Cyclopentene, hydroxylation

FIGURE 6 13 Mechanism of bromohydrin formation from cyclopentene A bridged bromonium ion is formed and is attacked by a water molecule from the side opposite the carbon-bromine bond The bromine and the hydroxyl group are trans to each other in the product... 

Rogers, J.D. (1989) Rate constant measurements for the reaction of the hydroxyl radical with cyclohexene, cyclopentene, and glutaraldehyde. Environ. Sci. Technol. 23, 177-181. 

OH radicals react very fast (almost in a diffusion-controlled rate) with simple alkenes (k = 7.0 x 109 for 1-butene or cyclopentene and 8.8 x 109 M 1 s 1 for cyclohexene) and there is almost no change for 1,3- or 1,4-cyclohexadiene. Cycloheptatriene reacts very fast with all the three radicals formed in the radiolysis of water k = 6 x 109 with eaq, 8 x 109 with H atoms and 1 x 101CI M 1 s 1 with hydroxyl radicals13. 

The cis form has been prepared by the syn hydroxylation of cyclopentene by osmium tetroxide as follows ... 

An alternative approach for generating the pentadienyl carbocation that is needed for the Nazarov cyclization has been demonstrated by de Lera and co-workers [20, 21] (Eq. 13.18). Vinylallene acetal 56 is converted to a ca 1 1 mixture of cyclopentenes 57 and 58 upon exposure to toluenesulfonic acid in acetone at room temperature. The reaction presumably involves initial generation of carbocation 59 that undergoes conrotation to give 60. Intramolecular trapping of the carbocation by the pendant hydroxyl group leads to the observed product. Depending on whether the conrotation in 59 takes place clockwise or counterclockwise, E- (57) or Z-(58) products are formed. 

Cycloalkylation (7, 148).2 Cycloalkylation of 2 with (Z)-l was used as one step in a total synthesis of (+ )-sesbanine (6), a constituent of Seshania drummondii seeds with antileukemic activity. The hydroxyl group was introduced into the cyclopentene ring of 4 by iodolactonization followed by reduction to give 5. Final steps included aminolysis of the lactone ring, intramolecular addition of the amide anion to the CN group, and hydrolysis to give 6. 

This is an alternative scheme for the hydroxylation of alkenes (see Section 11-7C). However, the overall stereochemistry is opposite to that in permanganate hydroxylation. For instance, cyclopentene gives trans-1,2-cyclopen-tanediol. First the oxirane forms by suprafacial addition and then undergoes ring opening to give the trans product ... 

Prostaglandins (= PG) constitute a class of hormones that are present in almost all human tissues and fluids in minute concentrations and are thought to play a dominant role in the control of pregnancy, hypertension, ulcers, asthma, and pain. Since they may be useful as pharmaceuticals, several total syntheses were developed, and the term to, prostaglandize a class of compounds, which means to exhaust its synthetic potentials, has become known. The principal structures of prostaglandins are shown below. The capital letters A,B,C,E,F denote the state of oxidation and the position of double bonds in the cyclopentane or cyclopentene ring the numeral subscript refers to the number of double bonds in the side-chains 1 = (trans)-13 2 (cis,trans)-6,13 3 = (cis,trans,cis)-6,13,17 a( = below) and fi( = above) indicate the position of the hydroxyl group on C-9. 

Nozaki reaction (12, 137 14, 96 15, 95). An intramolecular version of this reaction can effect cyclization even to a strained nine-membered ring. Thus 1 on treatment with CrCl2 at a low temperature cyclizes mainly to 2, a mixture of epimers in which a cyclononadiyne ring is fused to a cyclopentene ring. After acetylation of the secondary hydroxyl group, the mixture is dehydrated to 3, with a nine-membered enediyne ring. 

Because glycol formation involves a back-side attack on a protonated epoxide, the result is anti orientation of the hydroxyl groups on the double bond. For example, when 1,2-epoxycyclopentane ( cyclopentene oxide ) is treated with dilute mineral acid, the product is pure trans-cyclopentane- 1,2-diol. 

The Simmons-Smith reaction with cyclopenten-4-ol gives exclusively the cis isomer of bicyclo[3.1.0]hexan-3-ol (94, 540, 541). Cleavage of the Simmons-Smith reagent by the hydroxyl group would give a salt (VI), and intramolecular methylene transfer would then yield m-bicyclo-[3.1.0]hexan-3-ol after hydrolysis (42). An alternative mechanism was... 

Hydroxylation by hydrogen peroxide in t-butyl alcohol solution is catalyzed by osmium tetroxide. The catalyst is volatile and dangerous to handle but is conveniently used in a solution of the tertiary alcohol. The yields of diols are usually low (30-60%), and the process has not been adapted to large-scale preparations. In contrast to hydroxylation by performic acid, this procedure leads to cis addition of the two hydroxyl groups to the double bond. An extensive study of other catalysts has been made. Some catalysts, e.g., selenium dioxide and pertungstic acid, catalyze addition in the trans direction. Hydroxylation of cyclopentadiene takes place in the 1,4-positions to give 2-cyclopenten-l,4-diol. ... 

We shall begin with the glycol of cyclopentene, 1,2-cyclopentanediol. Using models, we find that we can anangc the atoms of this molecule as in I, in which both hydroxyls lie below (or above) the plane of the ring, and as in II, in which one hydroxyl lies above and the other lies belpw the plane of the ring. 

Irradiation of l,3-bisdiazocyclohexan-2-ones in hydroxylic solvents affords cyclopentene-1-carboxylic acids and esters regiochemical control is only moderate. Although the course of the reactions could be accommodated by sequential Wolff rearrangement and [1,2] H-shift, studies on l,3-bisdiazo-l,3-di-phenylpropan-2-one indicate the formation of the cyclopropenone intermediate (106). ... 

SAMPLE SOLUTION (a) The hydroxyl-bearing carbon in 2-cyclopenten-1-ol is a stereogenic center. There is no stereogenic center in 3-cyclopenten-1-ol, since the sequence of atoms 1 ->2->3 4 5is equivalent regardless of whether one proceeds clockwise or anticlockwise. 

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