Acyclic lactone

Hoye described an RCM-based total synthesis of the 20-membered marine macrolide dactylolide 288 and its subsequent conversion to the natural carbinolamide zampanolide 289 (Scheme 56), both feature a common highly unsaturated macrolide core, bridging a m-2,6-disubstituted 4-methylene tetrahydropyran unit. When the polyunsaturated acyclic lactone 286 (1 1 epimeric mixture around the /i r/-butyldimethylsilyl (TBS)-protected carbinol center) was in situ protected with bis-trimethylsilylacetamide (BSA) and then treated with catalyst G in benzene at 60 °C, each diastereomer smoothly cyclized to the corresponding cycloalkene 287 with exclusive ( )-geometry at the newly formed double bond. 

As with isolated rings, individual heterorings in fused systems which are synthetic equivalents of acyclic subunits, e.g. lactone, ketal, lactam, and hemiketal, can be disconnected. 

Esters undergo the same kinds of reactions that we ve seen for other carboxylic acid derivatives, but they are less reactive toward nucleophiles than either acid chlorides or anhydrides. All their reactions are equally applicable to both acyclic and cyclic esters, called lactones. 

Most heterocycles have the same chemistry as their open-chain counterparts. Lactones and acyclic esters behave similarly, lactams and acyclic amides behave similarly, and cyclic and acyclic ethers behave similarly. In certain cases, however, particularly when the ring is unsat lira ted, heterocycles have unique and interesting properties. 

It is important to note that the one-step conversion of 27 to 28 (Scheme 4) not only facilitates purification, but also allows differentiation of the two carbonyl groups. After hydrogenolysis of the iV-benzyl group (see 28—>29), solvolysis of the -lactone-ring in 29 with benzyl alcohol and a catalytic amount of acetic acid at 70 °C provides a 3 1 equilibrium mixture of acyclic ester 30 and starting lactone 29. Compound 30 can be obtained in pure form simply by washing the solid mixture with isopropanol the material in the filtrate can be resubjected to the solvolysis reaction. 

An example of a surprisingly facile and stereoselective formation of an eight-membered lactone from an acyclic precursor diene ester was observed during the total synthesis of the antitumor agent octalactin A (148) (Scheme 27) [81]. The dense substitution pattern in cyclization substrate 146 presumably imposes... 

NHC-promoted enolate formation from an enal, followed by a desymmetrising aldol event to generate P-lactones and loss of CO, has been exploited by Scheidt and co-workers to generate functionalised cyclopentenes 240 in high ee from enal substrates 238 (Scheme 12.52) [94]. Interestingly, the use of alkyl ketones in this reaction manifold allows the isolation of the p-lactone intermediates with acyclic diketones, P-lactones 239 are formed with the R group anti- to the tertiary alkox-ide, while with cyclic diketones the P-lactone products have the R group with a syn relationship to the alkoxide [95]. 

Acyclic diterpene-a-lactones Salix matsudan SiOj Hx + McjCO Hx + CHCI3 + UV Isolation 57 fD 0... 

This methodology has been applied to both acyclic esters and macrocyclic lactones. 

When the lactone silyl ketene acetal 18-1 is heated to 135° C a mixture of four stereoisomers is obtained. Although the maj or one is the expected [3,3] -sigmatropic rearrangement product, lesser amounts of other possible C(4a) and C(5) epimers are also formed. When the reaction mixture is heated to 100° C, partial conversion to the same mixture of stereoisomers is observed, but most of the product at this temperature is an acyclic triene ester. Suggest a structure for the triene ester and show how it can be formed. Discuss the significance of the observation of the triene ester for the lack of complete stereospecificity in the rearrangement. 

All these 3,4-dihydro-2H-1 -benzopyran-2-ones 17 and 18 are substrates of class A and class C (3-lactamases. They are thus the first 8-lactones that are hydrolyzed by [3-lactamases. The kcat values for these substrates are generally smaller than those of the analogous acyclic phenaceturates suggesting that the tethered leaving group obstructs the attack of water on the acyl-enzyme. Despite the apparent advantage of the long-lived acyl-enzymes, the irreversible inhibition by the functionalized compounds is no better than that of acyclic molecules 16. Thus, even the tethered QM cannot efficiently trap a second nucleophile at the [3-lactamase active site, at least as placed as dictated by the structure of compounds 18.70... 

