Highly Stereoselective Total Synthesis of Multiply Protected 3-Amino-3,6-dideoxyaldohexoses (Mycosamine/Mycaminose) by Aldol Condensation Reaction, Mediated by Tin Triflate, of Tricarbonyliron/α-Aminodienone Complexes

The divalent tin enol ether of the racemic complex between N-BOC-α-aminoheptadienone and tricarbonyliron reacts with both enantiomers of protected lactaldehydes to yield predominantly one optically active, easily isolable ketol diastereoisomer (45%). From the enantiomerically pure (S)-(+) complex and (R)-(+)-tert-butyldimethylsilyloxylactaldehyde, the major ketol is obtained almost exclusively (isolated yield 86%). From there, the multiply protected 3-amino-3,6-dideoxy-D-aldohexose mycosamine is obtained in a few high-yield steps (decomplexation, stereospecific reduction to an anti-1,3-diol, transformation into a diacetate and ozonolysis; absolute configurations S,S,S,R). Reduction of the ketol before decomplexation completely reverses the stereochemistry of the reaction [control by the Fe(CO)3 and not by the hydroxy group → syn-diol], also giving access to the (R,S,S,R) series (configuration of the N,N-dimethylated mycaminose). The key structures were determined by X-ray diffraction.