Ketal, isopropylidene removal

Difluorocyclopentadiene was prepared via a pyrolysis reaction the difluoro-methylene centre was installed (Eq. 119) using a DAST fluorination [318]. A fluorobutenolide building block was prepared by a Wadsworth-Emmons reaction of isopropylidene glyceraldehyde removal of the ketal protecting group led to the formation (Eq. 120) of the unsaturated lactone in acceptable overall yield... 

For the removal of toe isopropylidene ketal a variety of acid-catalyzed hydrolyses have been described. Typical conditions are Dowex 50-W (H l/HaO, 70 °C and 60-80% HOAc, 25 °C or reflux. 

Hydrolytic removal of acetals can be selective, as in the well-known production of 1,2-isopropylidene ot-D-glucofuranose from mild acid-catalysed hydrolysis of l,2 5,6-diisopropylidene ot-D-glucofuranose. °° The difference in reactivity of the two isopropylidene derivatives lies in the differing stabilities of the oxocarbenium ion intermediates. Under acid-catalysed conditions, a cyclic ketal with different electron demand at the alcohol sites will first open to give the most stable oxocarbenium ion leaving group ability is not an issue... 

During 0-isopropylidenation of the alditolyl chain of the 3-(alditol-l-yl)quinoxalin-2-one 745, the fused furan ring of 746 was formed as a result of acid-catalyzed cyclodehydration of the enolized C2 carbonyl and C2 OH groups. Removal of the ketal group of 746 gave 747 (93MI6) (Scheme 192). 

Being a ketal, this protective group shares with tetrahydropyranyl derivatives the property of being resistant to basic and nucleophilic reagents, but is readily removed by aqueous acid. The isopropylidene group can be introduced by acid-catalyzed condensation with acetone, or by acid-catalyzed exchange with 2,2-... 

These compounds have received considerable attention because of their importance in biosynthetic pathways and as precursors for the synthesis of enantiomeric phospholipids (cf. Sections 7.5 and 11.2). There are several procedures which involve blocking the 5n-3-hydroxyl in 1,2-isopropylidene- n-glycerol with various groups, followed by cleavage of the ketal, reacylation with specific fatty acids and then removal of the blocking group (Jensen, 1972). 

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