Lactone opening

Sn2 addition 413 Sn2 displacement 215 Sn2 lactone opening 412 ff. sodium bis(2-methoxyethoxy)-aluminium hydride see Red-Al sodium chlorite 516 - oxidation 772 sodium hypochlorite 550 f. sodium naphthalenide 739 f. sodium trimethoxyborohydride 390 SOMO 409... 

Anomeric radicals can be trapped by sugars having an exo-methylene group. This principle has been applied to C-2 exo-methylene lactones, opening the way to the interesting new class of carbon-linked disaccharides [113,114] (Scheme 30). 

The asymmetric catalytic hydrogenation of 4-aryl coumarins has been reported (Scheme 30) <1999TL3293>. Initial attempts to asymmetrically hydrogenate coumarin 240 directly proved unsuccessful. However, upon lactone opening, 241 was smoothly converted to hydrogenated product 242, which would relactonize to 243 gradually on standing or more quickly under catalysis with />-toluenesulfonic acid. 

Bioprecursors are distinguished from prodrugs by the lack of a promoiety, yet can be activated by oxidation (see Figure 20.5), reduction (see Figure 20.6), or hydrolysis (e.g., lactone opening) [14]. 

These conditions were used when dimethoxypropane was ineffective because of lactone opening as a result of the released methanol. ... 

Scheme 41. Selective lactone opening reactions with NaOMe.

Ring-fused 3-lactones open to deliver rram -2-organocycloalkanecarboxylic acids. 

Lactone opening A crucial operation in a synthesis of 12a-deoxytetracycline requires a mild alcoholysis of a lactone under conditions precluding closure of the B-ring, because intermediates with the linear tricarbocyclic array resist further cyclization. After successful lactone opening by treatment with Bu,SnOMe, the a-tetralone unit can be masked as a lactol silyl ether to permit a Dieckmann cyclization to form the A-ring. 

The temperature dependence of chemoselectivity for lactone opening or displacement of a y-alkoxy group has been discovered. 

This reaction scheme can be further extended when y-keto aldehydes 4 are employed as reaction partners to thiopyridyl ketene acetal lb (Scheme 8.2) [4]. In that case, the formed P-lactone 5 with the TIPS (triisopropylsilyl) group still attached to the lactone oxygen is attacked intramolecularly by the ketone group to furnish, upon lactone opening, a five-membered oxocarbenium ion 6. The in situ... 

In 2004, a similar methodology was applied by Harayama s group to the synthesis of another natural product, (-)-steganone, having antileukemic properties. Hence, the key step of the synthesis was the enantioselective lactone-opening reaction, performed with a combination of... 

S)-2-methyl-CBS-oxazaborolidine and BH3.THF, providing the corresponding lactone-opened product in an enantioselective manner (Scheme 5.46). 

Topotecan (9-[(dimethylammo)methyl]-10-hydroxy-CPT, 4) is the second approved semisynthetic, water-soluble CPT derivative. Similarly to irinotecan, topotecan undergoes equilibration at physiological pH, giving only small fraction of active lactone form and most of the introduced dose as inactive carboxylate [250, 251] (tartaric acid is used in new parenteral formulation to prevent the lactone opening [252]). 

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