T/ireo-selective

Z)-Crotylsilanes are inherently mildly //ireo-selective. The highest t/ireo-selectivity can be obtained when silicon bears an alkoxy substituent, but 2 equiv. of TiCl4 is then required, and yields are only moderate because of a secondary reaction (equation II). 

Stereoselective hydroboration of allylic alcohols. Acyclic secondary ally lie alcohols of the types 1 and (E)- and (Z)-2 undergo hydroboration stereoselectively to yield threo-, 3-dioh (3). Highest t/ireo-selectivity in hydroboration of 1 is obtained using 9-... 

Reaction of RAIC12 with /3-formylpropionamides.1 Reaction of (CHj)3A1 with the /3-formylpropionamides 1 shows no stereoselectivity, but the reaction with CHiAICI2 shows high t/ireo-selectivity. In contrast the reaction with CH3TiCI3 is erythro-seiectivc. 

A syn-selective asymmetiic nih o-aldol reaction has been reported for structurally simple aldehydes using a new catalyst generated from 6,6-bis[(tiiethylsilyl)ethynyl]BINOL (g in Scheme 3.18). The syn selectivity in the nitro-aldol reaction can be explained by steric hindrance in the bicyclic transition state as can be seen in Newman projection. In the favored h ansition state, the catalyst acts as a Lewis acid and as a Lewis base at different sites. In conbast, the nonchelation-controlled transition state affords anti product with lower ee. This stereoselective nitro-aldol reaction has been applied to simple synthesis of t/ireo-dihydrosphingosine by the reduction of the nitro-aldol product with H2 and Pd-C (Eq. 3.79). 

The 2,5-dibromo-2,5-dideoxy-D-xylono-l,4-lactone (6) (Scheme 2) can also be selectively reduced to give 5-bromo-2,5-dideoxy-D-t/ireo-pentonolactone (7) by reaction with either aqueous hydrazine as mentioned above, or by consumption of 1 mole of hydrogen [14]. Further reaction with hydrogen and Pd/C leads to 3-hydroxypentanoic acid by an initial reductive elimination [15]. Reduction of the primary bromine in these cases can be obtained by means of tributyltin hydride [25] to give 8. 

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