Furan enol ether route

The reaction is applicable to acyclic and cyclic enol ethers and to various (3-dicarbonyl compounds, but fails with silyl enol ethers and simple 1,2-disubstituted alkenes. When applicable, this route to furans is useful because the yields and regioselectivity are consistently satisfactory. The paper includes a preparation of the reagent by reaction of Mn(NO,)3 with Ac20 at 100° to give Mn,0(0Ac)7 H0Ac in 60% yield. 

An alternative to the above eliminations, which in principle offers a great many advantages, is the metal induced elimination of the elements of ROBr from a 2-halocyclopropyl ether. Thus the sequence of addition of dibromocarbene to an enol ether, reduction to a monobromide and reaction with magnesium in tetrahydro-furan provides a route to cyclopropene itself45). 

Furane synthesis.1 Copper-catalyzed decomposition of ethyl diazopyruvate (1) in the presence of enol ethers or of alkynes provides a useful route to ethyl a-furoates. 

In a route to polyhydroxylated quinolizidines, condensation between the silyl enol ether 182 and aldehyde 183 (Scheme 34) gave the aldol product 184 with high diatereoselectivity. Conditions were also found to obtain diastereoselective condensations between the enantiomer of 183 and either 182 or its precursor ketone to give stereoisomers of 184. Both 182 and 183 were obtained from furan. 

An alternative route starts from furan (19). Reaction with bromine in methanol leads to 2,5-dimethoxy-2,5-dihydrofuran (20) which is transformed to but-2-ene-1,4-dial bisdimethyl-acetal (21). Double enol ether condensation with 1-propenyl methyl ether (22), followed by acetal hydrolysis and elimination, provides crystalline (all- -Cio-dialdehyde 8 in an overall yield >50% [13]. 

Scheme 8 Enol ether ring-closing metathesis route to substituted furans.

According to route I (addition of water to the furan C-2/C-3 bond followed by disconnection O/C-2, that is, an enol ether hydrolysis according to steps a-c), the... 

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