Acknowledgment. This work was generously supported originally by NSF Grants No. CHE 85-06703 and 88-15052 and recently by NIH Grant No. GM39821-01A2. We greatly appreciate Dr. Tito Scaiano's continuing hospitality, and that of the NRCC in Ottawa, in the measurement of flash spectroscopic data.

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During our studies of Se-Se interactions in selenides, it was observed that halogen atoms X of C-X bonds were engaged in both a "head-on" and a "side-on" fashion to Se atoms. To understand such interactions, we have analyzed the crystallographic environment around halogen centers and find that, in general, "electrophiles" tend to approach halogens of C-X (X = Cl, Br, I) at an angle of ~100°and nucleophiles at ~165°and that C-X-X-C type interactions fall into two groups, one forming an "electrophile-nucleophile pairing" interaction and the other forming no such pairing. These interactions are interpreted in terms of HOMO and LUMO frontier orbitals centered on the halogens and the approaching atoms. Such "electrophile-nucleophile pairing" interactions are quite general for several systems like sulfides and selenides and no doubt are important in the interaction of small molecules containing halogens since halogen atoms often are in a situation to make short contact with a variety of other atoms, owing to their exposed positions in many molecules.

477 generated by the present method, since their precursors are readily accessible from o-t~lualdehyde.~ In a parallel line of experiments, cyclopentanone moiety 13 was prepared in an almost stereoisomerically pure form, starting from 2-methyl-2-cyclopentenone (12) by modifications of the reported method. l c i d , fThe assembly of the bromide 13 with 3 was carried out by adding 13 to the benzylic carbanion 5 at -75 "C to room temperature, which had been generated in situ from 3, according to the procedure described above. In the assembly of the bromide 13 with 4, HMPA was added to the carbanion 6, generated in situ prior to the alkylation. Deketalization of the coupling products 7 and 8 afforded the crucial precursors 14 and 15" as stereoisomeric mixtures [14, 2:l;" 15, 1:2 (the former is less polar)] in 94% and 95% yields, respectively, after preparative TLC [14: TLC on silica gel, Rf0.60 (6:4 AcOEt-C6H6); 15: TLC on silica gel, Rr 0.59 and 0.38 (6:4 AcOEt-CqH6),]. The final intramolecular cyclization of 14 and 15 via the corresponding o-quinodimethanes was exemplified with the estrone synthesis. To an acetonitrile solution (20 mL) of a diastereo-14, X = OMe; R = H 15, X = H; R = CH, 1-16, X = OMe; R = H 1 7 , X = H ; R = C H , R 18, X = OMe; R = H 2 , X = H ; R = C H ,

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