Cyclohexene oxidative ring contraction

Simple alkyl-substituted dihydro-1,4-thiazines are readily converted into the corresponding 5-monoxides atmospheric oxidation is sometimes sufficient, although side reactions may also occur. The cyclohexene derivative (99), for instance, affords not only the oxide (100) but also the spiro compound (101) probably generated by ring contraction of a hydroperoxide intermediate (Scheme 42) <73US(B)(8)34l>. 

Treatment of 2-methoxycyclohexyl phenyl tellurium oxide, prepared from cyclohexene and phenyl tellurium tribromide in methanol followed by hydrolysis of the tellurium dibromide, with 3-chloroperoxybenzoic acid in methanol caused ring contraction with formation of the dimethyl acetal of cyclopentanecarbaldehyde1,2. 

Cyclohexene is oxidized to cyclopentanecarboxaldehyde in 85% yield in fact this TTN-induced ring contraction is probably the method of choice for preparation of this aldehyde. This reaction was used by Corey and Ravindranathan- as one step in a synthesis of 11-desoxyprostaglandins for the conversion of (1) into (2). However, in this case, the use of the original conditions led to complex mixtures containing only a... 

An attempt d to repeat, with presumably a minor modification, an established procedure for the conversion of cyclohexene oxide to the cyclopropane (255 equation 108) gave instead the ring-contracted isomer (256). This reaction is fairly general and constitutes a convenient method for the preparation of acrylate esters, including some not easily made in other ways. 

Oxidation of olefins. Kabbe explored briefly the oxidation of cyclohexene, styrene, and o-allylphenol and expressed the view that thallium triacetate is intermediate between lead tetraacetate and mercuric acetate. Anderson and Winstein followed the oxidation of cyclohexene in acetic acid at room temperature (several days) by vapor-phase chromatography and accounted quantitatively for all the five products formed the cij-and rrans-diacetates (1 and 2), the ring-contracted diacetate (3) and aldehyde (4), and the product of allylic oxidation (5). In dry solvent the transdiacetate (2) predominated, and in moist solvent the cis-diacetate (I) predominated. 

Since then amine-catalyzed 5-enolexo aldolizations have been applied in a variety of other synthetic contexts (Scheme 4.10) [70-75]. Typically they constitute part of a strategy for the ring contraction of cyclohexenes to cy-clopentenes via oxidative cleavage and aldolization (Eq. (2)). 

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