Ketenes, preparation from 0-lactones

The methoxyketene 297, coordinated to Cr carbonyl, is formed from methoxy-carbene easily by insertion of CO under irradiation [90]. An ester is formed by the reaction of ketene with alcohol. The aminocarbene complex 298 was prepared from benzamide and converted to phenylalanine ester 300 under irradiation of sunlight in alcohol via ketene 299 [91]. The eight-membered lactone 304 was prepared in high yield by the reaction of the alkyne 301 having the OH group in a tether with Cr carbene without irradiation. The vinylcarbene 302 is formed at first and converted to the vinylketene intermediate 303 as expected. The keto lactone 304 is formed from 303 by intramolecular reaction with the OH group and hydrolysis [92],... 

Allene carboxylic acids have been cyclized to butenolides with copper(II) chloride. Allene esters were converted to butenolides by treatment with acetic acid and LiBr. Cyclic carbonates can be prepared from allene alcohols using carbon dioxide and a palladium catalyst, and the reaction was accompanied by ary-lation when iodobenzene was added. Diene carboxylic acids have been cyclized using acetic acid and a palladium catalyst to form lactones that have an allylic acetate elsewhere in the molecule. With ketenes, carboxylic acids give anhydrides and acetic anhydride is prepared industrially in this manner [CH2=C=0 + MeC02H (MeC=0)20]. 

Construction of the suitably substituted geranic acid for making the furan ring has been effected too. For example, Poulter et al. have prepared the substituted geranate 865 by reaction of 4-methyl-3-pentenylcopper with the acetylenic ester 866. The ester 865 then underwent cyclization in the presence of acid to the lactone 867, related to scobinolide (161), and the action of acid on the lactol produced from 867 with diisobutylaluminum hydride gave perillene (849). The lactone 867 has also been prepared by a slightly different method the C9 alcohol 868 was made (in poor yield) from isobutenol and prenyl chloride with butyllithium. The extra carbon atom was introduced by the action of sodium cyanide on the epoxide of 868, and hydrolysis of the cyano group followed by dehydration yielded the lactone 867. The dimethylthioacetal of 867 has been used to synthesize perillene (849). This thioacetal was made from the suitably substituted ketene thioacetal 869 and dimethylsulfonium methylide. Thus the ketene thioacetal 870 (readily prepared from acetone, carbon disulfide, and sodium amylate, followed by methylation °) can be prenylated with lithium... 

An interesting route to a-carboxy-8-lactones (81) and a-methylenelactones (80), based on hydrolysis of Knoevenagel products (79) of Meldrum s acid with cyclic aliphatic ketones (78), has been developed (Scheme 14). Reduction of 5-methylene derivatives of Meldrum s acid has been performed catalyti-caiiyi30 or by use of LAH. Imidoylation reaction of Meldrum s acid and subsequent solvolysis of the resulting (82) yields -enamino esters (83) in good yields.Flash vacuum pyrolysis of alkylidene derivatives of Meldrum s acid can be used to prepare methylene ketenes (84), a class of compounds difficult to prepare by conventional methods. By this procedure, methylene ketenes are obtained from aromatic aldehydes and ketones and from aliphatic ketones in only two steps. Intramolecular trapping of the methylene ketene obtained from the ketone (85) has been used successfully in the synthesis of the naphthol (86). ... 

The silyl ketene acetals 2, prepared from the lactone enolates, usually rearrange before or during workup of the corresponding silyl esters. However, the large-ring acetals (n = 4, 7 Table 21, entries 5 and 6) are isolable and require heating in toluene to effect the rearrangement328. 

Corey and Beames showed that a highly oxyphilic reagent, prepared from ethane-1,2-dithiol or propane-1,3-dithiol and trimethylaluminium, reacts with esters and lactones to give ketene dithioacetal intermediates. Ketene dithioacetals with suitably placed hydroxyl groups can then undergo acid-catalysed cyclisation to give spirocyclic QiS-orthoesters. The method was used to generate a protected lactone in the first synthesis of Rhizoxin [Scheme 2.119]. ... 

Furthermore, ketene dithioacetals 83 prepared from unsaturated ketones 82 by means of Peterson reactions are converted to the bicydic lactones 84 by treatment with cerium(IV) ammonium nitrate (CAN) in wet acetonitrile (Scheme 2.51) [142]. The oxidative cydization occurs stereosdectively in moderate yields. 

