Reactions of Enol Ethers and Esters

Reactions of Enol Ethers and Esters.—When enol acetylation of a steroid 4-en-3-one with acetic anhydride is catalysed by perchloric acid (or boron trifluoride ), C-acetylation of the enol acetate (373) occurs on prolonged reaction. This step, probably involving acetyl perchlorate ior airs, is akin to Friedel-Crafts acylation of aromatic compounds. Both 6-acetylation and 2-acetyIation are known, although each is prevented by the presence of a methyl group at the site. The products can be of various enolic types (e.g. 374 and 375). 

5-Dienolic ethers (379), in contrast with the acetates, are cleaved by peroxy-acid at the 3,4-unsaturated link under anhydrous conditions, giving carbo-methoxy-aldehydes (380). The reaction is probably mechanistically similar to the oxidative cleavage of dihydropyrans e.g. 381 — 382), although the 

Furost-20(22)-enes (384) react with peroxy-acid with introduction of a hydroxy-group at the 20a-position, and ring closure to give the 20a-hydroxy-spirostan (385). The isolation of up to 25% of the 20,23-diol (386) is attributed to contamination of the furost-20(22)-ene with the 22(23)-unsaturated isomer. Almost all 3,5-dienolic esters and ethers react with electrophiles at C-6, apart from rare instances of reaction at C-4 or C-2 (see above). Reaction between the dienolic ethers (387) and tetranitromethane in ether has now been shown to afford the 2 -nitro-derivatives (388). No mechanistic details are available, but it is tempting to suggest the involvement of free-radicals derived from the reagent. 

Hydride abstraction by dichlorodicyanobenzoquinone from the A -trienol benzoate (389), to give the 1,4,6-trienone (390), is accompanied by a novel electro- 

Hydroboronation of enol acetates of cyclohexanones is generally non-specific. The 2-en-3-ol acetate (392) gave a mixture containing the 2o,3j3-diol and the 

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