Chirality transfer in the aza-[2,3]-Wittig sigmatropic rearrangement

The aza-[2,3]-Wittig sigmatropic rearrangements of substrates derived from enantiomerically pure alanine, valine and serine with phenyl and ester anion stabilising groups were investigated for their efficiency in chirality transfer. It was found that a methyl substituent at the stereogenic centre of the rearrangement precursors was inadequate to control the alkene stereoselectivity and enantioselectivity of the rearrangement. Ester stabilised anions of valine and serine derivatives were the most successful with up to 66% yield, 14 : 1 alkene (E)-stereoselection and 88% chirality transfer. A limitation to the steric bulk of the stereogenic centre was noted in that the substituent has to be bulky enough to dictate alkene stereoselection, but not too large to compromise the directing effect of the activating phenyldimethyl silyl substituent on the anion stabilising group. Experimental evidence suggested a possible complimentary coordinating effect of an O-MOM serine substituent, which may assist alkene stereoselectivity and enantioselectivity.