguish them include high crystallinity and facile NMR and HPLC analysis of derivatives, favorable acylation and aldolization characteristics of derived Al-acyl enolates, and improved cleavage characteristics. 

Acylation, Alkylation, and Aldolization (Acyl Species-+ a-, P-, or a/fi-Functionalized Acyl Product) Alkylation reactions of sodium enolates of various lV-acyl-a-methyltoluene-2,a-sultams with selected (both activated and nonactivated ) alkyl iodides and bromides proceed with good C(a)-re stereocontrol (90-99% de). Analogous acylations with various acid chlorides can also be performed, giving p-keto products (97-99% de). Selective reduction of these latter products with Zinc Borohydride (chelate controlled, 82.6-98.2% de) or N-Selectride (nonchelate controlled, 95.8-99.6% de) can provide syn- and anft-aldol derivatives, respectively. ... 

Syn-aldol derivatives may also be obtained directly from bo-ryl enolates of the same M-acyl-a-methyltoluene-2,a-sultams by condensation with aliphatic and aromatic aldehydes (eq 4). The high C(a)-it topicity of these reactions parallels but exceeds that when using the 10,2-camphorsultam auxiliary and is the result of an analogous transition state. It is noteworthy, however, that al-dolizations of a-methyltoluene-2,a-sultam derivatives generally proceed to completion with just a small excess of aldehyde (1-1.2 equiv, cf. 2-3 equiv when 10,2-camphorsultam mediated). This may be ascribed to the lack of acidic protons a to the SO2 group in the Saccharine-derived auxiliary. 

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