Ether dialkyl trialkyloxonium salt

The starting trialkyloxonium salts can be rather simply prepared from the corresponding dialkyl ethers. However, only trimethyl-and triethyloxonium boroflu-orides have been used in the synthesis of nitronates to prepare O-methyl and O-ethyl nitronates, respectively. 

Trialkyloxonium salts are generally prepared by the alkylation of dialkyl ethers.1 90 Alkyl halides can be used as alkylating agents in the presence of strong halide acceptors such as BF3, PF5, SbCl3, and SbF5 [Eq. (4.13)].91... 

In many cases, the main step in the syntheses of trialkyloxonium salts is the alkylation of a dialkyl ether with a reactive intermediate oxonium ion formed in situ. Thus, the most widely used method for preparing trialkyloxonium tetrafluoroborates by the reaction of epichlorohydrin and BF3 is based on the intermediacy of the inner oxonium salt 3291 95 97 [Eq. (4.19)]. 

Lactams and some non-cyclic, secondary amides (RCONHR) can be alkylated with high regioselectivity either at nitrogen (Section 6.6) or at oxygen. N-Alkylations are generally conducted under basic reaction conditions whereas O-alkylations are often performed with trialkyloxonium salts, dialkyl sulfates, or alkyl halides/silver salts without addition of bases. Protonated imino ethers are formed these are usually not isolated but are converted into the free imino ethers with aqueous base during the work-up. Scheme 1.8 shows examples of the selective alkylation of lactams and of the formation of 2-pyrrolidinones or 2-iminotetrahydrofurans by cycli-zation of 4-bromobutyramides. 

No comparison between trialkyloxonium fluoroborates and dialkyl sulfates has been made, but analysis of available data shows that oxonium salts are more generally applicable reagents for the preparation of lactim ethers. The alkylation of 3-carbethoxypiperid-2-one (7)25 and morpholin-3-one (8)32 with dimethyl sulfate failed, but with triethyloxonium fluoroborate these compounds gave 2-ethoxy-3-carbethoxy-3,4,5,6-tetrahydropyridine (9) and 3-ethoxy-3,4-dehydro-morpholine (10) in excellent yield. The selective character of triethyloxonium fluoroborate is shown in its reaction with 3,3-diethyl-5-methylpiperidine-2,4-dione (11 ).25 Reaction of 11 with the calculated quantity of dimethyl sulfate resulted in alkylation of the carbonyl group in position 4 with formation of 12, but reaction of 11 with triethyloxonium fluoroborate gave the lactim ether (13). 

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