Reactions of Carbenes with Oxetane and with OxetaneJMethanol MixturesEthoxycarbonylcarbene, bis(methoxycarbonyl)carbene, phenylcarbene (17a), diphenylcarbene (l?b), fluorenylidene ( I~c ) , 2-furylcarbene (31 a], 2-furyl(phenyl)carbene (31 b), 4-oxo-2,5-cyclohexadienylidene (40), and 4,4-dimethyl-2,5-cyclohexadienylidene (53) were generated by photolysis of the appropriate diazo compounds. With neat oxetane, most of these carbenes react by competitive C -H insertion (B -+ A, Scheme 1) and ylide formation (B -+ C). 31a and 40 do not insert into C -H bonds; 31 b does not attack oxetane but rearranges exclusively with formation of 26. The ylides undergo Stevens rearrangement to give tetrahydrofurans (C -+ D) and a'$-elimination, leading to ally1 ethers (C + E). With oxetane/rnethanol mixtures, the intervention of oxonium ions (H) is indicated by the formation of 1,3-dialkoxypropanes (I). The oxonium ions arise either by protonation of the ylides (C -+ H) or by protonation of the carbenes (B -+ G ) , followed by electrophilic attack of the carbocations (G) at oxetane (G -+ H).The former route is followed by the alkoxycarbonylcarbenes and by 40; the ylides derived from the remaining carbenes do not react with methanol, owing to their rapid Stevens rearrangements. Protonation of the carbenes 17b, 31, and 53 is clearly indicated by their product ratios and, for 31, by the formation of isomeric ethers (33, 36). The more electrophilic carbenes discriminate but slightly between oxetane and methanol while the more nucleophilic carbenes (17b, 31,53) prefer the protic methanol strongly over the aprotic oxetane.Carbene reagieren mit den nichtbindenden Elektronenpaaren von Heteroaromatcn zu Yliden'). Wahrend die Bildung von Stickstoff-, Phosphor-und Schwefel-Yliden gut untersucht ist und auch stabile Vertreter dieser Verbindungsklassen dargestellt werden konnten, weiB man uber Sauerstoff-Ylide relativ wenig. Aus Alkylethern entstehen Ylide, die nicht umlagern, sondern durch Spuren von Wasser oder Alkoholen protoniert und dann nucleophil gespalten werden'). Formale C -0-Einschiebung erfolgt bei Benzylethern und Allylethern4); fur letztere weist Allyl-Inversion auf eine 2,3-sigmatrope Ylid-Umlagerung hin. Arylcarbene mit geeigneten Seitenketten zeigen intramolekulare Einschiebung in 0-Benzyl-und O-Allyl-Bind~ngen~). Tetrahydrofuran reagiert mit Methylen nur in der Gasphase, nicht aber in Losung unter Ringerweiterung6'. Dagegen beobachteten wir bei der Belichtung von Diazomethan in Oxetan (1) ca. 40% C -0-Einschiebung'). Weitgehende Racemisierung bei der Ringerweiterung von (S)-ZMethyloxetan (2) zu 3-Methyltetrahydrofuran (3) sprach fur einen radikalischen Mechanismus dcr Stevens-Umlagerung8).In dieser Arbeit berichten wir iiber die Reaktionen weiterer Carbene mit Oxetan und Oxetan/Methanol-Gemischen. Der Zusatz von Methanol dient zum Abfangen in-3 ( 2 7 % )termediarer Ylide, gibt aber auch Einblick in das Verhalten der Carbene gegenuber 0 -H-Bindungen. Alkox ycarbon ylcarbeneBei

Crown Ethers from o‐Quinone Diazides and Oxetane The carbenes 11, generated by photolysis of the di‐ and tetrachloro‐o‐quinone diazides 10, react with oxetane in a 1:3 ratio to give the 15‐membered crown ethers 19. In the presence of methanol, the oxonium ions 12 are trapped with formation of 1‐(aryloxy)‐3‐methoxypropanes (13). In the absence of additional nucleophiles, 12 reacts with oxetane until the chain length is appropriate for inverting intramolecular attack of the aryl oxide at the α‐carbon atom of the oxonium ion (18 → 19).

Reactions of Vinylogous Oxycarbenes with Methanol The carbenes 16 and 30 were generated in methanol/sodium methoxide by photolyses of 2‐methylchromone tosylhydrazone (14) and 3,5‐dimethoxy‐4,4‐dimethyl‐2,5‐cyclohexadienone tosylhydrazone (28), respectively. Neither 16 nor 30 accept methanol at the carbenic site to give products of a formal O – H insertion. Carbene 16 affords 2‐methoxy‐2‐methyl‐2H‐1‐benzopyran (19) by way of the benzopyrylium ion 17. The same ether was obtained from the independently prepared perchlorate of 17. Protonation of 30 was followed by demethylation of the delocalized carboxonium ion 31 to give 5‐methoxy‐6,6‐dimethyl‐2,4‐cyclohexadienone (33) as the only product.

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Reactions of Vinylogous Oxycarbenes with Methanol. -As expected the vinologous oxycarbene generated in the photolysis of ( I) shows protonation and affords only the benzopyrane (III). No product of a formal OH-insertion is observed. The protonation of carbene (IV) is followed by demethylation to give (VI). -(HOEMBERGER, G.; KIRMSE, W.; LELGEMANN, R.; Chem.