STEREOCHEMICAL FEATURES OF THE (2+2) CYCLOADDITION REACTIONS OF CHIRAL ALLENES .4. THE CYCLOADDITION OF ENANTIOENRICHED 1,3-DIMETHYLALLENE WITH METHYL PROPIOLATE

The cycloaddition of methyl propiolate (MP) with enantioenriched 1,3-dimethylallene (13DMA) has been investigated. Two cycloadducts, 16 and 17, are formed in which > 40% of the enantiomeric excess (ee) of 13DMA has been transferred to the cycloadducts. Ab initio calculations have been carried out on models for the diradical intermediates formed in this process. A single minimum-energy conformation appears to exist in which the alkenyl radical portion of the intermediates is oriented essentially perpendicular to the plane of the allyl radical. Energy barriers have been calculated for the 180-degrees rotation about the newly formed C-C bond in the intermediates, which would result in their racemization. These energy barriers appear to be higher than those for ring closure. Molecular modeling calculations have been carried out to determine the lowest-energy approach of the MP to the 13DMA which indicate a near-perpendicular approach of the reactants to the transition state for diradical intermediate formation directly leading to the lowest-energy conformations of the intermediates. A detailed description of the cycloaddition process is presented which indicates that the anti,syn diradical intermediate 14 is preferentially formed and undergoes predominant ring closure to the syn-methyl-substituted end of the allyl radical. On the basis of our prior successful molecular modeling calculations for predicting the preferred conformations for approach of the reactants to the transition states for diradical intermediate formation and the absolute configuration of the cycloadducts, the same configurations at C4 as shown are predicted for the two cycloadducts formed in this cycloaddition reaction.