Homoenolate anions

From the point of view of the synthesis of dissonant systems the most important finding reported by Reissig [19c] is the opening of cyclopropanes by fluoride ion-induced desilylation to give carboxylic ester stabilised "homoenolate" anions, from which a series of 4-oxoalkanoic esters (21a ). with a 1,4-D relationship, were prepared (Table 5.6) ... 

Marino and his coworkers [20a], on the other hand, studied the fluoride ion-induced desilylation of ethyl 2-silyloxycyclopanecarboxylates 24 and the resulting "homoenolate" anion 25 was allowed to react with different electrophiles, such as Michael acceptors, to give dissonant cyclopentene rings (26) via a [3 -i- 2] annulation strategy (Scheme 5.16). 

Let us now consider a dissonant 1,6-dicarbonyl system, which provides a good example of a [3,3]-sigmatropic rearrangement. The "illogical disconnection" would lead to an a,p-unsaturated ketone and a "homoenolate" anion ... 

Scheme 13.2. Reaction Sequences Involving Propanal Homoenolate Anion Synthetic... 

Richardson, S. K. Jeganathan, A. Watt, D. S. l-Vinyl-4-methyl-2,6,7-trioxabicyclo[2.2.2]oc-tanes unsaturated homoenolate anion equivalents. Tetrahedron Lett. 1987, 28, 2335-2338. 

Werstiuk, 1983). Moreover, such a homoenolate anion could eventually undergo ring closure leading to the formation of a cyclopropoxide anion (Chandrasekhar et al., 1981). 

Cyclopropanols in general, can well serve as homoenolate anion precursors, i.e., the P-anion of ethyl propionate 99), however, to avoid the easy base or acid induced ring opening the hydroxyl function of 42 must be protected when necessary. On simple addition of one equivalent of 3,4-dihydro-2H-pyran to a CH2C12 solution of the a-hydroxy acid 42, the tetrahydropyranyl ether 163 was obtained exclusively,... 

The title transformation provides an alternative pathway for the construction of the cyclopropane ring from a synthon of three carbons. The intramolecular cyclization of homoenolate anion equivalents is represented in Scheme 11, Eq. (ii). In practice, several routes involving metals such as Mg, Zn, and Ti, as well as different homoenolate precursors, have been developed [17a-e]. 

This homoenolate anion also acylates acid chlorides readily to give y-keto esters (equation I), but does not react with aldehydes or epoxides. 

Siloxycyclopropanes are known as homoenolate anion equivalents and have been favorably utilized in organic synthesis. Murai, Sonoda, and coworkers found that desi-lylative dimerization of the siloxycyclopropanes occurs on exposure to AgBp4 [26a], The reaction of 54 gives 1,6-diketone 55 in good yield (Sch. 13). The 8-silver ketone 56 is considered to be a key intermediate in the dimerization. The intermediary ver ketone, generated from siloxycyclopropane 57 and AgP, can be successfully trapped with allyl chloride to afford a (5,e-unsaturated ketone 58 [26b]. 

Some of these questions remain unanswered to this day. Homoenolate anions, however, have been studied since 1962, and presently their structure is understood not as a free 8-carbanion such as IV but as a much more stable homoanion (VI) (see Scheme 41.2). 

Although the anion is used as a means of derivatizing the sulfone the key step to this method is the generation of a homoenolate anion and its subsequent rearrangement. The subject of homoenolate anions and their synthetic applications has been extensively reviewed elsewhere. ... 

Reformatsky reagent which then adds to carbonyl derivatives according to Scheme 20. These authors also observed conjugate addition of the homoenolate anion to a,)5-unsaturated ketones in the presence of copper ion as shown ... 

Ring-opening reactions of cyclopropanone hemiketals are well known. Under appropriate conditions, cleavage of trimethylsilyl protected hemiketals can provide a synthetically useful route to homoenolate anions as noted earlier (Scheme 20). The reaction of an isopropoxy-titanium homoenolate (128) derived from 127 with an aldehyde has recently been used as the key step in the stereocontrolled construction of the steroidal side-chain of depresosterol (Scheme 49) ... 

The mechanism of the benzoin condensation, as depicted in Scheme 1, suggested that anions derived from a protected aldehyde cyanohydrin should function as nucleophilic acylating reagents. The use of protected cyanohydrins as carbanion equivalents has been studied by Stork and by Hunig and has found wide applicability in chemical synthesis. Such species may serve as either acyl anion equivalents or homoenolate anions. ... 

Heteroatom-substituted allylic anions as homoenolate anion equivalents 862... 

In accordance with this model one finds diastereoselectively anti products on reaction of aldehydes with ( )-allyl compounds, whereas allyl systems with the (Z)-configuration give mainly syn products and it is even possible to effect asymmetric induction. As the double bond of the product can be oxidatively cleaved to a CW3 group, the reaction can be regarded as a stereoselective aldol reaction, an aspect which explains the widespread interest in this type of reaction. With heterosubstituted allylic anions it is sometimes possible to effect predominantly y-attack with different electrophiles by the choice of the heteroatom.2 For instance it is well known that with sulfur substituents like —SR, —SOR or —SOjR the a-attack dominates, but doubly lithiated allenethiol possesses high y-reactivity and can be used as a homoenolate anion equivalent in reaction with electrophiles such as alkyl halides (Scheme 7). ... 

On the other hand the inherent a-selectivity of allylsulfur carbanions can sometimes be transformed to a y-reactivity by a sigmatropic rearrangement, earning in addition the stereoselectivity typical for such reactions (see Section 4.S.2.3). Due to their significance in synthesis a lot of work has been done on reactions of heteroatom-substituted allyl anions with special emphasis on their use as homoenolate anion equivalents. The more recent developments, with the possibility of introducing diastereoselectivity, will be discussed later in Section 4.S.3.2.I. 

There are two main synthetic applications where the reaction of an allyl system with electrophiles is accompanied by an allylic rearrangement. One consists of the use of heteroatom-substituted allylic anions as homoenolate anion equivalents and the other represents a synthetically valuable alternative to the aldol reaction by addition of allyl metal compounds to aldehydes. 

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