STEREOCONTROL AND REGIOCONTROL OF ELECTROPHILIC ADDITIONS TO CYCLOHEXENE SYSTEMS BY NEIGHBORING GROUPS - COMPETITION OF ELECTRONIC AND STEREOELECTRONIC EFFECTS AND COMPARISON OF THE REACTIVITY OF SELECTED ELECTROPHILES

Neighboring group participation in the addition of Br+, I+, PhSeX, Hg2+, Tl3+, and Pd2+ to a series of steroidal olefins 5–13 has been studied. The relative importance of the electronic (Markovnikov) and stereoelectronic effects has been assessed. In cases where these effects are in opposition the neighboring group participation may suppress the stereoelectronic control and alter the regioselectivity of the addition in favor of a diequatorial product (12 → 39–42). Iodination reagents generated in different ways (I2 + Ag+, Tl+, Ce4+, Cu2+, Bi3+, or KI) can often cleanly differentiate between the di- and trisubstituted double bond. Silver(I)-mediated iodocyclizations are followed by subsequent solvolysis with an overall retention of configuration, employing a push-pull mechanism (5 → 15 → 20). Departing iodine atom in this stereospecific Koenigs-Knorr-type reaction must be exocyclic to the newly formed heterocycle and antiperiplanar to the participating C-0 bond. Similarly, 5 can be converted to 20 on reaction with Tl(III) employing analogous 5(0)n-exo-trig cyclization. In contrast, the organothallium intermediate 34, arising from 9 by a 5(O)n-endo-trig process, gives 19-norsteroid 37 as the product of a novel, stereoelectronically controlled fragmentation. An improved procedure for cyclooxypalladation-carbonylation has been developed (5→19 → 24). © 1990, American Chemical Society. All rights reserved.