Organo copper and lithium enolates of cyclic ketones, lactones, and lactams or acyclic ketones are converted with acylimidazoles or imidazole-N-carboxylates into the corresponding / -diketones or / -ketoesters ... 

Inspired by the proline-catalyzed Robinson annulation pioneered by Wiechert, Hajos, Parrish and coworkers [39], they were able to construct cyclohexanones of type 2-107 with up to four stereogenic centers with excellent enantio- and di-astereoselectivity from unsaturated ketones 2-104 and acyclic (l-ketoesters 2-105 in the presence of 10 mol% phenylalanine-derived imidazohdine catalyst 2-106. The final products can easily be converted into useful cyclohexanediols, as well as y- and e-lactones. 

Cyclo-reversions proceed readily in reactions of enantiopure D7i and D7i ent with cyclic (551) and acyclic (585) nitrones. The sulfinyl group in lactones D7i and D7i ent controls the Jt-facial selectivity and is also controller of the endo/exo selectivity (Scheme 2.274) (788). 

In 1993, Alexakis et al. reported the first copper-catalyzed asymmetric conjugate addition of diethylzinc to 2-cyclohexenone using phosphorous ligand 28 (32% ee).36 An important breakthrough was achieved by Feringa et al. with chiral phosphoramidite (S,R,R)-29 (Figure 1), which showed excellent selectivity (over 98% ee) for the addition of 2-cyclohexenone.37 Since then, efficient protocols for the conversion of both cyclic and acyclic enones, as well as lactones and nitroalkenes, have been developed featuring excellent stereocontrol. 

D-Ribonolactone is a convenient source of chiral cyclopentenones, acyclic structures, and oxacyclic systems, useful intermediates for the synthesis of biologically important molecules. Cyclopentenones derived from ribono-lactone have been employed for the synthesis of prostanoids and carbocyclic nucleosides. The cyclopentenone 280 was synthesized (265) from 2,3-0-cyclohexylidene-D-ribono-1,4-lactone (16b) by a threestep synthesis that involves successive periodate oxidation, glycosylation of the lactol with 2-propanol to give 279, and treatment of 279 with lithium dimethyl methyl-phosphonate. The enantiomer of 280 was prepared from D-mannose by converting it to the corresponding lactone, which was selectively protected at HO-2, HO-3 by acetalization. Likewise, the isopropylidene derivative 282 was obtained (266) via the intermediate unsaturated lactone 281, prepared from 16a. Reduction of 281 with di-tert-butoxy lithium aluminum hydride, followed by mesylation, gave 282. 

In order to obtain lactones from natural alkenols, we investigated the cyclocarbonylation of monoterpenic alcohols. The catalytic precursor is [PdCl2L2] in the presence of a slight excess of tin chloride and phosphine ligands. Dihydromyrcenol, a representative acyclic terpene containing a termi-... 

The oxidation of an acyclic ketone to an ester, or a cyclic ketone to a lactone (the Baeyer-Vilhger reaction) can be achieved with the help of metal species of the... 

For example, Nakamura and Kuwajima [15] have described 1-alkoxy-l-trimethylsilyloxycyclopropanes (15) -prepared by reductive silylation of alkoxy 3-chloropropanoates-, which react with aliphatic aldehydes, but not with ketones, in the presence of one equivalent of TiCl4 to give good yields of y-lactones 17 through the acyclic derivative ethyl 4-hydroxybutanoate (16) (Scheme 5.10). With aromatic aldehydes and their acetals the reaction leads directly to acyclic 1,4-D derivatives. 

We were interested in applications of the high level of stereocontrol associated with the asymmetric Birch reduction-alkylation to problems in acyclic and heterocyclic synthesis. The pivotal disconnection of the six-membered ring is accomplished by utilization of the Baeyer-Villiger oxidation (Scheme 7). Treatment of cyclohexanones 25a and 25b with MCPBA gave caprolactone amides 26a and 26b with complete regiocon-trol. Acid-catalyzed transacylation gave the butyrolactone carboxylic acid 27 from 26a and the bis-lactone 28 from 26b cyclohexanones 31a and 31b afforded the diastereomeric lactones 29 and 30. ... 

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