From enol ethers

Yohimbenone has also been prepared by Benson and Winterfeldt (213) from enol ether 377, obtained by Birch reduction of 376, via subsequent acid hydrolysis to 378 and formaldehyde cyclization to 370. 

Figure 1. Synthesis of acetals from enol ethers and diols.

I. From Aldehydes and Ketones with 1-Alkoxy-l-lithiocyclopropanes and from Enol Ethers by Cyclopropanation... 

Thiazinium oxides (131) are formed in a two-step sequence from enol ethers (130) and sulfoximines (129 Scheme 52) (78CC197), whereas alkylsulfamoyl chlorides and triethyl-amine react with activated dienes to afford diketosulfonamides (132) which on heating afford 1,1-dioxides (133 Scheme 53) (79JOC305). 

Preparation from enol ethers of acyl silanes. 1623... 

Modem variants of the Mukaiyama aldol addition start from silyl enol ethers, not from enol ethers, and use an aldehyde instead of the acetal as the electrophile. Mukaiyama aldol additions of this kind have been included in the C,C coupling reactions that build the basic repertoire of modem synthetic chemistry and can even be performed in a catalytic enantioselective fashion. 

The [2 + 2]-photocycloaddition of nonconjugated alkenes was described in Chapter 5. This strategy can be used for synthesizing macrocyclic rings by using a long linker between the two alkene moieties (Scheme 9.47). Thus, bicyclic cyclobutanes can be obtained starting from enol ethers and cinnamates by means of electron-transfer sensitizers, such as DCN or DCA [80], and triplet photosensitizers such as benzo-phenone (BP) [81]. 

As can be expected, use of ethyl diazoacetate procides y-oxoesters15) ory-oxocarboxylic acids 16) from enol ethers. Emploging the Julia method with 25 leads to the p,y-unsaturated aldehyde 26. Thus, this sequence establishes an overall a-vinylation of a given aldehyde n 17 18). 

B. Synthesis of Enolates from Enol Ethers and Esters. 542... 

The reaction of enol ether radical cations with hydroxide is six orders of magnitude faster than with water [249], but products have not been identified. In contrast, reaction of 37 and 38 with acetonitrile or methanol is very slow. In a comparative study, it was demonstrated that enol ether reacted much slower than enol ester radical cations with fluoride [227] affording ot-fluoroke-tones only in poor yield. Hitherto, reports in the literature give no evidence for the loss of an alkyl group from enol ether radical cations which would allow to enter ot-carbonyl radical and ot-carbonyl cation chemistry. 

TABLE 3. Synthesis of a-benzylated ketones starting from enol ethers... 

Acyl Anions Derived from Enol Ethers... 

Epoxidations. Formation of chiral epoxides from enol ethers derived from protected glucopyranosyl derivatives has been reported. 1,2-Epoxyalkylphosphonates are obtained from epoxidation of vinylphosphonates. Generation of dimethyldioxirane at high pH (10.5-11.5) is advantageous. ... 

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