Lactone Claisen rearrangement

A highly successful route to stereoisomers of substituted 3-cyclohexene-l-carboxylates runs via Ireland-Claisen rearrangements of silyl enolates of oj-vinyl lactones. The rearrangement proceeds stereospeaifically through the only possible boat-like transition state, in which the connecting carbon atoms come close enough (S. Danishefsky, 1980 see also section 4.8.3, M. Nakatsuka, 1990). 

Some representative Claisen rearrangements are shown in Scheme 6.14. Entry 1 illustrates the application of the Claisen rearrangement in the introduction of a substituent at the junction of two six-membered rings. Introduction of a substituent at this type of position is frequently necessary in the synthesis of steroids and terpenes. In Entry 2, formation and rearrangement of a 2-propenyl ether leads to formation of a methyl ketone. Entry 3 illustrates the use of 3-methoxyisoprene to form the allylic ether. The rearrangement of this type of ether leads to introduction of isoprene structural units into the reaction product. Entry 4 involves an allylic ether prepared by O-alkylation of a (3-keto enolate. Entry 5 was used in the course of synthesis of a diterpene lactone. Entry 6 is a case in which PdCl2 catalyzes both the formation and rearrangement of the reactant. 

The C(9)—C(16) subunit was synthesized from the same starting material. The chain was extended by a boron enolate addition to 2-methylpropenal (Step D-2). After introduction of a double bond by selenoxide elimination in Step E-4, a Claisen rearrangement was used to generate an eight-membered lactone ring (Step E-6). 

A methylenation of cyclic carbonates such as 6/4-132 using dimethyltitanocene to give a ketene acetal, followed by a subsequent Claisen rearrangement, allowed the synthesis of medium-ring lactones such as 6/4-133 in good yields these are otherwise difficult to obtain. In this transformation, 6/4-133 is formed as a l l-mix-ture of the two atropisomers 6/4-133a and 6/4-133b (Scheme 6/4.33). The substrate... 

An instructive strategy to cyclize acyclic precursors to lactones was developed by Mulzer et al. (Scheme 7) [19]. The lactone 46 was formed via a halo-lactonization-dehalogenation [20] sequence from precursor 45, which in turn was build up by a Claisen rearrangement from the chiral allylic alcohol... 

Dehydrobromination of bromotrifluoropropene affords the more expensive trifluoropropyne [237], which was metallated in situ and trapped with an aldehyde in the TIT group s [238]synthesis of 2,6-dideoxy-6,6,6-trifluorosugars (Eq. 77). Allylic alcohols derived from adducts of this type have been transformed into trifluoromethyl lactones via [3,3] -Claisen rearrangements and subsequent iodolactonisation [239]. Relatively weak bases such as hydroxide anion can be used to perform the dehydrobromination and when the alkyne is generated in the presence of nucleophilic species, addition usually follows. Trifluoromethyl enol ethers were prepared (stereoselectively) in this way (Eq. 78) the key intermediate is presumably a transient vinyl carbanion which protonates before defluorination can occur [240]. Palladium(II)-catalysed alkenylation or aryla-tion then proceeds [241]. 

The Claisen rearrangement of lactonic enolates provides a new route to cycloalkenes. Cyclocitral was converted to the lactone (642) through a multistep sequence, the lactone deprotonated with LDA in THF at -78 °C, and the enolate quenched with f-butyldimethyl-chlorosilane (80JA6889, 6891). The crude ketene acetal (643) was heated at 110 °C for 10 h, and the product treated with fluoride ion to afford a single acid. Replacement of the quaternary carboxyl group by hydroxyl was accomplished through use of the carboxy inversion reaction (Scheme 147). The product (645) of this last reaction was identical with an authentic sample of widdrol in all respects excluding its optical rotation. 

BINAP, 127, 171, 191, 194, 196 olefin reaction, 126, 167, 169, 191 organic halides, 191 Pancreatic lipase inhibitors, 357 Pantoyl lactone, 56, 59 para-hydrogen, 53 Peptides, matrix structure, 350 Perhydrotriphenylene, crystal lattice, 347 Pericyclic reactions, 212 chiral metal complexes, 212 Claisen rearrangement, 222 Diels-Alder, 212, 291 ene reaction, 222, 291 olefin dihydroxylation, 150 Phase-transfer reactions asymmetric catalysis, 333... 

The (panial) description of the synthesis and coupling of the live fragments starts with the cyclohexyl moiety C21—CM The first step involved the enantio- and diastereoselective Sharpless epoxidation of l,4-pentadien-3-ol described on p 126f The epoxide was converted m four steps to a 3-vinyl 6-lactone which gave a 3-cydohexenecarboxylate via Ireland-Claisen rearrangement (cf p 87) Uncatalysed hydroboration and oxidation (cf. p 131) yielded the desired muis-2-methoxycydohexanol which was protected as a silyl ether The methyl car-... 

