Hemiacetals hemithioacetals

Heat of combustion, 113 Heat of hydrogenation, 186 table of, 187 Heat of reaction, 154 Helicase, DNA replication and, 1106 Hell-Volhard-Zelinskii reaction, 849 amino acid synthesis and. 1025 mechanism of, 849 Heme, biosynthesis of, 966 structure of, 946 Hemiacetal, 717 Hemiketal, 717 Hemithioacetal, 1148 Henderson-Hasselbalch equation,... 

Carb-29. Hemiacetals and hemithioacetals 2-Carb-30. Acetals, ketals and their thio analogues 2-Carb-31. Names for monosaccharide residues... 

When the hemiaminals A (Figure 9.12, Nu = NR3R4) are formed in a neutral or weakly basic solution, they also have the possibility to react further by an SN1 reaction albeit in a different manner than just described for the corresponding hemiacetals (Nu = OR3) and hemithioacetals (Nu = SR3). The OH group of hemiaminals A is then ejected without prior protonation (i.e., simply as an OH ion). This is possible because an especially well-stabilized carbocation is produced at the same time, that is, the iminium ion C (Nu = NR3R4). It reacts with the second equivalent of the N nucleophile. Proton loss affords the jVW-acetal B (Nu = NR3R4). 

Ordinary hemiacetals are less stable than acetals and, in fact, they have the tendency to disproportionate to give the parent carbonyl compounds and the acetals. Hemithioacetals are even less stable in comparison with hemiacetals and dithioacetals. 

Racemization via a reversible addition/elimination process under mild conditions can be used, with for example cyanohydrins, hemiacetals, hemiaminals and hemithioacetals. SiUca-supported benzyltrimethylammonium hydroxide (BTAH) was used to racemize cyanohydrins and effected an efficient DKR process in tandem with porous ceramic-immobilized lipase (lipase PS-C II) (Scheme 4.22) [57]. 

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