Rearrangement cyclobutane-1,2-diols

Three isomers of the substituted cyclobutanes 33, 34 and 35 are obtained by the [2+2] cycloaddition of isoprene, and separated at low temperature from other cooligomers without undergoing Cope rearrangement. When the mixture was subjected to hydroboration and oxidation, the alcohol 37 was obtained from the isomer 34, and easily separated from 35 and the diol 36. The alcohol 37 is a pheromone called grandisol [11]. Although overall yield was 15%, this is the shortest synthetic route to this pheromone. 

Reductive dehalogenation is an efficient method of synthesis of cyclopropanes spiroannulated to five- and higher-membered carbocycles (i.e. compounds in which spiroannulation does not result in accumulation of extra strain) . The required gem-(dihalomethyl)cycloalkanes are usually prepared by halogenation of the precursor diols (equation 1). The cyclization is most efficiently accomplished in the Zn-alcohol-water system . For example, spiro[2.5]octane 7 was prepared in 91% yield using this procedure. This method is useful even for a one-step preparation of bis-spirocyclopropyl compounds as exemplified in equation 2. However, the application of the reductive dehalogenation method to the synthesis of more strained SPC (i.e. spirohexane or spiropentane) often leads to rearranged products. For example, methylenecyclopentane was the only product obtained from bis(bromomethyl)cyclobutane (equation 3) ... 

By using a stoichiometric amount of the chiral titanium reagent prepared by mixing chiral diol, Titanium IV) Chloride, and titanium tetraisopropoxide, the asymmetric [2 + 2] cycloaddition reaction of 1,4-benzoquinones and styrenes gives the corresponding cyclobutane derivatives with high optical purity. These rearrange to 2,3-dihydrobenzofuran derivatives on mild acid treatment (eq 13). ... 

Ring contractions of cyclobutane-1,2-diols have been widely reported. Both cis- and fran -cyclo-butane-l,2-diols rearrange under acid catalysis to give the corresponding cyclopropanecarb-aldehydes or ketones, generally in high yield. ... 

Therefore, in the case of 1-alkylated cyclobutane-1,2-diols the tertiary hydroxy group is the preferred or exclusive leaving group. In the case of 1,2-dialkylated cyclobutane-1,2-diols the trans-isomers were normally found to rearrange somewhat more readily than the cts-isomers. 

Carbon-Oxygen Bond Formation. CAN is an efficient reagent for the conversion of epoxides into /3-nitrato alcohols. 1,2-cA-Diols can be prepared from alkenes by reaction with CAN/I2 followed by hydrolysis with KOH. Of particular interest is the high-yield synthesis of various a-hydroxy ketones and a-amino ketones from oxiranes and aziridines, respectively. The reactions are operated under mild conditions with the use of NBS and a catalytic amount of CAN as the reagents (eq 25). In another case, N-(silylmethyl)amides can be converted to A-(methoxymethyl)amides by CAN in methanol (eq 26). This chemistry has found application in the removal of electroauxiliaries from peptide substrates. Other CAN-mediated C-0 bondforming reactions include the oxidative rearrangement of aryl cyclobutanes and oxetanes, the conversion of allylic and tertiary benzylic alcohols into their corresponding ethers, and the alkoxylation of cephem sulfoxides at the position a to the ester moiety. 

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