AbstractThe intramolecular addition of unsaturated alkoxycarbenes leads in high yields and diastereoselectively to fused cyclopropanes (Scheme 1). Reaction of the halodiazirines 2, 10, 11, and 20 with the unsaturated phenolates 1, 8, and 9 yielded intermediate alkoxydiazirines, and hence the homobenzofurans 5, 12–16, 22, and 26 (Scheme 2). The intermediate alkoxydiazirine 25 was isolated at low temperature (Scheme 3). An equilibrium between the cyclopropane derivatives 12 and 27, and 14 and 28 was established at 120°. At 200°, 12 rearranged to the chromene 29, by disrotarory opening of the cyclopropane ring, followed by electrocyclization. Hydrogenation of 29 gave the (all‐cis)‐chroman 32 (Scheme 4). The homoindole 35 was obtained in good yields, presumably by an SRN1 reaction from 34 and 10 (Scheme 5).