Reduction with REDAL

The aziridinium-fused furanoside 221 was prepared from D-ribose derivative 220, and thence elaborated to acyclic aziridine 222 and compound 223, which are both 3,4-disubstituted L-glutamates (Scheme 16). [2+2] Cycloadditions of sugar vinyl ethers 224 and 225 to tosylcyanide followed by reduction with RedAl afforded the P-lactams 226 and 227. These were elaborated to the tricyclic systems 228 and 229. ... 

Reduction with Redal (bismethoxyethoxyaluminum hydride) at 4° of tbe lactone 16 supplied the hemiacetal 17 which was 0-methylated and subjected again to Redal reduction, this time at 20°. The product was the secondary amine 18 which was A -methylated to the trans-fused rhoeadine analog 19 ° ... 

The mechanism of the aluminium hydride reductions with LiAlff4 or RedAl involve a tram hydroalumination helped by coordination of A1 to the triple bond and external nucleophilic attack. The regioselectivity of the hydroalumination is again determined by silicon the electrophilic A1 attacks the alkyne on the carbon bearing the silyl group (the ipso carbon). 

The synthesis of 188 from epoxide 189 was done as shown in Scheme 25. Epoxide opening with a propargyl alane reagent [124], protection-deprotection of the alcohol functions, and Redal reduction led to the -allylic alcohol 194, also obtained by another route [116]. Asymmetric epoxidation-Redal epoxide opening [60-62] followed by silylation and debenzylation led to intermediate triol 195. Selective six-membered acetal formation and primary alcohol oxidation then furnished building block 188. 

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