Regioselective and Enantiospecific Synthesis of Dioxepines by (Cyclopentadienyl)ruthenium‐Catalyzed Condensations of Diazocarbonyls and Oxetanes

1,4‐Dioxepines result from the decomposition of α‐diazo‐β‐keto esters in the presence of oxetanes using the catalytic combination of the (cyclopentadienyl)ruthenium complex [CpRu(CH 3 CN) 3 ][BAr F ] and 1,10‐phenanthroline. The regioselective [4+3] insertions follow an S N 1‐like mechanism and occur yet enantiospecifically ( es 74%). The retention of configuration was ascertained by vibrational circular dichroism (VCD) and solid state analyses. Furans, products of [4+1] insertions, are only observed as traces in the above protocol. To promote their formation under CpRu catalysis, it is necessary to use a two‐step process with γ‐halogenated alcohols as substrates.