A-alkenylation

In the presence of a base such as l,4-diazabicyclo[2.2.2]octane (DABCO) or tri-alkylphosphines, conjugated carbonyl compounds such as esters and amides add to aldehydes via the a-carbon to give a-alkenyl-P-hydroxy esters or amides. This sequence is called the Baylis-Hillman reaction and a simple example is... 

Corrosion of steel during oil well acidizing or acid pickling treatments can be controlled effectively and economically with organic corrosion inhibitors. These additives interact with the steel surface to form an adherent barrier, the nature of which depends on the additives physicochemical properties. Work to date has established that acetylenic alcohols chemisorb and subsequently polymerize on steel surfaces (1-5"). a,/MJnsaturated aldehydes and a-alkenyl-phenones appear to behave in a similar manner (6j7"). The nature of Current address Amoco Production Company, Tulsa, OK... 

The synthesis of (5 5 5) fused heterocycle 62 has been achieved via intramolecular Diels-Alder reaction involving a series of A-alkenyl-substituted furanyl amides 494 (Scheme 112) <2006JOC5432>. 

Hydrogenation of 1,3-dienes to terminal olefins is catalyzed by HRh(PPh3)4 and [Rh(CO)2(PPh3)2]2 in the presence of excess phosphine diene insertion into a metal- hydride bond to give a-alkenyl rather than 7r-allyl intermediates was postulated for the initial step (141). Mechanistic studies of the HRh(PPh3)4 catalyst (142) and a more reactive phosphole analog (143) HRh(DBP)4 [5-phenyl-5//-dibenzophosphole (DBP), 7] for... 

It is observed that insertion into a zirconacyclopentene 163, which is not a-substituted on either the alkyl and alkenyl side of the zirconium, shows only a 2.2 1 selectivity in favor of the alkyl side. Further steric hindrance of approach to the alkyl side by the use of a terminally substituted trans-alkene in the co-cyclization to form 164 leads to complete selectivity in favor of insertion into the alkenyl side. However, insertion into the zirconacycle 165 derived from a cyclic alkene surprisingly gives complete selectivity in favor of insertion into the alkyl side. In the proposed mechanism of insertion, attack of a carbenoid on the zirconium atom to form an ate complex must occur in the same plane as the C—Zr—C atoms (lateral attack 171 Fig. 3.3) [87,88]. It is not surprising that an a-alkenyl substituent, which lies precisely in that plane, has such a pronounced effect. The difference between 164 and 165 may also have a steric basis (Fig. 3.3). The alkyl substituent in 164 lies in the lateral attack plane (as illustrated by 172), whereas in 165 it lies well out of the plane (as illustrated by 173). However, the difference between 165 and 163 cannot be attributed to steric factors 165 is more hindered on the alkyl side. A similar pattern is observed for insertion into zirconacyclopentanes 167 and 168, where insertion into the more hindered side is observed for the former. In the zirconacycles 169 and 170, where the extra substituent is (3 to the zirconium, insertion is remarkably selective in favor of the somewhat more hindered side. 

A similar reaction of pyrroles 20 with acrylates provides the C-2 substituted a-alkenyl derivatives 21 in 24-91% yield [29]. The 2,6-dichlorobenzoyl protecting group is noteworthy as it prevents cyclization of the phenyl group onto the pyrrole ring (vide infra). 

Kinetics of the acid hydrolysis of A-alkenyl derivatives of phenoxazine, phenoth-iazine, and carbazole in aqueous dioxane suggest an ASe2 mechanism, based on the activation parameters and isotope effects. ... 

Phosphines, as nucleophiles, add to many unsaturated substrates giving metal-lated ylides. Scheme 17 collects some representative examples of the addition of phosphines to carbyne complexes, giving (57) [132], to allenylidenes (58) [133], a-alkenyls (59) [134], or a-alkynyls (60) [135]. Moreover, reaction of phosphines with 7i-alkenes [136] and 71-aIkynes (61)-(64) [137-140] have also been reported. It is not possible to explain in depth each reaction, but the variety of resulting products provides an adequate perspective about the synthetic possibihties of this type of reactions. 

The above experimental results strongly support a ligand-coupling mechanism [62, 63] for bismuth(V)-assisted a-alkenylation. Thus, the enolate attacks the... 

The alkenylation of phenols also proceeds smoothly in the presence of TMG (Scheme 6). The major products are not aryl alkenyl ethers but a-alkenylated cyclohexa-2,4-dienones. That is, C-alkenylation occurs exclusively at the ortho position of phenols. When 2-naphthol reacts with two equivalents of the alkenylbismuthonium salt, a,a-dialkenyl ketone is obtained in good yield as the sole alkenylated product. 

