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Dioxycarbenes (4) have been generated by thermolysis of norbornadienone ketals (l),' by photolysis or thermolysis of 3,3dioxydiazirines (2),2 and by thermolysis of 2,2-dioxy-A3-1,3,4oxadiazolines (3),3 Scheme 1 .4 Among these approaches, that based on 1 has been limited to the generation of only a few dialkoxycarbenes.' Moreover, the byproducts from thermolysis of 1 can interfere with the isolation of products from reactions of 4.Diazirines 2, either with like (MeO, MeO) or with unlike (PhO, MeO) oxy groups, have been prepared.2 Their thermal or photochemical decompositions are relatively clean. As the only photochemical sources of 4 currently available, they are well suited for direct observation of carbenes 4 and for studies of the rates of their reactions.2c-8 However, diazirines 2 are not convenient for synthetic work with 4 because they are hazardous compounds available only as dilute solutions.Oxadiazolines 3, available by oxidation of (alkoxycarbony1)hydrazones of acetone (5) with lead tetraacetate (LTA)3 or by electrochemical ~x i d a t i o n ,~ can be obtained as pure compounds with long shelf lives6 (Scheme 2). They are attractive sources of 4 because they fragment quite cleanly in solution at ca. 100 OC to afford 4 and innocuous byproducts (Nz, acetone), but there are limitations to thnr preparation. For example, oxidation of 5 affords oxadiazoline in very poor yield in the presence of some alcohols (Scheme 2, RzOH = C F~C H Z O H , for example), and oxidation of 5 in the presence of phenols has very poor prospects, because many phenols are themselves oxidized easily.We now report a general approach to dioxyoxadiazolines (9a-1) from 2-acetoxy-2-methoxy-5,5-dimethyl-A3-1,3,4-0xadiazoline (7). The latter is formed by oxidation of the (methoxycarbony1)hydrazone of acetone ( 6 ) with LTA in dichloromethane (Scheme 3). The yield of 7 falls between 60 and 72%, and the byproduct is 8. Partial separation of 7 from 8 is possible by slow bulb-tobulb distillation under vacuum, but this tedious process is not necessary for the preparation of pure 9. Instead, the mixture of 7 and 8 can be stored in the refrigerator for later use after

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Cyclic 3-alkoxy-3-aryloxyvinylcarbenes can be generated by a highly exo-selective intramolecular cyclization of a dioxycarbene onto a tethered triple bond. Like their dialkoxy counterparts, the 3-alkoxy-3-aryloxyvinylcarbene intermediates are capable of interesting reactions. In contrast to 3,3-dialkoxyvinylcarbenes, which undergo a formal [3 + 2] cycloaddition with highly electron-deficient olefins such as benzylidenemalononitrile, 3-alkoxy-3-aryloxyvinylcarbenes undergo a highly diastereoselective [1 + 2] cycloaddition with benzylidenemalononitrile to give cyclopropylketene acetals. At high temperatures, those cyclopropylketene acetals undergo a clean vinylcyclopropane rearrangement in which the stereochemical integrity of the cyclopropane is retained. Keywords: carbene, alkoxyaryloxy, [1 + 2] cycloaddition, ketene acetal, vinylcyclopropane rearrangement, oxadiazoline.

Dialkoxycarbenes with a tethered triple bond, generated from the corresponding oxadiazolines, undergo an intramolecular cyclization with the tethered alkyne moiety to give 3,3-dialkoxyvinylcarbene/1,3-dipole intermediates. The regioselectivity of the carbene cyclization is highly dependent upon the nature of the alkyne substituent. In the cases where an exocyclic vinylcarbene is generated, the vinylcarbene intermediate can undergo a synthetically useful [3 + 2] cycloaddition with either an appropriate olefin or an alkyne. This approach allows the rapid construction of some interesting heterocyclic ring systems. Keywords: carbene, dialkoxycarbene, vinylogous dialkoxycarbene, heterocycle.

Thermolysis of 2-(3-butyn-l-oxy)-2-methoxy-5,5-dimethyl-A3-l,3,4-oxadiazoline in benzene or in toluene affords (la,laa,3aa,6aa,6ba)-methyl la-ethenyloctahydro-6a-methoxy-2,2-dimethyl-3,6dioxacyclobut[cd]indene-l-carboxylate in up to 74% yield. A reasonable mechanism involves about ten sequential steps and some novel intermediates including a dioxycarbene, a cyclopropenone ketal, and a vinylogous dioxycarbene. Various steps in the proposed mechanism were inferred from the results of deuterium-labeling experiments and from the incorporation of key intermediates from solution into the final product.Recently we reported the preparation of 2,2-dialkoxy-A3-l,3,4-oxadiazolines, such as 2, by oxidation of the

The crystal structure of the title compound, C54H36N2, shows that the anthracene rings in the unit cell do not stack in a face-to-face manner. Instead, the anthracene Acta Crystallographica Section C Abstract Methyl (+)-(1 a, 1 aa,3aa,6aa,6ba)-1 a-ethenyl-1,1 a,2,3a,-4,5,6a,6b-octahydro-6a-methoxy-2,2-dimethyl-3,6-dioxacyclobut[cd]indene-l-carboxylate, C15H2205, adopts a hemispherical conformation with the methoxy, methoxycarbonyl and vinyl groups pointing outwards from the hemisphere. The tetrahydropyran ring is in a chair conformation and the tetrahydrofuran ring is in an envelope conformation.