1,2-Cyclopentanediol, trans

Trans-1,2-cyclopentanediol has non-superimposable mirror images, and so can exist as two enantiomers. When equal parts of enantiomers are mixed, trans-1,2-cyclo-pentanediol is a racemic modification and appears to be optically inactive. This is because the equal parts of enantiomers rotate light to the same degree but in opposite directions, and the net effect is no observable optical activity. However, the trans compound can be resolved into its enantiomers by special methods, such as reacting them with reagents that are themselves optically active. 

The infrared spectrum of cis-1,2-cyclopentanediol has an 0-H stretching band at a lower frequency than for a free -OH group, and this band does not disappear even at high dilution, trans-1,2-Cyclopentanediol shows no such band. Can you suggest a possible explanation ... 

At this point, examine the structure of trans-1,2-cyclopentanediol as shown ... 

Acid-catalyzed hydrolysis (2) of 1,2-epoxycyclopentane yields a trans diol, trans-1,2-cyclopentanediol. Water acting as a nucleophile attacks the protonated epoxide from the side opposite the epoxide group. The carbon atom being attacked undergoes an inversion of configuration. We show here only one carbon atom being attacked. Attack at the other carbon atom of this symmetrical system is equally likely and produces the enantiomeric form of fra i-l,2-cyclopentanediol ... 

It is clear then that more than one mechanism is operative for glycol fission. In the case of c -cyclopentanediols and camphanediols a cyclic ester is a necessary intermediate. For tra/js-decalin-9,10-diol a non-cyclic mechanism must operate which cannot function for cholestane-3/ ,6j8,7a-triol and is inefficient for /rans-camphanediols. It is pertinent that while the fission of glycols capable of forming cyclic esters proceeds several hundred times faster in benzene than in acetic acid, the reactions of trans-decalin-9,10-diol and tra/ij-hydrindane-l,6-diol are 4-5-fold slower in benzene . ... 

Acid-catalyzed hydrolysis of 1,2-epoxy cyclopentane yields a trans-diol, trans-, 2-cyclopentanediol. 

Probtem 13.5 The ir spectra of trans- and cu-l,2-cyclopentanediol show a broad band in the region 3450-3570 cm On dilution with CCI, this band of the cis isomer remains unchanged, but the band of the trans isomer shifts to a higher frequency and becomes sharper. Account for this difference in behavior. M... 

Figure 58 Partial 13C NMR spectra for (R,R)-trans-1,2- diaminocyclohexane-( )-irani-l, 2-cyclopentanediol and (R)-l,l -bi-2-naphthol adducts. Left (a) (R,R)-trans-1, 2-diaminocy-clohexane (b) inms-( )cyclopentanediol (c) 1 1 adduct between (R,R)-trans-1, 2-diamino-cyclohexane and tranx-( )-cyclopentanediol (d) 0.5 1 adduct between (R,R)-trans-1, 2-diaminocyclohexane and Znms-(+/-)-cyclopentanediol. Right (a) (R,R)-trans-1, 2-diami-nocyclohexane (b) 1 1 adduct between (i )-l,l -bi-2-naphthol and trans-( )-1, 2-diaminocyclohexane (c) 1 1 adduct between R,R)-trans-1, 2-diaminocyclohexane and (2 )-1,1 -bi-2-naphthol (e.e. 35%) showing molecular recognition and chiral discrimination of the diastereomeric supraminols (d) 1 1 adduct between (R)-l,l -bi-2-naphthol and (R,R)-trans-1, 2-diaminocyclohexane [60],...

If we examine models of c/a- and /ra/75-K2-cyclopentanediol more closely, we find that each compound contains two chiral centers. We know (Sec. 4.18) that compounds containing more than one chiral center are often -but not always —chiral. Are these glycols chiral As always, to test for possible chirality, we construct a model of the molecule and a model of its mirror image, and see if the two are superimposable. When we do this for the trans-g yco, we find that the models... 

Aqueous acetic acid (rate of oxidation) [1, 145, after citation of ref. 5], Rocek and Westheimer58 reported that the oxidation of cm-l,2-dimethyl-1,2-cyclopentanediol (1) by chromic acid in water or 90% acetic acid to 2,6-heptanedione (2) is much faster than oxidation of the trans isomer (3). In water the oxidation of (1) to (2) is 17,000... 

You have two bottles labeled 1,2-Cyclopentanediol," one containing a compound of m.p. 30 , the other a compound of m.p. 55 both compounds are optically inactive. How could you decide, beyond any doubt, which bottle should be labeled "cis" and which "trans" ... 

Evidence that the reaction of the diol ester 5 with antimony pen-tachloride genuinely involves a neighboring-group reaction is provided by the stereospecificity of this reaction with esters of cyclic diols. trans -l,2-Cyclopentanediol diacetate (14) and Irons-1,2-cyclo-hexanediol diacetate (17) react " with antimony pentachloride to give the cis-acetoxonium salts 15 and 18, whereas the cis-diol esters (16 and 19) merely give difficultly soluble adducts from which unchanged starting material can be recovered after hydrolytic treatment. 

---