Further, medium-sized lactones have been prepared by a thermal elimination-Claisen rearrangement sequence, of unsaturated selenoxide cyclic acetals (equation 198)710. The reaction affords reasonable yields of these useful lactones upon treatment with DBU and a siloxy species at 185 °C. The reaction has been used as the key step in the synthesis of (-l-)-laurencin, which contains an 8-membered cyclic ether moiety711. 

Finally, using a variant of the Claisen rearrangement (Malherbe-Bellus712), nine-membered lactones may be synthesized in reasonable yields with some stereocontrol. 

Jones, G.B., Huber, R.S., and Chau, S. 1993. The Claisen rearrangement in synthesis Acceleration of the Johnson orthoester protocol en route to bicychc lactones. Tetrahedron, 49 369-80. 

As shown in Equation (179), a tandem Claisen rearrangement-lactonization involving the C-3 side chain of furan 292 was employed for a concise synthesis of hyperolactone C <2003JNP1039>. Although the product yield was low, the reaction created two contiguous quaternary carhon centers in one step. 

An intramolecular version of this Claisen rearrangement was used for the synthesis of a natural 10-membered lactone, phoracantholide J (8). The synthesis involves preparation of 6 by known reactions. Treatment of 6 with C MsSeBr in the presence of Hiinig s base gives the cyclic acetal 7 as a mixture of diastereoisomers in 71% yield. Decomposition of the selenoxide results in the rearranged y,B-unsaturated lactone 8 as the major product. The ready formation of a 10-membered ring is noteworthy because yields of rings of this size are low on lactonization of 0-hydroxy acids. 

Taking advantage of the likewise mild conditions of the Ireland version of the Claisen rearrangement, easily accessible ester or lactone bonds may be transformed via their enolates into C—C bonds. In the case of cyclic systems, the ring size may be varied by the position of the double bonds. Recently Funk et al. made use of this often applied principle for the synthesis of tricycle 76 (shown in Scheme 18) (76). 

In a series of elegant studies, Paquette and coworkers demonstrated the potential of the Claisen rearrangement for the stereocontrolled total synthesis of natural products. Dehydrative coupling of (2)-3-(trimethylsilyl)-2-propen-l-ol with cyclohexanone (51) under Kuwajima s conditions, followed by rearrangement of enol ether (52) in decalin, led in excellent stereoselectivity (>99 1) to aldehyde (53 Scheme 8). Concise construction of the eight-membered core of acetoxycrenulidine was achieved by intramolecular phenylseleno etherification of lactone (54), introduction of the exocyclic vinyl ether double bond by selenoxide elimination and subsequent Claisen rearrangement (Scheme 9, 66% from 54). ... 

As the ester enolate Claisen rearrangement allows for a stoichiometric combination of alcohol and acid components, it has been used for the formation of strategically important C—C bonds by esteriflcation or lactonization and subsequent rearrangement, such as is elegantly demonstrated in the synthesis of the antibiotic chlorothricolide (Scheme 24)." Radical decomposition of the selenoester of (132) leads to... 

Danishefsky and coworkers have demonstrated the conversion of lactones to carbocycles by the 3,3-sigmatropic shift of silylketene acetals. Jq the total synthesis of the Fusarium toxin equisetin, for example, keto lactone (138) was converted to its bissilyl derivative (139) by reaction with 2 equiv. of LDA and an excess of TMS-Cl. In situ thermolysis of ketene acetal (1 ) led to a very smooth transformation into ester (140), which was carried on to equisetin (Scheme 26). This methodology was also applied by Schreiber and Smith in the preparation of the cyclohexyl moiety of the immunosuppressive agent FK-506. Ireland-Claisen rearrangement of silylketene acetal (142), prepared by treatment with TBDMS-OTf and triethylamine at low temperature, provided, after hydrolysis of the silyl ester, the carboxylic acid (143) in 71% overall yield (Scheme 27). The strict translation of configuration via a boatlike transition state is typical for this permutation. 

In cyclic systems, however, conformational constraints can override the inherent preference for chairlike transition states in Cope as well as Claisen rearrangements and lead to a partial involvement if not a dominance of boat-like TS structures. In the Ireland rearrangement of lactones of type (247), for example, chair-like transition state (249) is accessible only when the diaxial bridging methylene chain becomes sufficient in length (n = 7, Scheme 44). The preference of boat-like transition state (250) over (251) is due to a serious A - -type interaction between the endocyclic oxygen atom and pseudoaxial substituent R in (251). 

A new approach to the synthesis of Prelog-Djerassi Lactonic acid (1) is reported. A key step in this synthesis involves an Ireland-Claisen rearrangement/silicon-mediated fragmentation sequence to provide the carbon framework in (1). 

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