Cyclopentenone preparation

Use of the chiral carbon pool for cyclopentenone preparation is also known. The fungal metabolite terrein [88] was selectively monoacetylated and then reduced with chromous chloride to enone [89]. Acetylation and olefin cleavage with ruthenium tetroxide aiwi sodium periodate led to aldehyde [90], which was readily decarbonylated to [65] (51). An alternative route (52) began with the less common S,S-tartaric acid [91], converted in four steps to diiodide [92]. Dialkylation of methyl methylthiomethyl sulfoxide with [92] gave the cyclopentane derivative [93]. Treatment of [93]... 

Certain variations in the a-chain were introduced by the use of the appropriate alkylating agent, BrCH2XCOOR (g2). The various cyclopentenones prepared in this way include the homologous series 9, W, and n and the precursor 12 (see Scheme 3) for the 3,3-dimethylprostaglandin series. 

Cyclic Ketones.—Many examples exist of reversible reactions that are not pre-paratively useful because of unfavourable equilibrium constants, and numerous other reactions are preparatively poor through self-condensation of the substrate. Both of these problems occur in the cyclopentenone preparation shown [equation (13)]. However, a cunning solution has been found whereby both the acid- and base-catalysed steps may be carried out concurrently using a mixed-ion-exchange resin consisting of sulphonic acid beads and quaternary ammonium hydroxide beads, giving the cyclopentenone in 87% (crude) yield. ... 

Several 1,4-dicarbonyl compounds are prepared based on this oxidation. Typically, the 1,4-diketone 10 or the 1,4-keto aldehyde 12 can be prepared by the allylation of a ketone[24] or aldehyde[61,62], followed by oxidation. The reaction is a good annulation method for cyclopentenones (11 and 13). Syntheses of pentalenene[78], laurenene[67], descarboxyquadrone[79], muscone (14 R = Me)[80]) and the coriolin intermediate 15[71] have been carried out by using allyl group as the masked methyl ketone (facing page). 

Reaction of (T)-(-)-2-acetoxysuccinyl chloride (78), prepared from (5)-mahc acid, using the magnesiobromide salt of monomethyl malonate afforded the dioxosuberate (79) which was cyclized with magnesium carbonate to a 4 1 mixture of cyclopentenone (80) and the 5-acetoxy isomer. Catalytic hydrogenation of (80) gave (81) having the thermodynamically favored aH-trans stereochemistry. Ketone reduction and hydrolysis produced the bicycHc lactone acid (82) which was converted to the Corey aldehyde equivalent (83). A number of other approaches have been described (108). 

Further utility of the Andersen sulphoxides synthesis is demonstrated by the preparation of optically active unsaturated sulphoxides which were first prepared by Stirling and coworkers359 from sulphinate 276 and the appropriate vinylic Grignard reagents. Later on, Posner and Tang360 prepared in a similar way a series of ( )-l-alkenyl p-tolyl sulphoxides. Posner s group accomplished also the synthesis of (+)-(S)-2-(p-tolylsulphinyl)-2-cyclopentenone 287, which is a key compound in the chiral synthesis of various natural products361 (equation 159). 

Conjugate addition of enolate anions to a, jS-unsaturated sulphoxides followed by a sulphoxide- ketone transformation were used for the preparation of 1,4-dicarbonyl compounds and cyclopentenone derivatives (equation 355)648. 

Alkenyl sulfoxides 177 and 178, which can be readily prepared from 1-alkynes222, provide synthones for the carbocations 179 and 180. These synthones are useful for the simple construction of cyclopentenones and also in providing an electrophilic precursor for the jS-side-chain on prostanoids223,224. 

When methyltriflone 248 was treated with two equivalents of n-butyllithium at — 50 °C in THF, a, a-dianion 249 is formed. After methylation of this dianion, further treatment with two equivalents of n-butyllithium gives the a, a -dianion 250. This dianion can be alkylated at the terminal carbon atom, like the dianions from ketosulfones. On reaction with three equivalents of n-butyllithium, 248 gives a, a, a -trianion 251 which, upon methylation with methyl iodide, afforded methylated product 252. Hendrickson and Palumbo328 prepared cyclopentenone derivatives using 248 as the starting material. 

