AbstractRing closure of 2‐alkoxyethyl diazomethyl ketones (VII) under the influence of a catalytic amount of hydrochloric acid gives 3‐ketotetrahydrofuran (VIII). The yields obtained in this reaction are greatly influenced by the choice of the alkoxy group. 3‐Ketotetrahydrothiophen (XX) is prepared from 2‐tert.butylthioethyl diazomethyl ketone (XIXa) by the same pr