Y-Hydroxyalkenes

The reaction between phenyl cations and alkenols represents another example of a one-pot, tandem synthesis of cyclic ethers. Accordingly, 2-ben2yltetrahydrofurans are obtained by the addition of photogenerated phenyl cations to y-hydroxyalkenes. For example, 4-penten-l-ol (a terminal alkene) reacts with phenyl chlorides bearing EDGs under irradiation in polar protic solvents, suchasTFE oramixture MeCN/H20 (Scheme 10.57) [72],... 

If the reaction is carried out in an emulsion of sodium hydroxide and hydrogen peroxide, the ozonide inteimediaies are convoted to carboxylic acids directly, with a consequent increase in yields. Oxidative cleavage of y-hydroxyalkenes results in the formation of lactones in good yields (equation... 

Reitz AB, Nortey SO, Maryanoff BE, Liotta D, Monahan R HI (1987) Stereoselectivily of electrophile-prmnoted cyclizations of y-hydroxyalkenes. An investigation of carbohydrate-dtaived and model substrates. J Org Chem 52 4197-4202... 

Oxidative cleavage of y-hydroxyalkenes results in the formation of lactones in good yields (equatitm 26). 28... 

Kang SH, Park CM, Lee SB, Kim M. Asymmetric iodocycU-zation of y-hydroxyalkenes. Synlett. 2004 7 1279-1281. 

Positive halogen reagents can cyclize y- and 8-hydroxyalkenes to tetrahydro-furan and tetrahydropyran derivatives, respectively.85 Iodocyclization of homoal-lylic alcohols generates 3-iodotetrahydrofiirans when conducted in anhydrous acetonitrile.86 The reactions are stereospecific, with the /(-alcohols generating the irons and the Z-isomer the cis product. These are endo-5 cyclizations, which are preferred to exo-4 reactions. 

However, as the formation of an intermediate simple chromate ester is not as favorable as the generation of the cyclic chromate ester, involved in the oxidation of 5,6-dihydroxyalkenes, this reaction, demands harsher conditions. Therefore, only tertiary 5-hydroxyalkenes may be normally used as starting compounds, otherwise a direct oxidation of the alcohol to an aldehyde or ketone would occur.287 Because of the harsher conditions involved, very often the resulting 1-hydroxyalkyltetrahydrofuran is further oxidized to a y-lactone or to a ketone.288... 

The intramolecular version of photoaddition of alcohols to alkenes was also reported. Mizuno and Otsuji reported the DCA-sensitized photocyclization of co-hydroxy-l,l-diphenylalkenes (n = 3-5) via exciplex in benzene [38]. The long chain cu-hydroxyalkenes (n = 6,10) do not give the corresponding cycHzed ethers. Gassman reported the photocyclization of unsaturated carboxylic acids by the use of a sensitizer system consisting of sterically hindered electron acceptors and biphenyl to give y-lactones (Scheme 10) [39]. 

Electrophilic iodine reagents have also been employed in iodocyclization. Several salts of pyridine complexes with such as bis(pyridinium)iodonium tetrafluoroborate and bis(collidine)iodonium hexafluorophosphate have proven especially effective. y-Hydroxy- and (3-hydroxyalkenes can be cyclized to tetrahydrofuran and tetrahydropyran derivatives, respectively, by positive halogen reagents. (see entries 6 and 8 in Scheme 4.2). 

Silyloxy allylic stannanes are isomerized with TBDMS triflate to (Z)--y-silyloxy allylic stannanes (eq 11). The resulting allylic stannanes undergo addition reactions with aldehydes in the presence of Boron Trifluoride Etherate to provide the 3-(r-butyldimethylsilyloxy)-4-hydroxyalkenes. ... 

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