This type of asymmetric conjugate addition of allylic sulfinyl carbanions to cyclopen-tenones has been applied successfully to total synthesis of some natural products. For example, enantiomerically pure (+ )-hirsutene (29) is prepared (via 28) using as a key step conjugate addition of an allylic sulfinyl carbanion to 2-methyl-2-cyclopentenone (equation 28)65, and (+ )-pentalene (31) is prepared using as a key step kinetically controlled conjugate addition of racemic crotyl sulfinyl carbanion to enantiomerically pure cyclopentenone 30 (equation 29) this kinetic resolution of the crotyl sulfoxide is followed by several chemical transformations leading to (+ )-pentalene (31)68. 

An interesting family of polycyclic pyrroles was described in 2005 using again the synthetic sequence of a Stetter reaction for the preparation of the starting 1,4 diketones followed by a microwave-assisted Paal-Knorr condensation [35]. For example, cyclopentenone 23 (obtained in a Pauson-Khand cyclization) reacted imder Stetter reaction conditions to give the amino ketone 25 (Scheme 8). The microwave-assisted Paal-Knorr cyclization of 25 with different amines gave a small collection of tricychc pyrrole 2-carbox-amides. 

Olefin hydrocarbonylation can be used in conjunction with oxidative addition to prepare indanones and cyclopentenones, but the reaction is limited to terminal alkenes.243... 

After preliminary evaluation of the original route, we identified several problems in the preparation of cyclopentenone 2 ... 

In order to prepare multi-kilogram quantities of 1 our efforts were strictly focused on the development of an asymmetric route. Our retrosynthetic approach was centered on the preparation of cyclopentenone 27 which, we envisioned, could be elaborated to chiral hydroxy acid 26 through a series of asymmetric transformations (Scheme 7.4). Etherification of the hydroxy group of 26 with benzylic alcohol 25 followed by installation of (P)-nipecotate 23 at the acid position of 24, would furnish the drug candidate 1. This section will address the following ... 

Selection of starting materials is always key for early development Starting materials should be readily available within a reasonable lead time. Cyclopentenone 27 was prepared by four different routes [6]. 

Because the a-nitroketones are prepared by the acylation of nitroalkanes (see Section 5.2), by the oxidation of (3-nitro alcohols (Section 3.2.3), or by the nitration of enol acetates (Section 2.2.5), denitration of a-nitro ketones provides a useful method for the preparation of ketones (Scheme 7.10). A simple synthesis of cyclopentenone derivatives is shown in Eq. 7.66.76... 

The [2+2+1] cycloaddition of an alkene, an alkyne, and carbon monoxide is known as the Pauson-Khand reaction and is often the method of choice for the preparation of complex cyclopentenones [155]. Groth and coworkers have demonstrated that Pauson-Khand reactions can be carried out very efficiently under microwave heating conditions (Scheme 6.75 a) [156]. Taking advantage of sealed-vessel technology, 20 mol% of dicobalt octacarbonyl was found to be sufficient to drive all of the studied Pauson-Khand reactions to completion, without the need for additional carbon monoxide. The carefully optimized reaction conditions utilized 1.2 equivalents of... 

Decenoic acid (143), easily prepared from the same telomer 142, was cyclized via acid chloride using A1C13 to give 2-pentyl-2-cyclopentenone (144). Michael addition of methyl malonate followed by removal of one ester group produced methyl dihydrojasmonate (145) (129) ... 

Castagnino, E., Corsano, S., and Strappavecchia, G.P., The preparation of a novel oxo-cyclopenten-2-phosphonate derivative, useful intermediate for 2-alkyl-substituted cyclopentenones synthesis, Tetrahedron Lett., 93, 1985. 

Eq. 71 becomes an effective cyclopentenone fusion. Such a sequence has proven to be the preferred preparation of polyquinanes exemplified in Eq. 72 52). 

This reaction can be used to effect a [3 + 2] or a [3 + 2 + 2] annelation to a cyclopentenone ketal or a cycloheptadienone ketal, respectively. Thus the adduct of a vinylcuprate with 1 rearranges thermally (240-290°) to 3-cyclopentenone ketals (3) in 50-80% yield (equation I). [3 + 2 + 2]Annelation involves preparation of... 

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. 

The groups of Liebeskind and Moore independently explored the ring expansion of allenyl-substituted cyclobutenol derivatives. This allows either the preparation of 5-alkylidene-2-cyclopentenones, which are substructures of naturally occurring bioactive compounds [59], or the generation of o-quinone methides, which are very attractive intermediates in the synthesis of hexahydrocannabinols [109]. As shown in Scheme 8.42, the ring expansion of the tetrasubstituted methoxyallene derivative 170 furnished the vinylhydroquinone 172 in high yield [109]. 

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