Boat-like transition structures, aldol reactions

Denmark et al. have reported that trichlorosilyl enolates also undergo non-catalyzed aldol reaction with aldehydes (Scheme 10.30) [95]. The sense of diastereoselectivity depends on the geometry of the enolates - ( )-enolate 33 adds to aldehydes with high. syn selectivity, whereas low anti selectivity is observed for (Z)-enolate 35 [96], The stereochemical outcomes can be rationalized by boat-like transition structures arranged by the Lewis acidity of the silicon atom, in which the configuration around silicon is trigonal bipyramidal with aldehyde binding in the apical position. In the transition structure from 35 there are severe steric interactions caused by the enolic Z substituent, which is attributable to the low anti selectivity. 

In connection with the synthesis of podophyllum lignans, ester (62) was deprotonated and the resulting enolate condensed with 3,4,5-trimethoxybenzaldehyde to give a 1 1 mixture of diastereomeric aldols (equation 68). The structure of (63) was established by X-ray analysis the other diastereomer was assigned the 2,3-anti relative stereochemistry (64) on circumstantial evidence. It was suggested that the 1 1 mixture of isomeric products results from a 1 1 mixture of the ( )- and (Z)-enolate, each of which shows complete simple and diastereofacial selectivity in its reactions with 3,4,5-trimethoxybenzaldehyde. For this to be true, it is also necessary that the ( )-enolate reacts through a non-Zimmerman , boat-like transition state, whereas the (Z)-enolate reacts through the normal chair-like transition state. 

The uncatalyzed aldol reaction of highly reactive enoxysilanes such as enoxysilacyclobutanes has previoudy been reported to proceed through six-membeied cyclic transition states (32,55), and the torsional structure of the difluoroketene acetal 5 is considered to be more suitable for the uncatalyzed aldol reaction tfian the planar geometry of 4 because the acetal 5 has a geometrical similarity to the ketene acetal in die cyclic transition states. As mentioned above (Scheme 1), a 60 40 mixture of the syn- and anri-aldols was obtained at 40°C from the bromofluoroketene silyl acetal 2 (E/Z =62/38), suggesting that the uncatalyzed aldol reaction of the fluorine-substituted ketene acetals 1 and 2 proceeds preferentially through boat-like cyclic transition states. Denmark et aL proposed diat the boat-like transition states are extremely predominant in the uncatalyzed aldol reaction of enoxysilacyclobutanes and trichlorosilyl enolates (55,54). 

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