Cyclopentyl Annulations via Vinylcyclopropanes

The intermediate enol silyl ether permits further regioselective substitutions such as bromination followed by dehydrobromination (Eq. 81)49) and alkylation (Eqs. 82 93) and 83 103)). Thus, in addition to activating the rearrangement, the oxygen substituent regioselectivity creates an enol silyl ether, a powerful enolate synthon. 

This sequence initiated by the nucleophilic cyclopropyl conjunctive reagent 9 allows facile annulation concommitant with regiocontrolled further structural elaboration as outlined in Eq. 84 29 . With the electrophilic conjunctive cyclopropyl reagent 58, this [3 + 2] annulation onto a vinyl organometallic (Eq. 85) has the availability of the 

Other cyclopropyl conjunctive reagents are also employed in such annulations including the parent (see Eq. 18). The closest in producing a similar structural change is the thio analogue. Some rate enhancement derives from the sulfur activation. As an annulation onto ketones, a sequence employing 37a accomplishes the same structural change (Eq. 87 a) when the annulated product 73 is hydrolyzed (TiCfj, 

room temperature)113). Alternatively, a simple cyclopentannulation results upon reduction of 73 (Eq. 87b, W-2 Raney-Ni, C2H5OH, reflux)114 . 

Whereas, the sulfur ylide 9 undergoes conjugate addition to enones thereby precluding 1,2-cyclopentyl annulation, the organolithium 37a undergoes clean carbonyl addition. Cyclopentenone participates smoothly in this sequence (Eq. 88)S1). 

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