Kinetic Resolution in 1,3-Dipolar Cycloaddition of Tartaric Acid-Derived Nitrones to 2,3-Dihydro-1-phenyl-1H-phospholes. An Enantioselective Approach to the 2,2'-Coupled Pyrrolidine-Phospholane Ring System

Enantiomerically pure five-membered ring nitrones derived from L-tartaric acid via C2-symmetric 0,O'-protected 3,4-dihydroxy pyrrolidines undergo highly regio- and stereoselective cycloaddition reactions withracemic2 ,3-dihydro-l-phenyl-VI-phosphol1e- oxide and 1-sulfide. In all cases formation of only two diastereomeric cycloadducts is observed and their ratio (up to 1O:l) is dependent on the size of the protecting groups in the nitrone and on the extent of conversion. The tricyclic cycloadducta feature 2,2'-connection of pyrrolidine and phospholane rings and six contiguous stereogenic centers of which three are created and the one at phosphorus is kinetically resolved during the cycloaddition process. It is established that in the studied kinetic resolutions the stereoselectivity factors = k s l k ~ exceeds the value of 10 (up to 14) in the most favorable cases. In a properly tuned reaction both the diastereomeric cycloadducts and the enantiomerically enriched dihydrophosphole derivative can be simultaneously obtained in satisfactory chemical and optical yields.