Some chiral 1,3,2-dioxastannolanes were used as catalysts in asymmetric Diels-Alder reactions of cyclopentadiene with methyl acrylate <90JCR(S)278>. A-Alkenyl- and -cycloalkenyl 1,3,2-oxaza-stannolanes, generated in situ from chiral amino alcohols, gave optically active 2-substituted aldehydes and ketones in modest to high chemical and optical yields after alkylation with methyl acrylate or acrylonitrile (which is usual for enamines) and subsequent hydrolysis <85CC504,85JOC3863>. 

Alkyl- or 3-aryl-2,4-oxazolidinediones via photochemical cyclization, ° organonickel-mediated carbonylation, ° cyclization of A-alkenyl-a-acet-amides, ° carboxylation and cyclization of 2-propynamides, °" cyclization of (9-carbamates of a-hydroxy acetic acids and esters,cyclization of a-hydroxy acetamides,and catalytic asymmetric dihydroxylation (ADH) of A-alkenoyl-2-oxazolidinones. ... 

The intramolecular Diels-Alder reaction of l-(A -a -alkenyl)-2(l//)-pyrazinones generates m-[l,7]naphthyridines in good yields (Scheme 62) <2001TL7397>. This method has been used in the development of a r-[l,7]naphthyr-idine-based scaffold on which to build a type VI external /3-turn mimic <2003EJ01868>, and for the synthesis and conformational analysis of [l,7]naphthyridine-based substance P antagonist analogues <2003T4721>. 

Essentially all triarylphosphines other than PPh3 are significantly more expensive than PPh3. Even so, some sterically and electronically modified monodentate triarylphosphines, such as tris(o-tolyl)phosphine (TTP)81a and tris(2-furyl)phosphine (TFP)81b, have exhibited catalytic reactivities that are clearly superior to those of PPh3 in many demanding cases of cross-coupling, such as a-alkenylation of a-haloenones82. 

SCHEME 57. Pd-catalyzed a-alkenylation and a-alkynylation of a-iodo-cr,fi-unsaturated carbonyl compounds... 

The reaction of /V-pentenyl-2-iodoindole in the presence of a palladium-triphenylphosphine catalyst led to the formation of a mixture of isomeric products in good yield (4.8.), Addition of thallium(I) acetate favoured the formation of an exocyclic double bond, while in its absence the product containing the endocyclic olefin moiety is formed preferentially. The shortening of the A-alkenyl chain by one carbon leads to the selective formation of a five membered ring.9 Starting from indole-carboxamide derivatives both /3-. and carbolinones are available in intramolecular Heck coupling. 

A variety of approaches have been employed to effect the preparation of a-alkenyl ketones and carbinols, including reactions of metallo alkenes with... 

Pyrroles can be a-alkenylated by acetylenedicarboxylic ester, as discussed in Section 3.3.1.8.1. 

The cyclization of A-alkenyl derivatives of resin-bound peptides by ruthenium-catalyzed ring-closing metathesis has been investigated [426-428]. These reactions do not proceed well on insoluble supports, and cyclizations in solution are generally more successful. 

Several (A/-alkenyl)iodoacetamides undergo cyclization when they are reacted with tetrakis(triphe-nylphosphine)palladium(O). The highest yields (which are generally only moderate) are obtained in DMF solution with 1,8-dimethylaminonaphthalene (Proton Sponge) to take up the hydrogen iodide formed. This reaction has been used to prepare piperidones, oxindoles, indolizidines, quinolizidines, pyrrolidines, indoles, quinolines53 and pyrrolizidines (equation 18).54... 

In the framework of Frustrated Lewis Pairs, Erker also investigated the hydroboration of a-alkenyl-phosphines with HB(C6F5)2.52/53 Starting from dimesityl(vinyl)phosphine, the CH2CH2-bridged PB 40a was first... 

Demethoxyerythratidinone (10), one of the simplest of the erythrina alkaloids, was isolated in 1973 by Barton et al. from Erythrina lithosperma [14]. A concise approach to such Erythrina alkaloids using a disfavored 5-endo-trig radical cyclization mediated by nickel powder was described by Zard and coworkers [15]. A-Alkenyl trichloroacetamide 7 was... 

Miller, J. A. Leong, W. Zweifel, G. Conformational instability of a-alkenyl and a-alkynyl vinyllithiums. Synthesis of stereodefined... 

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