Cyclopropylmethyl bromide reacts readily at -75° with a Mg/THF slurry formed by evaporation of Mg in a rotating-solution reactor to yield after carbonation cyclopropyl acetic acid and 4-pentenoic acid in a ratio of 11:1. Addition of benzaldehyde to the Grignard solution again predominantly yields the addition product containing the cyclopropylmethyl unit. Using the same low temperature Grignard method, benzocyclobutenyl acetic acid is isolated upon carbonation of the Grignard reagent derived from the corresponding bromide, whereas under normal conditions o-vinylphenylacetic acid, the product corresponding to ring cleaved Grignard is obtained.