Hemiacetal moiety

Step 2c Dilute HC1 depfotects the keto groups and facilitates the formation of the hemiacetal moiety in the TM. 

The oxidizing agent is a compound that, like ATP, constantly appears in these reactions nicotinamide adenine dinucleotide (NAD). The functional group here, we remember (Sec. 36.15), is the pyridine ring, which can accept a hydride ion to form NADH. Like the hemiacetal moiety, NAD is bound to the enzyme, and in a position for easy reaction (Fig. 37.3). 

Diethyl 2-oxo-3-alkenylphosphonates, which are readily accessible from diethyl 2-oxopropylphos-phonate, are useful heterodienes in Diels-Alder reactions. Cycloaddition reactions with vinyl ethers in ( I I in a sealed tube at 85-130°C give satisfactory yields (57-88%) of dihydro-27/-pyrans. The products are isolated as a mixture of 2,4-tra .v and 2,4-czT isomers. When treated with 4 M HCl in THF, the dehydropyran hemiacetal moiety is readily hydrolyzed to provide diethyl 5-formyl-2-oxopentylphosphonates in excellent yields (72-92%, Scheme 5.59). When R> = R =H, the aldehyde undergoes smooth cyclization leading to diethyl 2-oxocyclohexenephosphonate. ... 

The radical elongation process was involved in the another synthesis of KDO which started from D-lyxose bromide 159 and alkene 160 [108], (Scheme 34). The reaction proceeded in dry benzene under reflux to give a (3 1) diastereoisomeric mixture of 161 and 162. Conversion of the isomer 161 to the KDO salt required deprotection and reduction of the hemiacetal moiety, which led to 163. Ozonolysis process subsequent by treatment with NHjaq gave the desired KDO in high yield. 

The reduction of phthalidetetrahydroisoquinoline alkaloids was studied in an attempt to duplicate the hemiacetal moiety of rhoeagenine (154c) (706). The final product depends on the temperature and on the reducing... 

In the case of peraksine [RP-5 (27)] it was shown to have the formula C19H22N2O2, a fact not readily derived from combustion analysis since the free base crystallized from alcohol in a hydrated form (27). It has also been observed that this water of solvation could be displaced by chloroform (20). Peraksine has UV-absorption typical of a 2,3-disub-stituted indole and reacted with benzoyl chloride to form an 0-benzoyl derivative. The second oxygen was apparently present as a cyclic ether when it was found that although peraksine did not react with hydrazine derivatives, it was reducible with sodium borohydride to furnish a diol (mp 290°-291° [a]j) - -41° in Py diacetate, mp 103°-105°). This diol readily lost the elements of water upon acid treatment to afford a new ether, deoxyperaksine, 230° change in crystalline form (mp 255°-257°). Because of these properties peraksine was considered to possess a cyclic hemiacetal moiety. 

Unhke the other disaccharides we have seen thus far, sucrose is not a reducing sugar and does not undergo mutarotadon. This can be explained by noting that sucrose is comprised of two units that are linked to each other via their anomeric carbons. As such, neither unit has a hemiacetal moiety and neither unit is capable of adopting an open-chain form. 

As is shown in Scheme 13.5, the authors found that treatment of enol ether 32 with TEA resulted in both the removal of the Boc group and hydration of the enol ether to give lactol 33. After removal of the DNs (2-(2,4-dinitrophenyl)ethoxycarbonyl) group with pyrrolidine, the hemiacetal moiety reacted with the prereleased secondary amine to produce the cyclic enamine 34. Subsequently, the Michael addition of the enamine to the unsaturated ester and Mannich reaction of the indole to the resultant iminium ion furnished the desired 35 as the sole isomer. Finally, a regioselective dehydration and stereoselective epoxidation, followed by a deprotection step, gave the desired lower segment (-)-taberhanine, which was used for the total synthesis of conophylline. 

In carbohydrate chemistry, simple allyl groups were used to liberate hemiacetal moieties (Scheme 5-10). ... 

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