New Convenient Source of Precursors of Dioxycarbenes

Abstract

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

Methoxy-(2-trimethylsilyl)ethoxycarbene. Reactions with Michael acceptors, with hydroxylic compounds, and with miscellaneous functional groups

“…The ready availability of 2,2-dialkoxy oxadiazolines (I), particularly from a precursor such as 2-acetoxy-2-methoxy-5,5-dimethyl-~3-1,3,4-oxadiazoline (1) (2) by means of substitution reactions with hydroxylic nucleophiles, prompted us to synthesize (3) 2-methoxy-5,5-dimethyl-2-(2-trimethylsilyl)ethoxy-A3-1,3,4-oxadiazoline (2), Scheme 1. Analogous oxadiazolines undergo thermolytic fragmentation to N2, acetone, and a dialkoxycarbene (2), which, in the case of 2, would be methoxy(2-trimethylsilyl)ethoxycarbene (3,Scheme 1).…”

“…Analogous oxadiazolines undergo thermolytic fragmentation to N2, acetone, and a dialkoxycarbene (2), which, in the case of 2, would be methoxy(2-trimethylsilyl)ethoxycarbene (3,Scheme 1).…”

Intra‐ and intermolecular reactions of nucleophilic carbenes

“…The precursor was, in each case, an oxadiazoline, prepared by the general procedure (Ref. 15 ) of Scheme 4 for dioxycarbenes, and according to Scheme 5 for alkoxyaminocarbenes (Ref.…”

Oxadiazoles

“…This reaction route was further developed, transforming 767 into various D 3 -1,3,4-oxadiazolines 768 and 769 by reaction with alcohols or phenols. Notably, these compounds have been used as a carbene source, because they fragment quite cleanly in solution at about 100 C to give as by-products acetone and N 2 [559]. oxadiazolines in 60-75% yield (Scheme 13.243) [560].…”