Isopropylidene ketals

CYCLIC ACETALS AND KETALS 123 7. Acetonide (Isopropylidene Ketal) (Chart 3)... 

The present work involves the study of methyl glycosides and O-isopropylidene ketals of various isomeric deoxy sugars by mass spectrometry. Several of the compounds selected for the present study have free hydroxyl groups, and interpretation of their mass spectra shows the scope of the study of these and related deoxy sugar derivatives by mass spectrometry without prior substitution of all hydroxyl groups. Some of the candidates (compounds 4, 7, 8 and 10) are structurally related to biologically-derived deoxy sugars. 

The Mass Spectra of O-Isopropylidene Ketals 9 (Figure 6), 10, (Figure 7), and 11 (Figure 8). Three fragmentations characteristic of O-isopropylidene ketals are loss of a methyl radical from the ketal ring, cleavage of bonds adjacent to the ketal ring, and loss of acetone (12). 

Peaks at m/e 113 and 85 have been found in the mass spectra (12) of other O-isopropylidene ketals of sugars, as well as in Figure 7. Since these shift to m/e 119 and to m/e 88 and 91 in the mass spectrum of 10a as they did for the d6-analogs in Reference 12, the structures, 17, 18, and 19 from Reference 12 are shown as possible explanations. The peak at m/e 85 (91) could alternatively be from m/e 113 (119) by loss of carbon monoxide (28 mass units) from the six-membered-ring of structure 17b. 

The reaction is 100% stereoselective and affords 69 as a unique stereomer with the (/f)-configuration at C-6, assigned by X-ray analysis. Consequently, the configuration at C-6 results from the well-known chair transition state model for the (Z)-O-allyl enol 68 in which the aUyl unit is on the opposite side to the isopropylidene ketal at C3-C4 and reacts on the Si face of the trisubstituted carbon-carbon double bond. Such a transition state leads to the sole (/ )-configuration in 69. 

First the isopropylidene ketal at C-3b and C-4b is generated using acetone and an ion-exchange resin at first. There is no further ketal formation at other positions of the lactose. It is also possible to use 2,2-dimethoxypropane (Me)2C(OMe)2 instead of acetone. The problem in this step is the partial formation of hemi-ketals with the other hydroxy groups which lower the yield. They are destroyed by heating to 100 °C in glacial acetic acid.20... 

Another approach that led to differentiation is the combination of bulky group and isopropylidene ketal. With the ketal strongly favoring formation onto vicinal cis-diols, the 6-O-protected galactoside 47 [169,170] and mannoside 49 [171] provided the 2-alcohol 48 and 4-alcohol 50 in high yields, respectively (Scheme 2.18). 

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