A series of dichloro-/3-lactones (48) have been prepared from substituted benzaldehydes and dichloroketen yields are higher with electron-deficient aldehydes. Unsaturated /3-lactones of type (49) have been obtained by a [2ir + 2ir] addition reaction between isopropylideneketen and ketens (c/. 1, 50). 

In the [2 + 2]cycloaddition chemistry, enantioselective carbo[2 + 2]cycloaddition catalyzed by chiral Al(III) Lewis acids has not been reported. On the other hand, catalytic enantioselective [2 + 2]cycloaddition of ketenes and aldehydes is well known. As the pioneering work, Miyano and coworkers reported that substoichiometric amount of chiral Lewis acid (S)-(46a) prepared from (S)-BINOL derivative and Me3Al promotes [2 + 2]cycloaddition of aldehydes and ketene to give P-lactones in moderate optical yield (Scheme 6.106) [125]. The same research group also reported (R,R)-(116a) as a more effective Lewis acid catalyst. 

While diketene remains a very important synthetic precursor, there has been increasing interest in the chemistry of a-methylene-/3-lactones, 3-methylene-2-oxetanones. However, unlike diketene, which can be readily synthesized by the dimerization of aldehydic ketenes, there are few methods for the synthesis of a-methylene-/3-lactones in the literature. Recent strategies for the preparation of the compounds are discussed in Section 2.05.9.2. The kinetic resolution of racemates of alkyl-substituted a-methylene-/3-lactones has been carried out via a lipase-catalyzed transesterification reaction with benzyl alcohol (Equation 21) <1997TA833>. The most efficient lipase tested for this reaction was CAL-B (from Candida antarctica), which selectively transesterifies the (A)-lactone. At 51% conversion, the (R)-f3-lactone, (R)-74, and (A)-/3-hydroxy ester, (S)-75, were formed in very high enantio-selectivities (up to 99% ee). 

Recently, Mukaiyama and co-workers prepared cinchona alkaloid-derived chiral quaternary ammonium phenoxide-phenol complex 23 and used it as an efficient organocatalyst for the tandem Michael addition and lactonization between oc,f-unsaturated ketones and a ketene silyl acetal 24 derived from phenyl isobutyrate. This approach permits the highly enantioselective synthesis of a series of 3,4-dihydropyran-2-ones (25), as shown in Scheme 4.11 [17]. 

Naturally occurring sorbic acid may be extracted as the lactone (parasorbic acid) from the berries of the mountain ash Sorbus aucuparia L. (Fam. Rosaceae). Synthetically, sorbic acid may be prepared by the condensation of crotonaldehyde and ketene in the presence of boron trifluoride by the condensation of crotonaldehyde and malonic acid in pyridine solution or from 1,1,3,5-tetraalkoxyhexane. Fermentation of sorbaldehyde or sorbitol with bacteria in a culture medium has also been used. 

It took another 35 years until the first (and still the only known) enantioselective total synthesis of (/ )-ochratoxin a (326), and therefore of ochratoxins A and B, was published by Gill et al. in 2002 (264, 265). Scheme 6.1 shows six steps of the nine-step synthesis, which was achieved with 10% overall yield. The first three steps of the procedure are not shown and comprise the preparation of 327 from (/ )-2-methyloxirane according to ref. (266). Ketene dimethyl acetal and acetylenic ester 327 react in an intermolecular cycloaddition to give 328. This diene undergoes a Diels-Alder reaction with methyl propiolate to yield 329. Lactonization ( 330), demethylation ( 331), chlorination ( 332), and methyl ester cleavage finally furnished enantiomerically pure ochratoxin a (326) (267). 

The Baeyer-Villiger oxidation of l-methoxybicyclo[2.2.2]oct-5-enones, which are easily prepared by hydrolysis of the Diels-Alder adducts derived from dihydroanisoles and a-chloroacrylonitrile, provides an attractive route to 4-substituted cyclohex-2-en-l-one 4-acetic acid derivatives (Scheme 17)/ Substituted cyclohexenes of the type (124) are produced under mild conditions and in good yield by Claisen rearrangement of mixed ketene-acetals derived from the substituted lactone (123)." ... 

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