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Organic Lab Reactions 188

Organic Lab Reactions 188 - 18 After cleavage of the acetal...

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SELECTIVE REDUCTION OF CARBONYL GROUPS 183 Selective Reduction of Carbonyl Groups Some efforts have been made to employ the aluminum isopropoxide reduction in such a way that only one of two or more carbonyl groups in the molecule would be reduced. Most of this work has been based upon the conversion of one of the carbonyl groups to an enol ether or acetal, thus protecting it from the action of the reducing agent. In this manner it was possible to reduce the dimethyl acetal of phenylglyoxal (II) " to the corresponding hydroxy acetal (III) in 55% yield. This type of pro- tection also has been employed successfully in the sterol field. The a- CHO CHOAc keto acetal (IV) was reduced to the corresponding hydroxy acetal (V).
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Unformatted text preview: 18 After cleavage of the acetal linkage and conversion to the diacetate, the aldehyde (VI) was obtained in 68% over-all yield. The thioacetal [RCOCH(SC 2 H 5 ) 2 ] corresponding to IV was resistant to reduction and could not be employed for this purpose. The 3-enol ether (VIII) of 16-benzalandrostenedione (VII) was re-duced selectively 19 to the carbinol (IX). After reduction the 3-keto group was regenerated by treatment of the enol ether with acid. Another C,H 6 vn VJXl 18 Schindler, Frey, and Reichstein, Hdv. CHm. Ada, 24, 360 (1941). 19 Stodola and Kendall, / . Org. Chem., 6, 839 (1941)....
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