Abstract
The additions of six alkoxyallenes bearing carbohydrate-derived chiral auxiliaries
to imines were systematically studied. The reactions of three lithiated 1-alkoxypropa-1,2-dienes
with an N-tosyl imine revealed that the diacetone fructose-derived auxiliary provided the highest
diastereoselectivity of 91:9. The preferred absolute configuration of the newly formed
stereogenic center was determined by subsequent ozonolysis of the allene moiety, transesterification
and comparison with literature data. The analogous reactions of three axially chiral
3-nonyl-substituted 1-alkoxyallenes with these auxiliaries confirm these results and
also prove that the configuration of the generated stereogenic center was only steered
by the auxiliaries, whereas the chiral axis has essentially no influence. In general,
four diastereomers were obtained in various portions, depending on the ratio of the
two precursor allene diastereomers and on the auxiliary employed. The obtained diastereomeric
allenyl amines were cyclized under different conditions. As expected, under basic
conditions, a stereospecific cyclization occurred, whereas under silver nitrate catalysis
partial isomerization at the allene stage was observed. Under both conditions the
2,5-cis-disubstituted dihydropyrroles were formed faster than the trans-isomers. Several of the 2-substituted or 2,5-disubstituted dihydropyrrole derivatives
could be isolated in diastereomerically pure form and were subsequently converted
into the expected pyrrolidin-3-ones by removal of the carbohydrate-derived auxiliary
under acidic conditions. The desired products were obtained in good yield and with
high enantiopurity. They are suitable starting materials for the synthesis of enantiopure
pyrrolidine natural products.
Key words
alkoxyallenes - axial chirality - carbohydrate derivatives - imines - lithiation -
nitrogen heterocycles - pyrrolidines
