Cyclopentanol ring expansion

Reactions of this type (Scheme 3.44) are especially useful for the preparation of cyclic p,p-dihaloenones by ring expansions of alkynyl cyclopentanols or alkynyl cyclohexanols. 1-Bromoethynylcyclopentanols 111 react with equimolar amounts of iodine and PhI(OH)OTs under mild conditions to produce cyclohexanone derivatives 112 with a high degree of stereoselectivity (Scheme 3.45) [129]. 

Likewise, stereoselective ring expansions in l-iodoethynyl-2-methylcyclopentanols 113 and 115 affords (3,(3-diiodoenones upon treatment with PhI(OH)OTs and iodine (Scheme 3.46) [134]. Depending on the relative stereochemistry of the methyl and the hydroxyl groups in the starting cyclopentanol, the products are 2-(diiodomethylidene)-3-methylcyclohexanone (114), from c -cyclopentanol 113 and 2-(diiodomethylidene)-6-methylcyclohexanone (116) from fran -cyclopentanol 115 [134]. 

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